This booster study in Asian adults aged 18 to 60 years showed that one 3.75 μg HA dose of heterologous AS03A-H5N1 booster vaccine given at Month 6, 12, or 36 after two primary doses of AS03A-adjuvanted H5N1 primary vaccine, elicited robust HI antibody responses against both the booster and primary vaccine strains. Responses to heterologous booster vaccination given at Month 6 were markedly lower in subjects who were primed with non-adjuvanted versus AS03-adjuvanted H5N1 vaccine, despite receiving two versus one dose(s) of adjuvanted booster vaccine, respectively. Injection site reactions and solicited general symptoms after each booster dose were consistent with previous reports of AS03A-H5N1 vaccines, and no safety concerns were identified over the 48 month follow-up period.
Highly pathogenic avian-origin H5N1 influenza A viruses are endemic in domestic birds in many Asian countries, and since two human cases of H5N1 were reported in Hong Kong in February 2003, avian-origin influenza A viruses have continued to spread across Asia, Europe, and Africa . The majority of H5N1 cases reported in humans since 2003 have been classified as clade 1 or clade 2, with multiple clade 2 subclades emerging (e.g. 2.1 to 2.5, of which 2.1 and 2.3 are further classified as 2.1.1 to 2.1.3 and 2.3.1 to 2.3.4) . The vast majority of the 633 human cases of avian-origin influenza reported by the WHO between 2003 and July 2013 , were associated with bird to human transmission, and although H5N1 viruses do not transmit efficiently between humans, there have been reports of human-to-human transmission in people who have had sustained, close, and unprotected contact with infected patients . Indeed, the evolution of H5N1 viruses capable of human-to-human transmission remains a threat, and as such, pandemic preparedness is paramount. In this study, we provide evidence of robust, antigen-sparing, cross-clade immune responses following AS03A-H5N1 pandemic influenza vaccination in a population in Taiwan, Thailand, Singapore, and Hong Kong.
In the initial observer-blind study, 21 days after a second dose of AS03A-H5N1 influenza vaccine, HI antibody responses fulfilled the United States Center for Biologics Evaluation and Research (CBER) licensure criteria for the vaccine strain (A/Vietnam/1194/2004; clade 1), but not a drifted strain (A/Indonesia/5/2005; clade 2.1), although substantial HI and neutralizing SCRs against the drifted strain were observed (50.2% and 91.4%, respectively) . By comparison, the HI antibody responses after two doses of non-adjuvanted H5N1 vaccine did not fulfill licensure criteria for immunogenicity against either the vaccine or drifted strain . These findings support previous results in a European population of adults, which showed that AS03A-adjuvantation enhanced immunogenicity compared with a non-adjuvanted vaccine, and demonstrated that two 3.75 μg HA doses of AS03A-H5N1 (Prepandrix™) provided robust vaccine-matched and cross-clade immune responses [4, 5].
Antigen sparing vaccine formulations will maximize the number of doses available within the existing influenza vaccine infrastructure, yet this is only part of the challenge of providing adequate and rapid vaccine coverage in the event of the emergence and rapid global spread of a highly pathogenic H5N1 virus. The stockpiling of H5N1 pandemic influenza vaccines against clade 1 and clade 2 viruses to be deployed at the start of a pandemic may provide cross-protection against the emerging pandemic strain and prime a population in advance of the manufacture of a vaccine against the emerging strain [6, 7]. A previous study of AS03A-H5N1 vaccines in adults aged 18 to 60 years evaluated various vaccination schedules and established the feasibility of the prime–boost strategy; subjects received one dose of A/Vietnam/1194/2004 primary vaccine or two doses given 21 days apart, followed by a dose of A/Indonesia/5/2005 booster or another dose of A/Vietnam/1194/2004 vaccine given at 6 or 12 months after priming. The results showed that two doses of A/Vietnam/1194/2004 vaccine given 6 months apart provided similar immune responses against the vaccine strain and higher cross-clade immune responses compared with two doses of primary vaccine given 21 days apart . In subjects receiving one dose of primary vaccine, the heterologous booster at 12 months elicited robust immune responses against the primary and booster strains .
To further explore the flexibility of the time interval between priming and booster vaccine, we assessed a two-dose primary series of AS03A-H5N1 or non-adjuvanted H5N1 vaccine, followed by one dose of heterologous AS03A-H5N1 vaccine at Month 6, 12, or 36 in subjects in the AS03A-H5N1 primary vaccine group, or two doses of booster vaccine at Month 6 in the non-adjuvanted H5N1 primary group. In subjects who were primed with the non-adjuvanted vaccine, two doses of the adjuvanted heterologous booster vaccine elicited strong HI antibody responses against the primary and booster strain which fulfilled CBER licensure criteria after the second but not the first booster dose. In subjects who received AS03A-H5N1 primary vaccine, only one dose of booster at Month 6 months was needed to elicit booster responses that fulfilled CBER licensure criteria. Neutralizing antibody responses in the AS03A-H5N1 primary group supported the HI results, with a neutralizing booster SCR of 90.2% against the booster strain 21 days after the Month 6 booster dose.
Of note, HI antibody SCRs, SPRs, and GMTs after two doses of booster vaccine at Month 6 were lower in the non-adjuvanted H5N1 primary group compared with one dose of booster vaccine in the AS03A-H5N1 primary group. This phenomenon has been previously reported in a Phase II study which showed that HI antibody responses where markedly higher in subjects primed with AS03A-H5N1 vaccine compared with those primed with non-adjuvanted H5N1 vaccine . Moreover, the Phase II study showed that HI antibody responses after AS03A-H5N1 booster vaccine were stronger in un-primed subjects compared with subjects who had received non-adjuvanted H5N1 priming vaccine, which not only highlights the role of the adjuvant for enhancing the development of immune memory, but also suggests that non-adjuvanted vaccine may have a negative effect on priming . Consistent with this hypothesis are the findings from a study conducted during the 2009 swine-origin H1N1 influenza pandemic, which showed that HI antibody responses to AS03A-H1N1 vaccine were lower in subjects who had received a non-adjuvanted seasonal influenza vaccine during the preceding two seasons compared with those who had not . The apparent negative effect of previous non-adjuvanted seasonal influenza vaccination on responses to AS03A-H1N1 vaccination did not appear to be associated with HI antibody levels at baseline, and the cause of this phenomenon remains to be established .
This study also demonstrated the feasibility of a highly flexible booster schedule, with strong cross-clade anamnestic responses observed with the booster dose given at Month 6, 12, or 36 post-priming. HI antibody responses fulfilled CBER licensure criteria against the primary and booster strain after booster vaccination at Month 6, 12, and 36, and GMTs after booster vaccination appeared higher than those observed after primary vaccination, suggesting strong immune memory recall. Priming with AS03A-adjuvanted pre-pandemic vaccine induced immune memory, which will allow for low-dose, rapid cross-protection with boosting at various time-points, offering highly flexible heterologous prime–boost schedules that can be adapted according to global need, pandemic strain evolution, vaccine circulation, and logistical implications in the event of a H5N1 pandemic outbreak. This should help mitigate transmission and reduce the severity of disease during the different phases of a pandemic.
To evaluate the level of protection offered after priming and boosting, we assessed immune persistence at various time-points up to 48 months post-primary vaccination. At Month 48, HI SPRs against the booster strain were 39.2%, 61.2%, and 95.6% in subjects boosted at Month 6, 12, or 36 post-priming, respectively. Although immune responses decreased with time, this result suggests that vaccinated populations could be potentially protected for up to three years after primary or booster vaccination, which is likely to far exceed the peak of the pandemic.
The induction of T and B cells is an important component of the immune response, and CD4+ effector and memory T cells have various protective roles in the innate response to influenza infection . AS03A-adjuvanted H5N1 pandemic influenza vaccine has been previously shown to elicit strong T-cell and B-cell responses in adults, which indicates immune memory and antibody persistence, arising from both quantitative and qualitative changes in T-cell responses . In our study, we evaluated CMI based on Th1-specific activation marker expression after in vitro re-stimulation of influenza-specific CD4+ and CD8+ T-cells. There were no CD8+ responses, but antigen-specific CD4+ responses were observed, expressing CD40L, IFN-γ, IL-2, and/or TNF-α, after primary and booster vaccination. CMI responses at after boosting at Month 6 appeared markedly higher in those primed with AS03A-adjuvanted vaccine than in those who received non-adjuvanted vaccine, supporting previous reports suggesting that adjuvantation has a positive priming effect on CD4+ T-cell responses.
The reactogenicity profile after booster vaccination was consistent with that observed during the primary vaccination study . The most frequent injection site event was pain, and Grade 3 pain was uncommon, as were other local events such as redness, swelling, and ecchymosis. After booster vaccinations the most frequent general solicited events were myalgia, headache, and fatigue. The frequency of solicited symptoms observed after booster vaccination was consistent with that observed during the primary vaccination study . During the 30-day period after booster vaccinations, ≤24.5% of subjects reported at least one unsolicited AE, and the rate of Grade 3 unsolicited AEs was low (≤2.8%). No safety concerns were identified, and the safety profile of the AS03A-H5N1 booster vaccine appeared acceptable.