Pfizer‘s vaccine against respiratory syncytial virus maintained protection for older adults across two full seasons of the disease in an ongoing late-stage clinical trial, the company announced Thursday.

The shot’s efficacy declined slightly compared with the levels of protection after one RSV season, but the new data suggests that the jab generally offers durable protection for adults ages 60 and above, who are more vulnerable to severe illness from RSV. The launch of Pfizer’s vaccine, known as Abrysvo, and another RSV shot from GlaxoSmithKline last year proved to be a boon for both companies, with the jabs accounting for hundreds of millions in just half a year on the market. 

A single dose of Pfizer’s vaccine was 77.8% effective against more severe lower respiratory tract illness with three or more symptoms through a second season, down from the 88.9% efficacy following the end of one season. Those symptoms include wheezing, shortness of breath, rapid and shallow breathing, and mucus production.

The shot was roughly 55.7% effective against a less severe form of that condition with two or more symptoms after the end of season two, according to the initial data on more than 37,000 participants in the Northern and Southern Hemispheres. The jab showed 66.7% efficacy against that condition after one RSV season. 

Pfizer noted that the vaccine showed consistent efficacy against RSV A and RSV B, which are the two major subtypes of the virus, after the second RSV season. The shot was specifically 80% or more effective against each type in patients with the more severe form of lower respiratory tract illness.

No new adverse events were reported by patients after the two seasons.

The results come ahead of a meeting of an advisory panel to the Centers for Disease Control and Prevention on Thursday, which will consider whether seniors should take RSV shots annually or every other year.

Analysts don’t expect the committee to make a final recommendation until June – a decision that may have huge implications for Moderna, which is hoping to launch its own RSV jab this year. 

Jefferies analyst Michael Yee said in a note last week that the firm sees a 50% to 70% likelihood that the panel will recommend annual vaccination, which would put Moderna in a position to achieve “at least equivalent market share” to Pfizer and GSK. A biennial recommendation based on data on GSK’s shot across two seasons would “reduce competitive positioning” for Moderna, he noted. 

In older adults, GSK’s shot showed a cumulative efficacy of 67.2% against lower respiratory tract illness over two RSV seasons. That’s compared with 82.6% after one season of the virus. 

GSK’s vaccine booked around £1.2 billion ($1.5 billion) in sales last year. Meanwhile, Pfizer’s shot, which is also approved for expectant mothers who can pass on protection to their children, recorded about $890 million in revenue in 2023. 

RSV kills 6,000 to 10,000 older adults and hospitalizes 60,000 to 160,000 of them every year, according to the CDC. 

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