Pfizer’s vaccine against respiratory syncytial virus has shown promising results in maintaining protection for older adults across two full seasons of the disease in an ongoing late-stage clinical trial.

The vaccine, known as Abrysvo, has demonstrated a decline in efficacy compared to 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 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.

According to the initial data on more than 37,000 participants in the Northern and Southern Hemispheres, 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. The vaccine also showed roughly 55.7% effectiveness against a less severe form of the condition with two or more symptoms after the end of season two, compared to 66.7% efficacy after one RSV season.

Pfizer noted that the vaccine demonstrated consistent efficacy against RSV A and RSV B, 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. Additionally, no new adverse events were reported by patients after the two seasons.

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