A higher-dose, cell-derived adjuvanted flu vaccine produced stronger antibody responses after 50

Design

Phase 3, randomised, observer-blind, multicentre trial

Population

7,699 healthy adults aged 50 and over

Primary outcome

Day-29 haemagglutination-inhibition antibody titres and seroconversion

Effect

vs aQIV: GMT ratio 1.22 to 1.73 across strains; vs QIVr: H3N2 GMT ratio 0.69 (97.5% CI 0.65 to 0.74), non-inferiority not met

This phase 3 observer-blind trial randomised 7,699 healthy adults aged 50 and over, in the 2023 to 2024 season, to one of three vaccines: a new MF59-adjuvanted, cell-derived vaccine with 45 µg of haemagglutinin per strain (aQIVc), the existing adjuvanted egg-derived vaccine with 15 µg (aQIV), or a recombinant vaccine with 45 µg (QIVr).

At day 29, aQIVc met non-inferiority and superiority against aQIV for all four strains, with geometric mean titre ratios of 1.53 for H1N1, 1.22 for H3N2, 1.73 for B/Victoria and 1.71 for B/Yamagata, and seroconversion higher by 9.5 to 24.3 percentage points. Against the recombinant vaccine, it was non-inferior for H1N1 and both B strains but not for H3N2 (GMT ratio 0.69). Local and systemic reactions were more common with aQIVc, mostly mild to moderate; serious adverse events were similar across groups (2.8% to 3.5%).

This is an immunogenicity trial funded by the manufacturer. It shows bigger antibody responses, not fewer infections or admissions. The H3N2 result matters, because H3N2 seasons tend to hit older adults hardest, and here the recombinant vaccine did better.

• Keep vaccinating adults over 50 with whichever enhanced influenza vaccine your programme supplies; this trial does not change that.
• Warn recipients of adjuvanted, higher-dose vaccines to expect more sore arms and short-lived systemic symptoms.
• Note that the recombinant comparator gave a stronger H3N2 response than the new vaccine.
• Check national availability before discussing this vaccine; its regulatory status outside trials is not given here.

Why it matters

Older-adult flu vaccine choice is increasingly a comparison between enhanced vaccines rather than against standard dose.

Don't overread it

Antibody titres are a surrogate; the trial did not measure influenza cases, admissions or deaths.

The statistics, in plain English

A geometric mean titre ratio of 1.53 means antibody levels were about 50% higher on average. Higher titres usually go with better protection, but the size of that link varies by strain and season, so this is a surrogate result. Non-inferiority was judged by a pre-set margin; missing it for H3N2 means the trial could not rule out a meaningfully weaker response for that strain against the recombinant vaccine.

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