Abstract

Background

Children 6 through 35 months of age are recommended to receive half the dose of influenza vaccine compared with older children and adults.

Methods

This was a 6-site, randomized 2:1, double-blind study comparing full-dose (0.5 mL) trivalent inactivated influenza vaccine (TIV) with half-dose (0.25 mL) TIV in children 6 through 35 months of age. Children previously immunized with influenza vaccine (primed cohort) received 1 dose, and those with no previous influenza immunizations (naive cohort) received 2 doses of TIV. Local and systemic adverse events were recorded. Sera were collected before immunization and 1 month after last dose of TIV. Hemagglutination inhibition antibody testing was performed.

Results

Of the 243 subjects enrolled (32 primed, 211 naive), data for 232 were available for complete analysis. No significant differences in local or systemic reactions were observed. Few significant differences in immunogenicity to the 3 vaccine antigens were noted. The immune response to H1N1 was significantly higher in the full-dose group among primed subjects. In the naive cohort, the geometric mean titer for all 3 antigens after 2 doses of TIV were significantly higher in the 12 through 35 months compared with the 6 through 11 months age group.

Conclusions

Our study confirms the safety of full-dose TIV given to children 6 through 35 months of age. An increase in antibody responses after full- versus half-dose TIV was not observed, except for H1N1 in the primed group. Larger studies are needed to clarify the potential for improved immunogenicity with higher vaccine doses. Recommending the same dose could simplify the production, storage, and administration of influenza vaccines.

Influenza virus is associated with significant annual morbidity and mortality [ 1–3 ], with influenza hospitalization rates in children similar to those of high-risk adults [ 4 ]. However, unlike adults, nearly 50% of children hospitalized for influenza have no underlying medical comorbidities.

The main prevention strategy for influenza illness is vaccination. Since 2008–2009, universal influenza vaccination has been recommended for all children 6 months through 18 years of age [ 5 ]. The United States advisory bodies recommend that children 6 through 35 months of age receive half the antigenic dose, 7.5 μg of each hemagglutinin antigen (HA) in a total volume of 0.25 mL. This recommendation is based primarily on the results of large multicenter trials of monovalent and bivalent whole-virus influenza vaccines conducted in children during the 1970s [ 6 ]. These earlier studies demonstrated that adverse reactions were significantly more common in children than in adults after vaccination with these whole-virus vaccines [ 7–12 ]. It was also demonstrated that with careful adjustment of the dose, the severity and rates of reactions following whole-virus vaccines could be reduced to acceptable levels in children [ 9 , 12 , 13 ]. However, the introduction of split-virus influenza vaccines, which are less reactogenic than whole-cell vaccines, may have changed the necessity for dosage adjustments in children 6 through 35 months of age [ 7–12 ].

Although randomized, controlled trials have established the efficacy of split-virus trivalent inactivated influenza vaccine (TIV) in older children, efficacy data for younger children (eg, <5 years of age) are limited. Reports of protective efficacy against laboratory-confirmed influenza illness have varied between 60% and 95% when the influenza vaccine strains matched the predominant circulating influenza virus [,14–16 ] and between 44% and 57% when there was a partial match of circulating and vaccine strains. Lower efficacy has been reported in children <5 years of age compared with older children [ 16–20 ]. Furthermore, in these young children (eg, <36 months of age), the percentage that achieve seroprotective titers to different influenza virus vaccine strains varied from 30% to 100% after 2 doses [ 10 , 17 , 21–23 ]. A number of immunologic mechanisms have been proposed to explain these observations. One possible explanation of the poor immune response in young children may be the lower antigen doses used. Therefore, the primary aim of our study was to determine whether a higher dose of influenza vaccine would be safe in the 6 through 35 months age group. In addition, we sought to determine whether immunization with 0.5 mL doses of TIV (15 μg of each HA) would improve the immunogenicity without increasing the reactogenicity of TIV when administered to children 6 through 35 months of age with and without a history of previous TIV vaccination.

METHODS

Subjects

Healthy children 6 through 35 months of age (naive cohort) or 12 through 35 months of age (fully primed cohort) who were available for the entire study period and whose parents or guardians provided informed consent were eligible to participate. Children who were eligible in the fully primed cohort also required a history of receiving 2 doses of 2009–2010 H1N1 influenza vaccine and 2 doses of TIV at any time in the past. Influenza vaccine-naive subjects were recruited in year 1 (2010–2011) and year 2 (2011–2012) of the study. Influenza vaccine-primed subjects were only recruited in year 1 of the study (2010–2011). Subjects were recruited from private and hospital-based practices as well as surrounding communities. Exclusion criteria are included in Appendix 1.

Vaccines

Season 2010–2011 and 2011–2012 TIV vaccines (Fluzone; sanofi pasteur, Swiftwater, PA) were provided in prefilled thimerosal-free syringes containing either 0.25 mL or 0.5 mL and contained 7.5 μg and 15 μg, respectively, of each of the following: A/California/7/09 (H1N1)-like virus, A/Perth/16/2009 (H3N2)-like virus, and B/Brisbane/60/2008-like virus.

Study Design

This was a prospective, phase I, 2-arm, 2:1 randomized, double-blinded trial comparing the safety and immunogenicity of 0.5 mL (full dose) with 0.25 mL (half dose) of TIV in children 6 through 35 months of age with and without a history of previous TIV vaccination (ClinTrials.gov: NCT01164553). Children with a history of previous TIV vaccination received 1 dose, whereas previously unvaccinated children received 2 doses of TIV. The study was approved by Institutional Review Boards of all 6 institutions: Vanderbilt University, Nashville, TN; Cincinnati Children's Hospital Medical Center, Cincinnati, OH; University of Maryland, Baltimore, MD; Saint Louis University, St. Louis, MO; University of Iowa, Iowa City, IA; and Emory University, Atlanta, GA. The studies were conducted before the 2010–2011 and 2011–2012 influenza seasons. For each season, enrollment ended when the influenza season began in that community. The influenza season was defined by identification of at least 2 respiratory positive tests for influenza with at least 10% of the diagnostic tests positive during 2 consecutive weeks in the clinical or research laboratory at each respective site.

An electronic data system (EMMES Corporation, Rockville, MD) assigned participants to receive study vaccine containing either 0.25 mL or 0.5 mL, stratified by trial site and prior vaccine status (1:2 allocation, in random block sizes of 3 or 6 within stratum). Designated unblinded nurses administered the requisite vaccine and were excluded from any other study activity.

To determine safety outcomes, parents documented local and systemic reactions for 7 days after each vaccination on a memory aid, grading each event as mild, moderate, or severe according to a predefined scale (Appendix 2). To determine immunogenicity outcomes, serum samples were obtained before each vaccination and 28 days after final vaccination in both cohorts in year 1. In year 2, serum samples were obtained before the first vaccination and 28 days after the second vaccination, whereas only a subset of subjects had sera obtained before the second dose of TIV to facilitate an increase in enrollment numbers.

Laboratory Assays

Serum samples were treated with receptor-destroying enzyme and were heat-inactivated before analysis for influenza-specific antibody using a standard hemagglutination inhibition (HAI) assay and egg-derived antigen from the vaccine strain obtained from the Centers for Disease Control and Prevention [ 24 ]. Each sample was tested in a blinded fashion in duplicate at a central laboratory (Cincinnati, OH). Sera were tested beginning at an initial dilution of 1:10, with titers <10 assigned a value of 5 for analysis.

Statistical Analysis

The sample size for this study was selected to provide a safety assessment of subjects receiving a full dose (0.5 mL) of TIV. A target of 180 naive subjects to receive the full dose of vaccine was selected to provide an 80% probability of observing at least 1 event if the true underlying probability was 1.00% or greater.

Adverse events (AEs) were tabulated by MedDRA categories, severity, and relationship to vaccination for group. The proportion of subjects in each dose group who experienced AEs was compared using Fisher's exact test. Solicited reactions were summarized using the most severe response recorded during the 7-day period postvaccination. These derived “maximum severity” variables were dichotomized into none or mild versus moderate or severe, and Fisher's exact test was used to compare among groups.

Fisher's exact test was used to compare the differences in rates of seroprotection for HAI antibody responses between the high-dose and standard-dose groups. Age was dichotomized into age <1 year or age ≥1 year. Geometric mean titers (GMTs) were analyzed using analysis of variance; the seroprotection (HAI titer ≥ 1:40) and seroconversion (4-fold or greater rise in titers to a postvaccination titer of at least 1:40) were analyzed using logistic regression. Only results with P value <.05 were reported as significant. No corrections were made for multiple comparisons. The estimate and 95% confidence interval (CI) of differences in GMTs were based on 100 000 bootstrap simulations.

RESULTS

A total of 243 healthy children aged 6 through 35 months of age (naive cohort, n = 211) or 12 through 35 months of age (fully primed cohort, n = 32) were enrolled between October 5, 2010 and March 2, 2012 (Figure 1 ). In year 1 only, 32 children were enrolled in the primed group, with 23 receiving full dose and 9 receiving the half dose. Of the 211 subjects enrolled in the naive group, 140 received the full dose and 71 received half dose: 67 in year 1 (44 full dose and 23 half dose) and 144 in year 2 (96 full dose and 48 half dose).

Figure 1.

Children assessed for eligibility, randomized, vaccinated, and included in the analysis of safety and immunogenicity.

Figure 1.

Children assessed for eligibility, randomized, vaccinated, and included in the analysis of safety and immunogenicity.

Table 1 includes demographic information for the primed and naive groups. No statistical differences were noted between the full-dose and half-dose groups. Among subjects enrolled in the 2010–2011 season, the mean age ± standard deviation of subjects in the fully primed cohort was 24.8 ± 5.9 months, and in the naive cohort the mean age ± standard deviation was 15.2 ± 8.1 months. The mean age of naive subjects enrolled in the 2011–2012 season was 11.3 ± 5.2 months. Among all subjects enrolled, 48% were male and 52% female. Overall, 98% of subjects were of non-Hispanic ethnicity. The racial distribution included 67% white, 26% black/African American, 5% multiracial, and 1% Asian.

Table 1.

Demographics

  2010–2011 Influenza Season
 
2011–2012 Influenza Season
 
All
(N = 243)  
Naive Cohort 0.25 mL TIV
(N = 23)  
Naive Cohort 0.5 mL TIV
(N = 44)  
Fully Primed Cohort 0.25 mL TIV
(N = 9)  
Fully Primed Cohort 0.5 mL TIV
(N = 23)  
Naive Cohort 0.25 mL TIV
(N = 48)  
Naive Cohort
0.5 mL TIV
(N = 96)  
Gender, n (%)  
 Male 14 (61) 18 (41) 3 (33) 13 (57) 21 (44) 48 (50) 117 (48) 
 Female 9 (39) 26 (59) 6 (67) 10 (43) 27 (56) 48 (50) 126 (52) 
Ethnicity, n (%)  
 Non-Hispanic 23 (100) 43 (98) 9 (100) 23 (100) 45 (94) 94 (98) 237 (98) 
 Hispanic 1 (2) 3 (6) 2 (2) 6 (2) 
Race, n (%)  
 Asian 2 (2) 2 (1) 
 African American 7 (30) 11 (25) 1 (11) 1 (4) 18 (38) 26 (27) 64 (26) 
 White 14 (61) 30 (68) 8 (89) 20 (87) 29 (60) 62 (65) 163 (67) 
 Multiracial 2 (9) 3 (7) 2 (9) 6 (6) 13 (5) 
 Other/unknown 1 (2) 1 (0) 
Age (months)  
 Mean (standard deviation) 13.1 (6.4) 16.4 (8.6) 23.4 (6.4) 25.3 (5.8) 11.8 (5.6) 11.1 (5.0) 14.2 (7.6) 
 Median 10.7 13.8 21.2 24.4 9.9 9.4 10.8 
 Min, max (7.5, 34.9) (6.3, 34.8) (15.6, 33.7) (15.9, 34.0) (6.0, 28.9) (6.2, 29.0) (6.0, 34.9) 
  2010–2011 Influenza Season
 
2011–2012 Influenza Season
 
All
(N = 243)  
Naive Cohort 0.25 mL TIV
(N = 23)  
Naive Cohort 0.5 mL TIV
(N = 44)  
Fully Primed Cohort 0.25 mL TIV
(N = 9)  
Fully Primed Cohort 0.5 mL TIV
(N = 23)  
Naive Cohort 0.25 mL TIV
(N = 48)  
Naive Cohort
0.5 mL TIV
(N = 96)  
Gender, n (%)  
 Male 14 (61) 18 (41) 3 (33) 13 (57) 21 (44) 48 (50) 117 (48) 
 Female 9 (39) 26 (59) 6 (67) 10 (43) 27 (56) 48 (50) 126 (52) 
Ethnicity, n (%)  
 Non-Hispanic 23 (100) 43 (98) 9 (100) 23 (100) 45 (94) 94 (98) 237 (98) 
 Hispanic 1 (2) 3 (6) 2 (2) 6 (2) 
Race, n (%)  
 Asian 2 (2) 2 (1) 
 African American 7 (30) 11 (25) 1 (11) 1 (4) 18 (38) 26 (27) 64 (26) 
 White 14 (61) 30 (68) 8 (89) 20 (87) 29 (60) 62 (65) 163 (67) 
 Multiracial 2 (9) 3 (7) 2 (9) 6 (6) 13 (5) 
 Other/unknown 1 (2) 1 (0) 
Age (months)  
 Mean (standard deviation) 13.1 (6.4) 16.4 (8.6) 23.4 (6.4) 25.3 (5.8) 11.8 (5.6) 11.1 (5.0) 14.2 (7.6) 
 Median 10.7 13.8 21.2 24.4 9.9 9.4 10.8 
 Min, max (7.5, 34.9) (6.3, 34.8) (15.6, 33.7) (15.9, 34.0) (6.0, 28.9) (6.2, 29.0) (6.0, 34.9) 

Abbreviations: TIV, trivalent inactivated influenza vaccine.

Of the 243 participants enrolled, 29 subjects were excluded from the immunogenicity analysis as described in Figure 1 . All subjects receiving at least 1 dose were included in the safety summaries to the extent data were available.

Safety

Proportions of full-dose and half-dose recipients by cohort experiencing local and systemic reactions over time are displayed in Figure 2 A and B and Figure 3 A and B. Summary tables of maximum local and systemic reactogenicity for primed and naive subjects can be found in Appendix 3 . No safety differences between the full-dose or half-dose groups were noted for either the fully primed or naive cohorts for systemic reactions and local reactions when both seasons were combined. When separating the naive cohort by seasons, for the 2010–2011 influenza season, elevated temperatures were more common in the half-dose group with 7 of 23 (30.4%) compared with 2 of 44 (4.5%) in the full-dose group ( P < .01). Likewise, in this cohort, 12 of 23 (52.2%) subjects in the half-dose group compared with 11 of 44 (25%) in full-dose group had decreased appetite ( P = .03). In the 2011–2012 influenza season, the incidence of elevated temperature was reversed so that 3 of 48 (6.3%) subjects in half-dose group compared with 21 of 94 (22.3%) in the full-dose group had elevated temperatures ( P < .02). To further explore these differences in elevated temperature, we also examined antipyretic use and the incidence of elevated temperatures by day ( Appendix 4 ). For the naive group in 2010–2011, the antipyretic use for half dose after vaccination 1 and 2, respectively, was 21% (4 of 19) and 26% (5 of 19) compared with the full-dose group, 11% (4 of 38) and 3% (1 of 37). In 2011–2012, the antipyretic use for half dose after vaccination 1 and 2, respectively, was 18% (8 of 44) and 10% (4 of 40) compared with the full-dose group, 30% (27 of 91) and 20% (17 of 83). Thus, the differences in elevated temperature for each season was also seen when antipyretic use was compared. The only other significant difference in the 2011–2012 season was that 8 of 48 (16.7%) subjects in the half-dose group compared with 32 of 96 (33.3%) in the full-dose group had increased redness at the injection site ( P < .05).

Figure 2.

(A) Proportion of children in the naive group who experienced solicited local reactogenicity during the 8 days after receiving 2 doses of either 0.5 mL or 0.25 mL of TIV. (B) Proportion of children in the primed group who experienced solicited local reactogenicity during the 8 days after receiving 1 dose of either 0.5 mL or 0.25 mL TIV.

Figure 2.

(A) Proportion of children in the naive group who experienced solicited local reactogenicity during the 8 days after receiving 2 doses of either 0.5 mL or 0.25 mL of TIV. (B) Proportion of children in the primed group who experienced solicited local reactogenicity during the 8 days after receiving 1 dose of either 0.5 mL or 0.25 mL TIV.

Figure 3.

(A) Proportion of children in the naive group who experienced solicited systemic reactogenicity during the 8 days after receiving 2 doses of either 0.5 mL or 0.25 mL TIV. (B) Proportion of children in the primed group who experienced solicited systemic reactogenicity during the 8 days after receiving 1 dose of either 0.5 mL or 0.25 mL TIV. Vac, vaccine.

Figure 3.

(A) Proportion of children in the naive group who experienced solicited systemic reactogenicity during the 8 days after receiving 2 doses of either 0.5 mL or 0.25 mL TIV. (B) Proportion of children in the primed group who experienced solicited systemic reactogenicity during the 8 days after receiving 1 dose of either 0.5 mL or 0.25 mL TIV. Vac, vaccine.

The majority of local reactions occurred within the first 3 days of vaccination for both cohorts. The most common local symptom recorded was redness, and the most common systemic reaction was irritability/fussiness. Only 26 of 241(11%) of all subjects (7 of 80 [9%] in the half-dose group and 19 of 161 [12%] in the full-dose group) and 12 of 196 (6%) of all subjects (4 of 65 [6%] in the half-dose group and 8 of 131 [6%] in the full-dose group) experienced an increased temperature after the first and second doses, respectively. Only 8 of 241 (3%) of all subjects (5 in the full-dose group and 3 in the half-dose group) and 2 of 196 (1%) of all subjects (1 in each group) had a temperature >39°C after the first and second doses, respectively.

For severity of reactions, there were no significant differences between the full- and half-dose groups in occurrence of severe solicited reactions. No severe (Grade 3) systemic or local reactions were reported for subjects in the fully primed cohort (Figure 2 B). There were no significant differences in unsolicited AEs or serious adverse events (SAEs) or onset of chronic medical conditions between the dose groups in either the naive or fully primed cohorts, and none of the SAEs were deemed related to the vaccine.

Immunogenicity

Table 2 includes the 3 immunogenicity endpoints (percentage with final titer ≥ 1:40, percentage with a 4-fold or greater increase in titers, and GMT) for the primed and naive cohort. In subsequent post hoc analyses, the naive cohort is further subdivided by age group: 12 through 35 months of age compared with 6 through 11 months of age. In the primed cohort, the only significant difference between groups was that the H1N1 GMT in the full-dose group was significantly greater than in the half-dose group (445.1 vs 172.8; difference and 95% CI of the difference, 267.5 [3.9, 527.9]). In the naive cohort, increasing the antigen content of the vaccine did not significantly increase the HAI antibody response to any of the 3 vaccine components (H3N2, H1N1, or B). This finding was also true when analysis was restricted to those under 1 year of age. The poor response to the H3N2 antigen in both naive cohorts is unexplained.

Table 2.

Immune Responses Before and After Vaccination(s) in the Primed and Naive Cohorts as Measure by HAI

Vaccine Strain   Primed Cohort
 
Naive Cohort
 
Naive Cohort: Age 6 Through 11 Month
 
Naive Cohort: Age 12 Through 35 Month
 
0.25 mL Group 0.5 mL Group 0.25 mL Group 0.5 mL Group 0.25 mL Group 0.5 mL Group 0.25 mL Group 0.5 mL Group 
A/California/7/2009 H1N1 N 21 55 119 38 76 17 43 
% with ≥1:40 89 100 85 89 82 86 94 95 
(95% CI) (0.52, 1.00) (0.84, 1.00) (0.73, 0.94) (0.82, 0.94) (0.66, 0.92) (0.76, 0.93) (0.71, 1.00) (0.84, 0.99) 
% with ≥4-fold 89 90 78 85 74 82 88 91 
increase (95% CI) (0.52, 1.00) (0.70, 0.99) (0.65, 0.88) (0.77, 0.91) (0.57, 0.87) (0.71, 0.90) (0.64, 0.99) (0.78, 0.97) 
Prevaccine GMT 13.6 35.1 10.4 8.8 7.9 6.7 17.9 14.3 
(95% CI) (5.8, 31.7) (17.9, 68.8) (7.0, 15.4) (6.9, 11.0) (5.2, 11.9) (5.4, 8.1) (7.6, 42.2) (8.5, 23.9) 
Postvaccine GMT 172.8 445.1 181.5 187.2 154.3 120.6 261.0 407.5 
(95% CI) (65.1, 459.0) (283.0, 700.3) (119.2, 276.4) (142.5, 246.1) (94.7, 251.4) (88.3, 164.7) (109.3, 623.5) (261.5, 635.2) 
GMT difference 159.2 410.0 171.1 178.4 146.4 113.9 243.1 393.2 
estimate (95% CI)* (69.2, 350.0) (263.8, 614.3) (110.2, 260.5) (134.9, 234.8) (88.6, 235.1) (82.2, 156.2) (100.7, 533.0) (252.6, 599.8) 
GMT difference between SD and HD −267.5 −5.7 33.7 −146.6 
estimate (95% CI)* (−527.9, −3.9) (−94.9, 90.2) (−38.2, 128.9) (−402.2, 178.0) 
A/Perth/16/2009 H3N2 % with ≥1:40 89 90 15 15 11 11 24 23 
(95% CI) (0.52, 1.00) (0.70, 0.99) (0.06, 0.27) (0.09, 0.23) (0.03, 0.25) (0.05, 0.20) (0.07, 0.50) (0.12, 0.39) 
% with ≥4-fold 78 86 11 18 19 
increase (95% CI) (0.40, 0.97) (0.64, 0.97) (0.02, 0.18) (0.06, 0.18) (0.00, 0.14) (0.02, 0.15) (0.04, 0.43) (0.08, 0.33) 
Prevaccine GMT 18.5 12.2 10.5 8.2 11.4 8.5 9.0 7.7 
(95% CI) (4.8, 71.0) (8.6, 17.3) (8.3, 13.2) (7.3, 9.2) (8.4, 15.3) (7.4, 9.8) (6.2, 13.0) (6.1, 9.7) 
Postvaccine GMT 93.3 111.3 12.9 14.8 10.8 13.6 19.2 17.0 
(95% CI) (32.4, 268.9) (67.8, 182.6) (10.2, 16.3) (12.9, 16.9) (8.7, 13.4) (11.8, 15.8) (10.7, 34.4) (12.9, 22.4) 
GMT difference 74.8 99.1 2.4 6.6 −0.6 5.1 10.2 9.3 
estimate (95% CI)* (31.5, 167.6) (59.9, 159.1) (−0.6, 5.6) (4.7, 8.7) (−4.5, 2.5) (3.3, 7.3) (4.1, 22.5) (5.5, 14.4) 
GMT difference between SD and HD −11.0 −1.9 −2.9 2.2 
estimate (95% CI)* (−105.1, 122.2) (−2.0, 5.2) (−5.8, 0.2) (−7.2, 16.8) 
B/Brisbane/60/2008 % with ≥1:40 33 14 .44 50 39 45 53 58 
(95% CI) (0.07, 0.70) (0.03, 0.36) (0.30, 0.58) (0.40, 0.59) (0.24, 0.57) (0.33, 0.57) (0.28, 0.77) (0.42, 0.73) 
% with ≥4-fold rise 22 10 31 42 32 34 29 56 
(95% CI) (0.03, 0.60) (0.01, 0.30) (0.19, 0.45) (0.33, 0.51) (0.18, 0.49) (0.24, 0.46) (0.10, 0.56) (0.40, 0.71) 
Prevaccine GMT 6.3 7.2 12.0 8.6 11.6 9.0 12.9 7.9 
(95% CI) (4.3, 9.2) (5.0, 10.2) (9.1, 15.8) (7.6, 9.7) (8.5, 15.8) (7.7, 10.5) (7.1, 23.3) (6.4, 9.8) 
Postvaccine GMT 17.1 14.4 27.4 31.1 25.4 28.0 32.6 37.5 
(95% CI) (9.6, 30.7) (9.0, 22.9) (21.4, 35.1) (26.1, 37.2) (18.8, 34.3) (22.4, 35.1) (20.4, 52.1) (28.1, 50.1) 
GMT difference 10.8 7.2 15.4 22.5 13.8 19.0 19.7 29.6 
estimate (95% CI)* (5.0, 19.4) (3.4, 13.3) (9.5, 22.3) (17.7, 28.3) (7.1, 22.1) (13.7, 25.7) (8.6, 33.4) (20.6, 41.1) 
GMT difference between SD and HD 3.0 −3.7 −2.7 −4.9 
estimate (95% CI)* (−7.5, 14.8) (−5.1, 12.0) (−12.0, 7.4) (−21.3, 14.1) 
Vaccine Strain   Primed Cohort
 
Naive Cohort
 
Naive Cohort: Age 6 Through 11 Month
 
Naive Cohort: Age 12 Through 35 Month
 
0.25 mL Group 0.5 mL Group 0.25 mL Group 0.5 mL Group 0.25 mL Group 0.5 mL Group 0.25 mL Group 0.5 mL Group 
A/California/7/2009 H1N1 N 21 55 119 38 76 17 43 
% with ≥1:40 89 100 85 89 82 86 94 95 
(95% CI) (0.52, 1.00) (0.84, 1.00) (0.73, 0.94) (0.82, 0.94) (0.66, 0.92) (0.76, 0.93) (0.71, 1.00) (0.84, 0.99) 
% with ≥4-fold 89 90 78 85 74 82 88 91 
increase (95% CI) (0.52, 1.00) (0.70, 0.99) (0.65, 0.88) (0.77, 0.91) (0.57, 0.87) (0.71, 0.90) (0.64, 0.99) (0.78, 0.97) 
Prevaccine GMT 13.6 35.1 10.4 8.8 7.9 6.7 17.9 14.3 
(95% CI) (5.8, 31.7) (17.9, 68.8) (7.0, 15.4) (6.9, 11.0) (5.2, 11.9) (5.4, 8.1) (7.6, 42.2) (8.5, 23.9) 
Postvaccine GMT 172.8 445.1 181.5 187.2 154.3 120.6 261.0 407.5 
(95% CI) (65.1, 459.0) (283.0, 700.3) (119.2, 276.4) (142.5, 246.1) (94.7, 251.4) (88.3, 164.7) (109.3, 623.5) (261.5, 635.2) 
GMT difference 159.2 410.0 171.1 178.4 146.4 113.9 243.1 393.2 
estimate (95% CI)* (69.2, 350.0) (263.8, 614.3) (110.2, 260.5) (134.9, 234.8) (88.6, 235.1) (82.2, 156.2) (100.7, 533.0) (252.6, 599.8) 
GMT difference between SD and HD −267.5 −5.7 33.7 −146.6 
estimate (95% CI)* (−527.9, −3.9) (−94.9, 90.2) (−38.2, 128.9) (−402.2, 178.0) 
A/Perth/16/2009 H3N2 % with ≥1:40 89 90 15 15 11 11 24 23 
(95% CI) (0.52, 1.00) (0.70, 0.99) (0.06, 0.27) (0.09, 0.23) (0.03, 0.25) (0.05, 0.20) (0.07, 0.50) (0.12, 0.39) 
% with ≥4-fold 78 86 11 18 19 
increase (95% CI) (0.40, 0.97) (0.64, 0.97) (0.02, 0.18) (0.06, 0.18) (0.00, 0.14) (0.02, 0.15) (0.04, 0.43) (0.08, 0.33) 
Prevaccine GMT 18.5 12.2 10.5 8.2 11.4 8.5 9.0 7.7 
(95% CI) (4.8, 71.0) (8.6, 17.3) (8.3, 13.2) (7.3, 9.2) (8.4, 15.3) (7.4, 9.8) (6.2, 13.0) (6.1, 9.7) 
Postvaccine GMT 93.3 111.3 12.9 14.8 10.8 13.6 19.2 17.0 
(95% CI) (32.4, 268.9) (67.8, 182.6) (10.2, 16.3) (12.9, 16.9) (8.7, 13.4) (11.8, 15.8) (10.7, 34.4) (12.9, 22.4) 
GMT difference 74.8 99.1 2.4 6.6 −0.6 5.1 10.2 9.3 
estimate (95% CI)* (31.5, 167.6) (59.9, 159.1) (−0.6, 5.6) (4.7, 8.7) (−4.5, 2.5) (3.3, 7.3) (4.1, 22.5) (5.5, 14.4) 
GMT difference between SD and HD −11.0 −1.9 −2.9 2.2 
estimate (95% CI)* (−105.1, 122.2) (−2.0, 5.2) (−5.8, 0.2) (−7.2, 16.8) 
B/Brisbane/60/2008 % with ≥1:40 33 14 .44 50 39 45 53 58 
(95% CI) (0.07, 0.70) (0.03, 0.36) (0.30, 0.58) (0.40, 0.59) (0.24, 0.57) (0.33, 0.57) (0.28, 0.77) (0.42, 0.73) 
% with ≥4-fold rise 22 10 31 42 32 34 29 56 
(95% CI) (0.03, 0.60) (0.01, 0.30) (0.19, 0.45) (0.33, 0.51) (0.18, 0.49) (0.24, 0.46) (0.10, 0.56) (0.40, 0.71) 
Prevaccine GMT 6.3 7.2 12.0 8.6 11.6 9.0 12.9 7.9 
(95% CI) (4.3, 9.2) (5.0, 10.2) (9.1, 15.8) (7.6, 9.7) (8.5, 15.8) (7.7, 10.5) (7.1, 23.3) (6.4, 9.8) 
Postvaccine GMT 17.1 14.4 27.4 31.1 25.4 28.0 32.6 37.5 
(95% CI) (9.6, 30.7) (9.0, 22.9) (21.4, 35.1) (26.1, 37.2) (18.8, 34.3) (22.4, 35.1) (20.4, 52.1) (28.1, 50.1) 
GMT difference 10.8 7.2 15.4 22.5 13.8 19.0 19.7 29.6 
estimate (95% CI)* (5.0, 19.4) (3.4, 13.3) (9.5, 22.3) (17.7, 28.3) (7.1, 22.1) (13.7, 25.7) (8.6, 33.4) (20.6, 41.1) 
GMT difference between SD and HD 3.0 −3.7 −2.7 −4.9 
estimate (95% CI)* (−7.5, 14.8) (−5.1, 12.0) (−12.0, 7.4) (−21.3, 14.1) 

Abbreviations: CI, confidence interval; GMT, geometric mean titer; HAI, hemagglutination inhibition; HD, high dose; SD, standard dose.

*Note: GMT difference estimate is the difference between prevaccine GMT (day 0) and postvaccine GTM (day 28 for primed cohort, day 28 after second vaccination for naive cohort). These CIs are based on 100 000 bootstrap simulations.

For the naive cohort, after 2 doses of TIV, subjects 1 year of age and older had significantly higher HAI titers than subjects less than 1 year of age to all the vaccine strains. For the H1N1 strain, the difference was also significant after 1 dose ( P < .0001). Subjects enrolled and vaccinated in the 2010–2011 season had significantly lower titers than subjects enrolled in the 2011–2012 season to the B strain ( P = .05) and to the H1N1 strain after 2 doses ( P = .04).

DISCUSSION

Our study demonstrated that the administration of full-dose TIV to infants 6 through 35 months of age was safe and not associated with increased local or systemic reactions. The reported frequency and severity of reactions were consistent with previous influenza vaccine trials in this age group. No evidence of increased toxicity was noted with the full-dose group compared with the half-dose group in the either the fully primed or naive cohort when both seasons were combined. When the influenza seasons were separated by individual seasons, differences in number of solicited reactions in the naive cohort were observed. In 2010–2011 season, significantly more systemic reactions in the half-dose group (elevated temperature and decreased appetite) compared with the full-dose group were observed, whereas in the 2011–2012 there were more reactions in the full-dose group (increased temperature and redness). The use of antipyretics correlated with the higher frequency of fever for that influenza season; the half-dose group had higher use in the 2010–2011year compared with full-dose group, and the reverse was true in 2011–2012 year. It is unclear why these differences were detected. In both seasons, the vast majority of the reactions were mild and resolved within 3 days of vaccination. In addition, none of the SAEs reported were associated with vaccine administration.

Our findings in regards to safety are consistent with another trial comparing the full-dose with half-dose TIV in unprimed children 6 through 23 months of age, in which 252 participants were evaluated [ 25 ]. Fever, defined as axillary temperature 38°C or higher, was detected in <10% of subjects, and no differences in reactions between the full or half doses were noted. A lower frequency of local reactions were reported in the half-dose compared with full-dose TIV; however, none of the differences were statistically significant [ 25 ].

In our study, increasing the antigen content of the vaccine did not significantly increase the HAI antibody response to any of the 3 vaccine components, except for the primed group for H1N1. This result was also true when analysis was restricted to those under 1 year of age. However, our study differs from the results reported by Skowronski et al [ 25 ], who noted that infants (6 through 11 months) administered the full-dose TIV had higher HAI antibody titers to all 3 vaccine components compared with half dose. In addition, the differences in those achieving seroprotective titers were statistically significant for 2 antigens (H3N2 and B) [ 25 ]. Postimmunization seroprotection rates for the toddlers (12 through 23 months) exceeded 85% for all 3 vaccine components but without a significant difference by dose. Esposito et al [ 26 ] investigated an increased dose (15 μg) of a virosomal-adjuvanted subunit influenza vaccine administered to unprimed children 6 through 35 months of age compared to a dose of 7.5 μg. They noted an increased HAI immune responses in the high-dose group after 1st dose and 6 months later for H1N1 and H3N2, in the absence of an increase in systemic reactions or local reactions [ 26 ]. Subsequently, a phase III study by the same group further supported that a single dose of 0.5 mL (15 μg) of a virosomal-adjuvanted subunit influenza vaccine could effectively and safely provide long-term immunogenicity to all 3 influenza strains in unprimed children 6 through 35 months of age [ 27 ].

In our study, both the full-dose and half-dose cohorts achieved serological criteria by World Health Organization biological standards for influenza vaccines (>40% for seroconversion, GMT >2.5-fold increase, and >70% with ≥ 1:40) for H1N1, but only the primed cohort met these criteria, regardless of dose, for H3N2 (Table 2 ) [ 28 ]. Our study also detected age differences in postimmunization titers in the naive cohort, with children 12 through 35 months of age having statistically significantly higher titers than infants 6 through 11 months of age for all 3 strains. Other strategies, such as including adjuvants, have been investigated to improve immunogenicity in these young children [ 26 , 27 , 29–32 ], but currently no adjuvanted vaccines are available in the United States.

Finland, similar to the United States, recommends universal vaccination for all children 6 through 35 months of age but uses full-dose TIV for all children. Using this dose, Heinonen et al [ 33 ] reported an overall 66% effectiveness in the prevention of influenza in all age groups, with similar effectiveness in younger age groups. These data, along with other studies of effectiveness, suggest that TIV is effective against influenza in young children when there is a good antigenic match between the vaccine and the circulating strains; however, other studies using lower doses for younger age groups reported important age-specific differences in TIV vaccine efficacy, with higher efficacy in older children [ 6 , 20 ]. Furthermore, reduced benefits of influenza vaccination are often found, particularly in <2 years of age [ 20 ]. The use of 0.5 mL for all children in Finland may explain the similar effectiveness against influenza disease in the younger children. Additional countries, including the United Kingdom and Canada, also recommend the 0.5 mL dose for all ages for whom influenza vaccine is recommended, based on the favorable reactogenicity and immunogenicity data from the Finnish study [ 33 ].

Our study has limitations. This trial was conducted over 2 influenza seasons, and some individuals in the naive group may have had influenza disease that was not medically attended, which may have influenced immunogenicity data; however, the safety comparisons from both years should be comparable. This study was originally powered for safety, so our sample size for detecting immunogenicity differences in all the subgroups may not have been adequate. Furthermore, a limitation of the study is the small number of children enrolled in the primed cohort. The response to the H3N2 strain was unexpectedly low for the naive cohort in both groups, and this result is unexplained. We enrolled children who were primed with TIV in year 1 only; therefore, the enrollment numbers were small and were not powered to detect immunogenicity differences. Enrollment stopped when each site met the definition for the start of influenza season, so it is possible that some of children may have acquired natural influenza disease, affecting immunogenicity results.

CONCLUSIONS

Our study confirms earlier findings that full-dose TIV in children 6 through 35 months of age is safe [ 26 ], but the failure to confirm improved immunogenicity in unprimed children during different influenza seasons leaves unresolved the issue of whether a full dose may provide more protection. One advantage of using the same dose of influenza vaccine for all children would be the simplification of the production, purchase, storage, and administration of influenza vaccines. Likewise, extended use of the half dose of vaccine could have similar advantages, especially in times of vaccine shortage [ 34 ]. Given the safety results of this study, further studies are warranted. In addition, larger studies are needed to clarify the potential for immune differences in this population.

Acknowledgments

We thank the participants, their families, Kristen Buschle, and Stacie Wethington at Cincinnati Children's; the staff at Pediatrics at the Harbor, Annapolis Pediatrics, and The Pediatric Center of Frederick: Kimberly (Rincavage) Wilhelmi, Linda Wadsworth, Nancy Wymer, Ginny Cummings, Susan Feigelman, and Samer El-Kamary; Irene Graham; Wendi McDonald; Shanda Phillips; Gayle Johnson; Alice O'Shea; Cathryn Richmond, Gretchen Cress; Joseph Hilinski, Andrea Shane, and Paul Spearman; Vaccine Treatment and Evaluation Unit and Clinical Research Unit staff at the University of Iowa.

Disclaimer. The contents of this report are solely the responsibility of the authors and do not necessarily represent official views of the National Center for Advancing Translational Sciences or the National Institutes of Health.

Financial support . The Clinical Research Unit is supported by the National Institutes of Health Clinical and Translational Science Awards (CTSA) Program, (Grant 2 UL1 TR000442-06); and CTSA award No. UL1TR000445 from the National Center for Advancing Translational Sciences.

This work was supported by contracts from the National Institute of Allergy and Infectious Diseases (Grant numbers: HHSN27220080000-3C, 5C, 6C, 7C, 8C, 13C, and 57C).

Potential conflicts of interest. N. B. H. received grant funding from Sanofi Pasteur, Gilead, and Pfizer.

All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.

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APPENDIX 1

Exclusion Criteria: Children were excluded if they had known allergy to eggs or to other components of the vaccine; known or suspected latex allergy; history of bronchodilator use more than 2 times per week within 28 days of vaccination; significant underlying chronic illness; immunodeficiency disease; known infection with hepatitis B or hepatitis C; use of immunosuppressive therapy; long term use of glucocorticoids; exposure to an investigational drug or investigational vaccine within 28 days before vaccination in this trial; history of Guillain-Barré syndrome or any other neuromuscular disease; history of more than 1 febrile seizure or any afebrile seizure; or they were former premature infants (<37 weeks gestation in season 1 and <35 weeks in season 2). Specifically for the naive cohort, any prior influenza vaccination or documented laboratory-confirmed influenza infection was also considered exclusionary.

The following conditions were temporary or self-limiting, and a subject was included in the study once the condition(s) were resolved, provided that the subject was otherwise eligible: receipt of blood products in the previous 90 days; fever (axillary temperature >100.0°F/37.8°C); an acute illness within 48 hours of enrollment; and/or receipt of any live vaccines within 2 weeks or any inactivated vaccines within 2 weeks of study vaccination.

Local Reaction Mild (Grade 1) Moderate (Grade 2) Severe (Grade 3) 
Pain/tenderness: hurts only when injection site is touched. Minor reaction to touch Cries/protests to touch Cries when limb is moved/spontaneously painful 
Erythema/redness* <5 mm 5 mm–20 mm >20 mm 
Induration/swelling* <5 mm 5 mm–20 mm >20 mm 
Systemic (subjective) Mild (Grade 1) Moderate (Grade 2) Severe (Grade 3) 
Irritability/fussiness Crying more than usual Some interference with normal activity Significant interference, prevents daily activity 
Drowsiness Drowsiness that does not interfere with normal activity Some interference with normal activity Significant interference, prevents normal activity 
Loss of appetite Drinking or eating slightly less than usual Drinking or eating much less than usual Not eating or drinking 
Systemic (quantitative) Mild (Grade 1) Moderate (Grade 2) Severe (Grade 3) 
Fever (°C)* ≥37.8 to <38.3 ≥38.3 to <39 ≥39 
 ≥100 < 101°F ≥101 to <102°F ≥102°F 
Local Reaction Mild (Grade 1) Moderate (Grade 2) Severe (Grade 3) 
Pain/tenderness: hurts only when injection site is touched. Minor reaction to touch Cries/protests to touch Cries when limb is moved/spontaneously painful 
Erythema/redness* <5 mm 5 mm–20 mm >20 mm 
Induration/swelling* <5 mm 5 mm–20 mm >20 mm 
Systemic (subjective) Mild (Grade 1) Moderate (Grade 2) Severe (Grade 3) 
Irritability/fussiness Crying more than usual Some interference with normal activity Significant interference, prevents daily activity 
Drowsiness Drowsiness that does not interfere with normal activity Some interference with normal activity Significant interference, prevents normal activity 
Loss of appetite Drinking or eating slightly less than usual Drinking or eating much less than usual Not eating or drinking 
Systemic (quantitative) Mild (Grade 1) Moderate (Grade 2) Severe (Grade 3) 
Fever (°C)* ≥37.8 to <38.3 ≥38.3 to <39 ≥39 
 ≥100 < 101°F ≥101 to <102°F ≥102°F 

*Axillary temperature [Note: A fever can be considered not product-related if an alternative etiology can be documented and it is confirmed to be not product-related by the Independent Safety Monitor].

APPENDIX 2

Elevated temperature was defined as an axillary temperature ≥37.8°C. Solicited local reactions included erythema, induration/swelling, and pain/tenderness at the injection site. Solicited systemic reactions included fever (a daily temperature was recorded), irritability/fussiness, drowsiness, and loss of appetite. Study personnel contacted the subjects by telephone between 1 and 3 days and at 8 and 10 days after each vaccination to review any AEs and SAEs. Unsolicited AEs were collected through 28 days after the last vaccination; SAEs and new-onset chronic medical conditions were collected until 6 months after the final vaccination. All AEs and SAEs were assessed by a licensed study investigator and classified as being either associated with or not associated with TIV administration. Concomitant medication use was also collected for 28 days postvaccination.