The Potential of Live, Attenuated Influenza Vaccine for the Prevention of Influenza in Children

LAIV, vaccine efficacy, influenza vaccine for children

(See the Major article by Lewis et al on pages 777–85 and Major article by Brickley et al on pages 786–94)

In this issue of CID, 2 articles provide new insights into the use of the Russian-backbone live, attenuated influenza vaccine (LAIV; manufactured by the Serum Institute of India) in children to prevent influenza [1, 2]. The Russian-backbone version of LAIV (LAIV-R) has many similarities to the US and European version of LAIV (LAIV-AA, AstraZeneca). LAIV-R influenza A components are grown in eggs from seed virus produced by a classical reassortant technique from the attenuated backbone A/Leningrad/134/17/57 (H2N2) [3]. The LAIV-AA also produces the influenza A components in eggs, but the seed virus is produced using reverse genetics from the A/Ann Arbor/6/60 (H2N2) backbone and hemagglutinin (HA) and neuraminidase (NA) consensus RNA sequences. The results of the present study using LAIV-R are strikingly similar to the recent experience with LAIV-AA for the H1N1 component made from California/O9 pdm H1N1. Little or no efficacy was seen in the United States, and low efficacy against H1N1 was observed in several countries with a LAIV-AA containing the California/09 pdm H1N1 virus antigens; the present study with the LAIV-R version of California/09/pdm H1N1 reassortant demonstrated 0 shedding of the H1N1 vaccine virus in young children. In contrast, the H3 and B components of LAIV-R were shed with high frequency, as expected in a study of young children. LAIV-R did stimulate some local immunoglobin A (IgA) to H1N1, but systemic immunoglobin G developed only to the H3 and B components. By design, protection could not be assessed in the present study. Nevertheless, these 2 papers guide us in understanding what needs to be done to continue to improve both versions of LAIV.

The full potential of the 2 LAIVs have yet to be realized on a regular basis. In some years, LAIV is clearly superior to inactivated influenza vaccine (IIV) in children. LAIV-AA clinical trials conducted by the National Institute of Allergy and Infectious diseases revealed the potential for LAIVs to control influenza in children. Vaccine efficacy against antigenically-mismatched H3N2, as well as closely-matched H3N2, was high (88% and 94%, respectively) in the placebo-controlled, pivotal, efficacy field trial [4]. In a post-marketing head-to-head trial comparing LAIV-AA to IIV in children 6–60 months of age, the relative superior efficacy of LAIV-AA vs IIV was 55% overall, regardless of an antigenic match between the vaccine and circulating viruses, and LAIV-AA performed even better vs individual influenza A subtype viruses (its relative efficacy was 79% for H3N2 and 89% for H1N1) [5].

Results of these early successes indicated that secretory IgA and, possibly, cellular immune responses, in addition to serum antibody, were important for protection against influenza infection and disease. The replication of influenza in mucosa suggests that local immune responses will be critical for more complete protection, beyond the limited protection provided by the serum antibody induced by inactivated influenza vaccines.

More recent observational studies with LAIV-AA have not recapitulated those earlier, positive clinical trial results. As illustrated in the current reports, both serum antibody and secretory IgA are correlated with protection from vaccine virus replication, and these are probably the correlates of protection from wild-type virus as well. Similar observations on LAIV-AA–vaccinated children who were subsequently challenged with monovalent, attenuated H1N1 (LAIV-AA expressing A/Shenzhen 225/95-like H1N1, homologous to the previous vaccine) demonstrated the independence of serum and secretory antibodies in protection from reinfection [6]. The correlates of immune protection against influenza probably include both serum immunoglobin G and secretory IgA, directed against HA and possible NA, and we need to develop vaccines that induce both systemic and local antibodies. The development of LAIVs is moving in that direction, but how do we identify instances when the circulating HA and NA do not perform well as a vaccine on an LAIV backbone?

Some progress is being made in understanding the biology of the reassortant viruses made for use in LAIVs, but in some instances, newer techniques for measuring vaccine titers and generating seed viruses may have unexpected, negative consequences for LAIV vaccine manufacture. Historically, the quantitation of LAIVs has been done by 50% tissue culture infectious dose, 50% egg infectious dose, or plaque assay, but fluorescent focus assay, which has been used more recently to quantitate LAIV-AA, may detect non-infectious vaccine antigens and mislead in calculating the dose to be used. In other words, we might be giving too low a dose with some vaccine virus reassortants. Secondly, reverse genetics may not be the best way to generate seed viruses. Traditional reassortant methods, in use for LAIV-AA prior to 2009 and still used in seed production by LAIV-R, cocultivate a wild-type virus (not a clonal population) that expresses the desired vaccine type HA and NA, along with the LAIV backbone virus, to generate a mixture of progeny viruses [6]. The seed vaccine virus is identified by plaque selection of a robust plaque phenotype. In contrast, the use of reverse genetics to produce seed reassortant viruses may not select the HA sequence that replicates optimally on the LAIV backbone. These differences in seed virus manufacturing might explain some of the anomalous results with H3 vaccine strains used in recent LAIV-AA vaccines. These recent vaccines used fluorescent focus assay quantitation and reverse genetics, rather than the classical laboratory methods that had been used to produce the LAIV-AA vaccines evaluated in the pivotal, efficacy field trail and head-to-head comparison studies noted above.

The A/California/09 pdm H1N1 vaccine is not the first H1 virus to be poorly immunogenic on an LAIV backbone. In an early clinical trial of trivalent LAIV-AA (study AV-002), shedding of the H3 and B viruses was vigorous, but few children shed H1N1 or made antibodies [7]. At the time, 2 doses of vaccine were suggested to try and overcome the weak responses.

Bivalent H1 and H3 LAIV-AA studies in seronegative young children have demonstrated that the H3 component may suppress the growth of the H1 vaccine component in about half of the children and, therefore, the recommended use of LAIV-AA in the United States is 2 doses for children who have not previously been vaccinated with either LAIV-AA or IIV [8, 9]. The vaccine schedule is, fortuitously, the same for LAIV-AA and IIV, but for different biologic reasons. It takes a prime-and-boost response for immunologically-naive children to respond with antibodies to IIV. For LAIV, antibodies form to the H3 component and then, with the second dose, H1 can replicate and induce antibodies. This is an alternate explanation for the anomalous results seen in the present reports of LAIV-R; I favor the hypothesis that the LAIV-R H1N1 reassortant was overly attenuated, since 0 of the children shed H1.

A simple laboratory technique for identifying overly-attenuated LAIV reassortants needs to be developed, so that these strains can be eliminated and a more vigorous reassortant selected for manufacture. Evaluating LAIV reassortants in human epithelium tissue culture may be predictive of the replication of the vaccine in children and, if true, this technique could be used in vaccine strain selection. In the meanwhile, there is no substitute for a small clinical trial in young children, modeled on the larger trial reported in this issue, to assess the safety, replication, and immune responses of each new reassortant in children. A small study of 20 children who have little prior experience with influenza vaccines should be sufficient to identify reassortants that are replication-deficient in children. In order not to delay the release of an annual vaccine, it would seem practical to conduct such a study annually in the summer and help guide strain selection for future years. In 2009, a 20-child study of LAIV-AA containing the reassortant vaccine for A/California/09 pdm H1N1 to assess shedding and immune responses would probably not have yielded results in time to fix the replication-deficient vaccine in that same year, but the problem would have been recognized and addressed 8 years earlier than the drawn-out drama of waiting year to year for observational efficacy data on LAIV compared to IIV, and grappling with confounding speculation as to the cause of low LAIV efficacy.

Note

Potential conflicts of interest. R. B. B. has been a consultant and/or speaker for Medimmune, Sanofi, Flugen, Novavax, and Moderna; owns stock/stock options in Flugen; is a scientific board member for Flugen; and has served on data safety monitoring board for GSK, MedImmune, Vical, and Vaxart. The author has 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.