RSV vaccines typically begin building protection within two weeks of the injection, with strong antibody levels detectable by about one month. That’s why the CDC recommends getting vaccinated in late summer or early fall, ideally August through October, so your immune response is fully developed before RSV season peaks in winter.
How Quickly Protection Develops
After a single dose, your immune system starts producing antibodies against the virus’s surface protein within days. By day 14, antibody levels are climbing significantly. Clinical trial data shows that neutralizing antibody levels for both major RSV strains (RSV-A and RSV-B) rise above protective thresholds within the first 31 days after vaccination. Most immunologists consider the two-to-four-week window the period when you transition from unprotected to meaningfully protected.
The vaccine works by training your immune system to recognize a specific shape on the virus’s surface, the protein RSV uses to fuse with and enter your cells. A single dose triggers a diverse set of antibodies that target multiple spots on this protein, which is why the immune response tends to be broad and effective even against different RSV strains.
Best Time of Year to Get Vaccinated
Because protection takes a few weeks to fully develop, timing matters. The CDC recommends vaccination during August through October for most of the continental United States. This positions your immune response to be at its strongest right as RSV begins circulating heavily, which typically runs from November through March. If you miss that window, you can still get vaccinated at any time of year, but the benefit is greatest when your antibody levels are high during peak season.
How Well the Vaccine Works
Once protection is established, the results are substantial. In clinical trials pooling data from both approved vaccines (Arexvy by GSK and Abrysvo by Pfizer), adults 75 and older saw a 69% reduction in RSV-related lower respiratory tract disease and a 77% reduction in cases severe enough to require medical attention. For adults aged 60 to 74 with conditions that put them at higher risk, the numbers were similar: a 73% reduction in lower respiratory disease and a 73% reduction in medically attended cases.
These are strong numbers for a respiratory virus vaccine, especially in older adults whose immune systems are generally less responsive to vaccination.
Who Should Get the Vaccine
The CDC currently recommends a single dose of RSV vaccine for all adults 75 and older and for adults 50 to 74 who face increased risk of severe RSV illness. That includes people with chronic lung disease, heart disease, weakened immune systems, and other conditions that make respiratory infections more dangerous. It is not currently an annual vaccine. One dose is considered complete, and no booster is recommended at this time.
Timing for Pregnant Women
Pregnant individuals have a different timeline. The maternal RSV vaccine (Abrysvo) is given between 32 and 36 weeks of pregnancy, during September through January in most of the U.S. The goal is for the mother’s body to produce antibodies that cross the placenta and protect the baby during its first months of life. This transfer takes time. If a baby is born fewer than 14 days after the mother’s vaccination, there may not have been enough time for adequate antibody transfer. In those cases, the infant may be offered a monoclonal antibody product instead, which provides immediate, passive protection rather than relying on the immune system to build its own response.
Getting It Alongside Other Vaccines
If you’re planning to get a flu shot around the same time, you don’t need to space them out. Clinical studies found that receiving the RSV vaccine and a flu vaccine simultaneously produced an immune response to influenza that was just as strong as getting the flu shot alone. The RSV vaccine’s antibody response also remained robust when the two were given together. Co-administration was well tolerated, so you can simplify your fall vaccination schedule into a single visit without sacrificing effectiveness or adding extra time to the protection timeline.
How Long Protection Lasts
Current evidence supports meaningful protection through at least one full RSV season following vaccination, which is the primary goal. Because the vaccine is not yet recommended as an annual shot, the CDC considers a single dose sufficient for now. Protection does gradually decline over time as antibody levels wane, which is typical of most vaccines. Ongoing surveillance will determine whether a second dose is eventually recommended, but for the time being, one dose is the complete course.
The practical takeaway: if you get vaccinated in September, expect solid protection by early to mid-October, well before the worst of RSV season hits.