A single dose of RSV vaccine provides strong protection for at least two full RSV seasons, though the level of protection varies by vaccine type and how it’s measured. The CDC currently recommends only one dose for eligible adults, with no booster or annual shot needed at this time.
Protection in Older Adults: Two Seasons of Data
Three RSV vaccines are approved for older adults, and each has slightly different durability data. Arexvy (GSK’s protein-based vaccine) showed 74.5% efficacy against RSV-related lower respiratory tract disease across two full seasons from a single dose. That number held remarkably steady from the first season into the second, suggesting the protection doesn’t drop off sharply in year two.
Abrysvo (Pfizer’s protein-based vaccine) performed similarly well. In its first season, efficacy against severe lower respiratory illness (defined by three or more symptoms) was 88.9%. By the end of the second season, it was 77.8%. Across both seasons combined, protection averaged 81.5%. When measuring less severe illness (two or more symptoms), efficacy dropped from 65.1% after season one to 55.7% after season two. So the vaccine holds up better against serious disease than mild illness, which is exactly the pattern you’d want to see.
mResvia (Moderna’s mRNA-based vaccine) showed more noticeable waning. Against lower respiratory illness with two or more symptoms, efficacy dropped from 56% in the first year to 30% in the second. Against more severe disease (three or more symptoms), it went from 55% to 36%. The CDC noted that participants in the second-year analysis had an average of only seven months of follow-up, so the true decline by month 24 could be steeper than what’s been reported so far.
Why Protection Fades at Different Rates
The pattern across all three vaccines is consistent: protection weakens over time, but it weakens faster for milder illness. This makes biological sense. Preventing a severe infection requires a lower threshold of immune readiness than preventing any infection at all. As antibody levels naturally decline after vaccination, the body retains enough immune memory to fight off serious disease longer than it can prevent every mild case.
The protein-based vaccines (Arexvy and Abrysvo) appear to maintain higher efficacy into the second season compared to the mRNA vaccine. Whether this reflects a true difference in durability or differences in trial design and populations studied isn’t fully settled. What’s clear is that all three vaccines offer their strongest protection in the first RSV season after the shot.
No Booster Recommended Yet
Despite the evidence of waning, the CDC does not recommend a second dose. If you’ve already received one RSV vaccine, you’re considered fully vaccinated. The current guidance applies to all adults 75 and older, and to adults 50 to 74 who are at increased risk of severe RSV. The advisory committee is actively evaluating whether a booster dose will be needed down the line, but no timeline has been set for that decision.
This means the practical answer for most people is straightforward: one shot, and then wait for updated guidance. If you were vaccinated before the 2023-2024 season, you still have meaningful protection heading into the next season, though it won’t be as robust as it was in year one.
Maternal Vaccination Protects Infants for 6 Months
The timeline is quite different for babies. When a pregnant person receives the RSV vaccine (Abrysvo, given during weeks 32 through 36 of pregnancy), antibodies cross the placenta and protect the newborn for approximately six months after birth. That window aligns with the period when infants are most vulnerable to severe RSV, which peaks in the first few months of life before a baby’s own immune system matures enough to handle the virus.
Six months is a hard ceiling, not a gradual fade. Maternal antibodies are borrowed, not generated by the infant’s own immune system, so they decline steadily from birth. For infants who need protection beyond that window or who weren’t covered by maternal vaccination, a separate antibody injection (nirsevimab) is available and works through a different mechanism, providing direct antibody protection for a single RSV season.
What This Means for Timing
If you’re eligible and haven’t been vaccinated yet, the ideal time to get the shot is late summer or early fall, before RSV season typically ramps up in November. This ensures your antibody levels are at their peak during the months when the virus circulates most heavily. RSV season in most of the United States runs from fall through early spring, though the exact timing shifts year to year.
For those who were vaccinated one or two seasons ago, the remaining protection is real but reduced. The strongest case for revaccination will likely come from people who received the mRNA vaccine, given its steeper decline, but until the CDC updates its guidance, the recommendation remains one dose only. Surveillance studies tracking how well vaccinated populations fare in subsequent seasons will shape whether annual or periodic boosters eventually become standard.