Nirsevimab RSV protection is designed to last through an entire respiratory syncytial virus (RSV) season after a single injection. CDC currently describes that protection as lasting at least 5 months, while newer randomized data from HARMONIE show substantial protection against RSV hospitalization through 180 days, or about 6 months. The key point is that “duration” depends on the outcome being measured: antibody levels persist for months, but protection against infection, emergency visits, hospitalization, and critical illness are not identical endpoints.
Nirsevimab RSV protection was first tested across a 150-day season
Nirsevimab was developed as a long-acting monoclonal antibody that could cover a typical RSV season with one dose. In the phase 2b trial in preterm infants and the phase 3 MELODY trial in late-preterm and term infants, efficacy was assessed through 150 days after injection.
In MELODY, 1,490 infants were randomized in a 2:1 ratio to nirsevimab or placebo. Through day 150, medically attended RSV-associated lower respiratory tract infection occurred in 12 of 994 infants receiving nirsevimab and 25 of 496 receiving placebo, corresponding to 74.5% efficacy. The expanded MELODY population later showed efficacy of 76.4% against medically attended RSV lower respiratory tract infection and 76.8% against RSV hospitalization through the same 150-day window.
The earlier phase 2b trial in preterm infants also showed protection throughout 150 days, with 70.1% efficacy against medically attended RSV lower respiratory tract infection and 78.4% efficacy against RSV hospitalization.
Why one dose can last months
Nirsevimab targets the prefusion form of the RSV F protein, blocking the virus from entering cells. Unlike a vaccine, it does not teach the infant’s immune system to make antibodies. It supplies the antibody directly.
The molecule was engineered to remain in circulation longer than older monoclonal antibodies. Clinical studies estimate an extended half-life of roughly 63 to 73 days, with HARMONIE citing about 71 days. A half-life does not mean protection ends after 71 days. It means the antibody concentration falls gradually, with about half remaining after one half-life under simplified pharmacokinetic assumptions.
That extended persistence is what allows one injection to cover months rather than requiring monthly dosing. Our original nirsevimab approval explainer covers how this differs from older RSV antibody strategies.
HARMONIE extended the evidence to 180 days
The phase 3b HARMONIE trial was designed to approximate routine practice in France, Germany, and the United Kingdom. More than 8,000 infants were randomized to receive nirsevimab or standard care.
In the 180-day analysis, 12 of 4,038 infants in the nirsevimab group and 68 of 4,019 infants in the standard-care group were hospitalized for RSV-associated lower respiratory tract infection. That corresponded to an efficacy of 82.7% (95% confidence interval, 67.8% to 91.5%; P<0.0001).
The investigators described the result as showing “consistent and sustained protection” through at least 6 months. Importantly, that conclusion applies to the hospitalization endpoint studied in HARMONIE. It should not be interpreted as proof that every type of RSV protection remains unchanged for exactly 180 days.
Real-world U.S. data show strong protection, with more uncertainty later after dosing
A 2026 U.S. analysis published in Pediatrics evaluated RSV prevention during the 2024-2025 season. Estimated nirsevimab effectiveness was 62% against RSV-associated emergency-department encounters, 77% against hospitalization, and 79% against critical illness.
The median time from nirsevimab administration to illness was about 2 to 2.5 months for those endpoints, but observations extended as far as 180 days. Effectiveness point estimates for emergency visits and hospitalization were higher at 7 to 59 days after dosing than at 60 days or more, although the confidence intervals overlapped.
That is a more realistic way to think about duration. Protection can remain clinically meaningful for months while gradually changing as antibody concentrations fall. Observational studies also have fewer events at longer post-dose intervals, making late-season estimates less precise.
Critical-illness data also suggest protection persists beyond the first two months
CDC’s 2024-2025 multicenter evaluation of infants admitted to intensive care units found nirsevimab was 80% effective against RSV-associated ICU admission overall and 83% effective against acute respiratory failure.
When ICU effectiveness was divided by time since dosing, the estimate was 86% at 7 to 59 days and 66% at 60 to 183 days. The latter estimate still suggested substantial protection, but the confidence interval was wider. This pattern is compatible with waning antibody levels, although observational data cannot define a precise day when protection becomes inadequate.
“At least 5 months” is more useful than saying protection lasts exactly 5 months
CDC uses the phrase “at least 5 months” for a reason. The pivotal trials were designed around a 150-day season, and subsequent evidence has shown protection against hospitalization through 180 days. There is no biological cliff at day 150.
At the same time, it would be misleading to tell families that a single dose guarantees the same degree of protection indefinitely. Antibody concentrations fall over time, RSV exposure varies by season and geography, and clinical studies become less statistically precise as follow-up extends.
For infants entering their first RSV season, timing therefore matters. In most of the continental United States, CDC recommends giving an infant RSV antibody shortly before or during the RSV season, generally October through March. Infants born during the season should usually receive the antibody within the first week of life when indicated.
Maternal vaccination changes the timing question for many infants
For most infants, CDC recommends protection through either maternal RSV vaccination during pregnancy or an infant RSV antibody, not both. If maternal vaccination was given at least 14 days before delivery, most infants do not also need nirsevimab.
Our nirsevimab vs maternal RSV vaccine guide explains how the timing and duration of those two strategies differ. We also compare the two available long-acting infant antibodies in our nirsevimab vs clesrovimab article.
A second-season dose is not given simply because the first dose “wore off”
Nirsevimab can also be given to selected high-risk children entering a second RSV season, but that is a separate eligibility decision. It does not mean routine first-season protection failed or that every child needs a booster the following year.
CDC recommends second-season nirsevimab only for certain children aged 8 through 19 months who remain at increased risk for severe RSV disease. Our Beyfortus dosing guide explains the 50 mg, 100 mg, and 200 mg regimens and who qualifies.
What the evidence cannot yet answer
The strongest randomized evidence shows protection through 150 days and, for hospitalization in HARMONIE, through 180 days. Those studies do not identify an exact endpoint beyond which protection disappears.
HARMONIE was open-label and funded by Sanofi and AstraZeneca, the manufacturers developing nirsevimab. The MELODY and phase 2b programs were also industry funded. The randomized designs strengthen the efficacy evidence, but longer-term comparisons after a typical RSV season involve fewer events and greater uncertainty.
Real-world studies add important evidence because they include routine clinical populations, but they are observational and can be affected by differences in who receives nirsevimab, timing of administration, health-care seeking behavior, and circulating RSV intensity.
One dose is designed to cover a season, not a lifetime
The most evidence-based answer is that nirsevimab provides strong RSV protection for at least 5 months, with randomized evidence showing sustained protection against hospitalization through 6 months. Protection likely declines gradually rather than ending suddenly. For most infants, that is long enough to span a typical RSV season when the dose is timed appropriately.
References
- Munro APS, Drysdale SB, Cathie K, et al. 180-day efficacy of nirsevimab against hospitalisation for respiratory syncytial virus lower respiratory tract infections in infants (HARMONIE): a randomised, controlled, phase 3b trial. Lancet Child Adolesc Health. 2025;9:404-412. DOI: 10.1016/S2352-4642(25)00102-6.
- Hammitt LL, Dagan R, Yuan Y, et al. Nirsevimab for Prevention of RSV in Healthy Late-Preterm and Term Infants. N Engl J Med. 2022;386:837-846. DOI: 10.1056/NEJMoa2110275.