Abstract
Avian-origin H5/H7 influenza has the potential to cause the next influenza pandemic. Availability of effective vaccines is an essential part of pre-pandemic preparedness. However, avian influenza surface antigens are poorly immunogenic to humans, which necessitates the use of adjuvants to augment the immunogenicity of pre-pandemic influenza vaccines. Aluminum salts are approved, safe, and affordable adjuvants, but their adjuvanticity for influenza vaccines remains unverified. We conducted the first meta-analysis on this issue. A total of nine randomized controlled trials (2006–2013, 22 comparisons, 2,467 participants in total) compared aluminum-adjuvanted H5N1 vaccines versus non-adjuvanted counterparts. The weighted estimate for the ratio of the seroprotection rate after a single dose of H5N1 vaccine is 0.66 (95% CI: 0.53 to 0.83) by hemagglutination-inhibition assay or 0.56 (95% CI: 0.42 to 0.74) by neutralizing titer assay. The weighted estimate for the risk ratio of pain/tenderness at injection sites is 1.85 (95% CI: 1.56 to 2.19). The quality of evidence is low to very low for seroprotection (due to indirectness and potential reporting bias) and moderate for pain/tenderness (due to potential reporting bias), respectively. The significantly lower seroprotection rate after aluminum-adjuvanted H5N1 vaccines and the significantly higher risk of pain at injection sites indicate that aluminum salts decrease immunogenicity but increase local reactogenicity of pre-pandemic H5N1 vaccines in humans.
Highlights
Avian-origin H5/H7 influenza has the potential to cause the influenza pandemic
Our results showed an inferior seroprotection rate after aluminum-adjuvanted H5N1 vaccines compared with that conferred by non-adjuvanted counterparts
The absence of an increase in seroprotection rates of aluminum salts-adjuvanted vaccines indicates that aluminum salts are not suitable to serve as adjuvants for pre-pandemic H5N1 influenza vaccines for humans
Summary
Avian-origin H5/H7 influenza has the potential to cause the influenza pandemic. Availability of effective vaccines is an essential part of pre-pandemic preparedness. Avian influenza surface antigens are poorly immunogenic to humans, which necessitates the use of adjuvants to augment the immunogenicity of pre-pandemic influenza vaccines. Safe, and affordable adjuvants, but their adjuvanticity for influenza vaccines remains unverified. The significantly lower seroprotection rate after aluminum-adjuvanted H5N1 vaccines and the significantly higher risk of pain at injection sites indicate that aluminum salts decrease immunogenicity but increase local reactogenicity of pre-pandemic H5N1 vaccines in humans. Randomized controlled clinical trials of aluminum salts-adjuvanted H5N1 influenza vaccines in humans are either statistically inconclusive – likely due to the insufficient sample sizes in each of these clinical trials – or not designed to evaluate the adjuvanticity of aluminum salts[8]. To maximize the pandemic preparedness, it is necessary to clarify whether aluminum salts, the approved, safe, and affordable adjuvant, are effective in enhancing the immunogenicity of pre-pandemic influenza vaccines. To overcome the limitation of insufficient statistical power in single randomized controlled trials, we conducted the first meta-analysis on this issue
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