In a recent announcement, Pfizer revealed that its vaccine against respiratory syncytial virus (RSV) has maintained protection for older adults during an ongoing late-stage clinical trial spanning two full seasons of the disease. While the shot's efficacy declined slightly compared to the levels of protection observed after one RSV season, the new data suggests that the jab generally offers durable protection for adults aged 60 and above, who are more vulnerable to severe illness from RSV.

Known as Abrysvo, Pfizer's RSV vaccine, along with another RSV shot from GlaxoSmithKline, proved to be highly successful for both companies since its launch last year. Together, these vaccines accounted for hundreds of millions in sales in just half a year on the market.

Based on the initial data, a single dose of Pfizer's vaccine was found to be 77.8% effective against more severe lower respiratory tract illness with three or more symptoms through a second season. This efficacy rate was slightly lower than the 88.9% observed after the end of one season. The shot also showed approximately 55.7% effectiveness against a less severe form of the condition with two or more symptoms after the end of season two. Notably, the vaccine consistently demonstrated efficacy against both major subtypes of RSV, RSV A and RSV B, in patients with the more severe form of lower respiratory tract illness.

Crucially, no new adverse events were reported by patients over the course of the two seasons, further emphasizing the safety of Pfizer's RSV vaccine.

The results of the clinical trial come ahead of an advisory panel meeting scheduled by the Centers for Disease Control and Prevention (CDC) on Thursday. During this meeting, the panel will discuss whether seniors should receive RSV shots annually or every other year. Analysts anticipate that the committee will not make a final recommendation until June, a decision that could have significant implications for Moderna, which plans to launch its own RSV vaccine this year.

If the panel recommends annual vaccination, there is a possibility for Moderna to achieve market share similar to that of Pfizer and GlaxoSmithKline. On the other hand, a biennial recommendation based on GlaxoSmithKline's shot's data across two seasons could potentially limit Moderna's competitive positioning.

In older adults, GlaxoSmithKline's RSV shot demonstrated a cumulative efficacy of 67.2% against lower respiratory tract illness over two RSV seasons, compared to an efficacy rate of 82.6% after just one season. GSK's vaccine generated approximately £1.2 billion ($1.5 billion) in sales in the previous year, while Pfizer's shot, which is also approved for expectant mothers to pass on protection to their children, recorded around $890 million in revenue in 2023.

According to the CDC, RSV poses a significant threat to older adults, with 6,000 to 10,000 deaths and 60,000 to 160,000 hospitalizations occurring annually due to the virus.

The decision regarding the frequency of RSV shots for older adults remains pending, as experts review the data and assess the best strategy to protect this vulnerable population from RSV.