CATEGORY: IMMUNE HEALTH
One dose of an RSV vaccine cut cases of RSV-related chest illness in adults 60 and older by 62.9% across three full RSV seasons—but the shield weakened winter by winter, from 82.6% protection in the first season to 56.1% in the second and 48.0% in the third, in a randomized, placebo-controlled trial of nearly 25,000 people published in The Lancet Respiratory Medicine.1 The trial also tested a second dose given a year after the first, and it added no clear benefit.
Twenty-Five Thousand Adults, Assigned by Chance, Watched for Three Winters
RSV—respiratory syncytial virus—is a common winter virus that most people shrug off as a cold, but in older adults it can descend into the lungs and turn dangerous. The trial behind these numbers, an international study funded by the vaccine’s maker, GSK, assigned 12,468 adults to receive the RSVPreF3 vaccine and 12,498 to receive a placebo, entirely by chance—the design that lets the difference between groups be credited to the vaccine itself. Neither the participants nor the staff assessing their illnesses knew who had received which. For a median of about two and a half years, every case of RSV-related lower respiratory tract disease—illness that reaches the airways of the chest, with symptoms like new wheezing, deep cough or shortness of breath—was laboratory-confirmed and logged, season by season.
From Four-Fifths Protection to About Half
Season by season, the pattern was consistent: strong protection at first, then a gradual slide—82.6%, then 56.1%, then 48.0%. Across all three seasons combined, one dose prevented 62.9% of RSV chest illness, and 51.1% of RSV respiratory illness of any severity. The vaccine’s protection behaved less like a switch that flips off than like a phone battery aging: by the third winter it no longer held its original charge, but it still carried enough to matter. Serious side effects judged related to vaccination were rare, occurring in fewer than 1% of participants.1
The Second Dose That Added Little
The researchers built a revaccination question directly into the trial: before the second season, part of the vaccinated group received another dose to test whether an annual top-up would restore the first winter’s peak protection. It did not clearly do so—across three seasons, protection in the revaccinated group was 67.8%, in the same general range as a single dose. That unfulfilled expectation is itself a useful result, and it lines up with how the shot is currently handled in the United States, where the RSV vaccine is given as a one-time dose rather than repeated each year.
A Rival Vaccine’s Trial Tells a Similar Story
The durability question is not unique to one product. In its own randomized trial, a competing RSV vaccine made by a different manufacturer also kept a substantial share of its protection through a second season without another dose.2 Together, the two trials point the same way: for older adults, one dose of an RSV vaccine keeps working well past its first winter, with protection that fades gradually rather than disappearing.
Key Takeaways
- In a randomized trial of nearly 25,000 adults 60 and older, one dose of an RSV vaccine prevented 62.9% of RSV-related chest illness across three seasons—82.6% in the first winter, easing to 48.0% by the third.
- A second dose given one year after the first did not clearly improve on single-dose protection, which is consistent with current U.S. practice of giving the vaccine once rather than annually.
- A separate randomized trial of a competing RSV vaccine also found protection carrying into a second season, suggesting multi-winter durability is a feature of this class of vaccines rather than one product.
- Serious side effects judged related to the vaccine were rare, at fewer than 1% of participants.
References
- Ison MG, et al. Efficacy, safety, and immunogenicity of the AS01E-adjuvanted respiratory syncytial virus prefusion F protein vaccine (RSVPreF3 OA) in older adults over three respiratory syncytial virus seasons (AReSVi-006): a multicentre, randomised, observer-blinded, placebo-controlled, phase 3 trial. The Lancet Respiratory Medicine. 2025;13(6):517–529. link
- Walsh EE, Pérez Marc G, Falsey AR, et al. RENOIR Trial—RSVpreF Vaccine Efficacy over Two Seasons. The New England Journal of Medicine. 2024;391(15):1459–1460. link