With rising global reports of H5 avian influenza and the ongoing threat of pandemic emergence, induction of cross‑reactive antibody responses is a key vaccine objective. This phase 2 extension study assessed whether a heterologous booster with an MF59‑adjuvanted, cell‑culture derived H5N6 vaccine (aH5N6c) could elicit cross‑reactive immunity in adults previously primed with an MF59‑adjuvanted, cell‑culture derived H5N1 vaccine (aH5N1c), and compared those responses with unprimed adults receiving the same H5N6 vaccine.
The study was conducted as an extension of the parent study V89_18. Adults who had been primed previously with two doses of aH5N1c in the parent trial were randomized into two groups: Group 1 received two doses of aH5N6c and Group 2 received one aH5N6c dose and one placebo dose, administered 3 weeks apart. Unprimed adults (Group 3) received two doses of aH5N6c, also 3 weeks apart. A total of 258 participants were exposed in the study population described in the source.
The investigational product evaluated was an MF59‑adjuvanted, cell‑culture derived H5N6 vaccine (aH5N6c). The priming formulation used in the parent study was an MF59‑adjuvanted, cell‑culture derived H5N1 vaccine (aH5N1c). Doses in the extension were administered on a 0 and 3‑week schedule according to group assignment. The source does not report additional formulation details such as antigen content per dose or exact adjuvant composition beyond MF59 and cell‑culture derivation.
Humoral immune responses were evaluated using hemagglutination inhibition (HI) and microneutralization (MN) assays. Serum was collected and tested against both the priming strain (H5N1) and the booster strain (H5N6) at prespecified timepoints: Days 1, 8, 22, 43, and 202. Outcomes reported in the source include geometric mean titers (GMTs) for HI assays, seroconversion rates, and MN responses.
Primed participants (Groups 1 and 2) showed markedly higher HI GMTs against both the H5N1 and H5N6 strains than unprimed participants (Group 3). On Day 43, heterologous responses against the priming H5N1 strain in primed participants were strong, with HI GMTs reported in the range of 333–343 and seroconversion rates exceeding 89%. In contrast, unprimed subjects had minimal heterologous responses to H5N1 following H5N6 vaccination. MN assay results mirrored the HI findings, with enhanced neutralizing responses in the primed groups compared with unprimed subjects.
Responses in the primed cohorts persisted to Day 202, indicating sustained antibody levels for at least the time window captured in the study. The authors interpret these findings as evidence that heterologous H5N6 boosting in H5N1‑primed adults elicits strong cross‑reactive immunity and reflects long‑lasting immune memory attributed to the earlier H5N1 priming.
Solicited adverse events following vaccination were reported as mild or moderate in severity and were comparable across the three groups. The safety profile was described as consistent with other MF59‑adjuvanted pandemic influenza vaccines. Importantly, the source reports that no vaccine‑related serious adverse events occurred among study participants. The article does not provide detailed counts or percentages for specific solicited reactions within this summary.
The study reports persistence of cross‑reactive antibody responses to Day 202 in primed participants and states that the findings demonstrate long‑lasting immune memory for at least 6 years following the initial H5N1 priming in the parent study. This durability supports the potential utility of heterologous prime‑boost strategies to maintain preparedness against emerging H5 influenza subtypes.
A heterologous booster with an MF59‑adjuvanted, cell‑culture derived H5N6 vaccine in adults previously primed with an MF59‑adjuvanted H5N1 vaccine produced robust cross‑reactive HI and MN antibody responses against the priming strain, with high seroconversion rates and sustained titers through Day 202. The vaccine was well tolerated, with mostly mild to moderate solicited adverse events and no vaccine‑related serious adverse events reported. These results support consideration of heterologous prime‑boost approaches using MF59‑adjuvanted, cell‑culture derived H5 vaccines as part of pandemic preparedness strategies for emerging H5 outbreaks.
The source abstract does not report certain methodological details such as exact antigen doses, full demographic breakdowns, numerical safety event rates, or statistical analyses beyond the GMT and seroconversion summaries. If more granular data are needed (for example, per‑reaction frequencies, confidence intervals, or subgroup analyses), those details were not reported in the provided source abstract.