Antibody Against RSV Prevents Cases for 1 Year: Study

Beyfortus is approved for young children.

A package of Beyfortus, in an undated file photograph. Fred Tanneau/AFP via Getty Images

An antibody that protects against respiratory syncytial virus (RSV) works well for one year but possibly not beyond, according to newly published research.

Nirsevimab, the antibody, “may not be effective after 12 months of administration,” Japanese researchers reported in the study, which was published on Nov. 6 by the Journal of Infection.

Sanofi, which manufactures nirsevimab with AstraZeneca, did not respond to a request for comment.

Nirsevimab, a monoclonal antibody, is approved in multiple countries for the prevention of RSV in young children. RSV usually causes mild, cold-like symptoms.

It can trigger severe problems that, in a subset of cases, can result in death. The elderly and young children are particularly susceptible.

In the United States, nirsevimab, marketed as Beyfortus, is approved and recommended for all infants aged eight months and younger, as well as certain children aged 8 to 19 months.

Trial data showed that the antibody prevented RSV in child recipients, when compared to placebo, the Food and Drug Administration said when approving it in 2023.

Data from health care systems also indicate it is effective against RSV-associated emergency department encounters and hospitalization, according to the Centers for Disease Control and Prevention. Data for effectiveness beyond 180 days are scant, a CDC scientist said at a meeting earlier this year.

In the new study, the Japanese researchers analyzed records from a global database for children who were tested for RSV between July 2023 and June 2025. They compared children who had received nirsevimab before testing to children who had not. The study included about 6,000 children who had received the antibody and some 210,626 who had not.

Children who received nirsevimab within six months of testing were about half as likely to test positive for RSV as children who had not received the antibody, the researchers found. Children who received nirsevimab between six and 11 months before a test were also less likely to test positive than the children who had not received the product.

Children who received the antibody at least 12 months before the test were more likely to test positive, versus the group that had not been injected.

“In this study, the preventive effect of nirsevimab against RSV infection was observed not only for the first 6 months, but also potentially for up to 12 months after the last dose in children younger than 24 months, whereas the preventive impact of nirsevimab for longer than 12 months was not evident,” the researchers said.

Limitations included a smaller sample size of children who had received nirsevimab at least months before a test, they added.

The researchers included Dr. Taito Kitano from the Nara Prefecture General Medical Center. Declarations of competing interests included grants from the Japan Foundation for Pediatric Research and the Japan Agency for Medical Research and Development. Neither Sanofi nor AstraZeneca was involved in the study.