GSK gets the vaccine edge on Pfizer

US experts favour GSK’s RSV vaccine over Pfizer’s, but both could be on the market soon.

GSK is in need of good news, and yesterday it got some, with a US adcom seemingly giving its adult respiratory syncytial virus vaccine Arexvy the edge over Pfizer’s rival Abrysvo. As the panel endorsed both vaccines the FDA will likely now approve them both in time for this winter’s northern hemisphere RSV season.

However, the unanimous 12-0 vote in favour of GSK’s vaccine came in contrast to Pfizer’s 7-4 endorsement the day before, and important issues have yet to play out in terms of safety, durability and reimbursement. Billion-dollar sales are at stake, and by this time next year both companies could be facing at least one other competitor armed with strong data.

That competitor is Moderna, whose mRNA-1345 put up pivotal data in January that arguably rivalled those backing Arexvy on a cross-trial basis. However, given that mRNA-1345 has yet to be filed, Moderna will likely miss the coming RSV season, and the market will be a straight head-to-head battle between GSK and Pfizer until 2024.

With the limitations of cross-trial comparison, Arexvy and mRNA-1345 look similar in terms of vaccine efficacy, and better than Abrysvo. However, superior efficacy was not the key reason why the US adcom had doubts over Pfizer’s offering.

Rather, the main question related to safety, in particular Guillain-Barré syndrome, which was seen in two Abrysvo patients in phase 3; the FDA has requested a postmarketing study to evaluate this risk. Panellists also criticised a lack of re-vaccination/co-administration studies, and Pfizer’s limited data in elderly, frail and immunocompromised people.

The 7-4 backing for Abrysvo was the same for efficacy and safety. For Arexvy, meanwhile, efficacy got a 12-0 vote while safety was backed by a 10-2 margin. Both vaccines have May Pdufa dates, with GSK specifying that its one falls on 3 May.

Support for GSK

In contrast to Pfizer’s experience, there was support yesterday for GSK’s co-administration data and results across age groups, though one case of Guillain-Barré syndrome was also noted. However, GSK has not yet been asked for a postmarketing study, though it has agreed to monitor the syndrome as part of pharmacovigilance efforts.

If US approval is forthcoming in May a reimbursement decision should follow shortly afterwards, and could determine how much of this multi-billion dollar market the RSV vaccines might seize by age group. Durability data could also swing usage; current pivotal trials will by May include a second RSV season, so could shed light on this.

Evaluate Pharma sellside consensus forecasts Arexvy and Abrysvo selling $1.8bn and $1.2bn respectively in 2028, and remarkably Berenberg analysts estimate that the two vaccines account for 9% and 3% of the respective two companies’ total NPVs. Others are less sure, with Wolfe's Tim Anderson cautioning that investor expectations for RSV are likely too high.

How much of the potential each company will realise depends on the regulators as well as on two rivals. If Moderna files mRNA-1345 soon, and if Johnson & Johnson’s Ad26.RSV.preF yields positive data from its Evergreen study, the adult RSV market could feature four rival vaccines in time for the 2024/25 season.

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