RSV Season Extends: Implications for Infant Immunization

Respiratory syncytial virus (RSV) is spreading later into spring than typically seen, prompting many states to prolong the immunization period for eligible infants and toddlers.

While RSV usually causes mild illnesses reminiscent of a cold, it can lead to severe complications for younger children. Typically, the RSV season commences in the fall, peaking during winter and continuing into spring. While most states recommend immunization until the end of March, this year, nearly all have extended this period through the end of April.

Data from the third week of March reveals that 7.5% of RSV tests were positive, a notable increase compared to the 5% positivity rate recorded at the same time last year and lower rates of preceding years.

“This year, RSV peaked later than usual. Many regions are still dealing with high levels of emergency department visits and hospitalizations,” Dr. Susan Kansagra, chief medical officer for the Association of State and Territorial Health Officials, noted in an email. She emphasized the importance of continuing monoclonal antibody treatments through April due to ongoing transmission rates.

She elaborated, “This recommendation is crucial because RSV is a leading cause of hospitalizations among infants, and these antibodies can significantly mitigate that risk.”

According to the US Centers for Disease Control and Prevention, annually, 2 to 3 out of every 100 infants under three months are hospitalized due to RSV. This season, tens of thousands of children have already faced hospitalization due to the virus.

To help protect infants and young children from RSV, the American Academy of Pediatrics advocates for immunizations using monoclonal antibodies. Additionally, a vaccine is available during pregnancy. These antibodies provide effective, albeit passive, immunity. A recent CDC study found that RSV-related hospitalization rates among infants up to seven months during the 2024-25 season were significantly lower than in prior seasons when those immunizations weren’t available.

This season marks only the third time monoclonal antibodies have been accessible against RSV. Dr. Michelle Fiscus, a pediatrician and chief medical officer for the Association of Immunization Managers, refers to these immunizations as “game-changers,” a sentiment she doesn’t express lightly.

“After 30 years in pediatrics, I’ve finally seen the opportunity to prevent RSV in babies,” she shared. “It’s essential that as pediatricians and public health professionals, we ensure everyone understands the importance of protecting babies from RSV disease.”

Despite a recent overhaul of childhood immunization recommendations by the CDC—aimed at reducing the number of vaccines for most American children—the US Department of Health and Human Services maintains existing guidelines for RSV immunizations. Infants born to unvaccinated mothers should receive one dose of the antibody shot.

However, US health regulators have initiated inquiries into the RSV shots, even though their safety and efficacy have been documented.

In mid-March, the CDC advised states to evaluate their local RSV data. The Association of Immunization Managers has been in coordination with 66 federally funded immunization programs across the US to track their responses to ongoing virus circulation. As of Wednesday, 48 jurisdictions had extended their RSV seasons through at least April 30.

This extension allows states and other regions to order immunizations through the federal government’s Vaccines for Children program for an additional month. It also encourages healthcare providers to carry on with RSV immunizations while activity levels remain high.

“Our hope is that extending the administration period for RSV immunizations will keep infants from needing hospitalization as disease activity remains elevated,” noted Dr. Tao Sheng Kwan-Gett, Washington state health officer. In Washington, RSV activity began to rise in December, a month later than in past seasons.

Interestingly, ten jurisdictions, including Florida, Hawaii, and Oregon, have year-round RSV seasons or epidemiological data suggesting no need for extended immunization periods. Louisiana and Washington, D.C., have opted not to prolong the immunization period, while Missouri and Virginia are reviewing provider requests on a case-by-case basis.

Experts remain uncertain about the reasons behind this atypical RSV season. Various factors, from environmental to behavioral, could come into play. For instance, differences in the timing of doctor visits for immunizations or changes in climate that lead to increased indoor gatherings might contribute.

“Ultimately, it’s vital that our responses to these viral outbreaks align with actual disease trends, not just the calendar,” said Dr. Jennifer Nuzzo, an epidemiologist and director at Brown University’s Pandemic Center.

“Real-time disease surveillance in our communities is crucial,” she emphasized. “We must ensure RSV monoclonal antibodies are available as long as the virus is still circulating.”

This extension of the immunization period may mark a first in managing RSV, according to Fiscus—though it’s worth noting that many other viruses don’t have the same seasonal patterns as RSV.

Fiscus believes even an extra month for immunizations against RSV could be significant.

“Ten thousand babies are born every day,” she pointed out. “We’re doing all we can to shield those newborns. Preventing RSV disease means avoiding hundreds of thousands of potential hospitalizations if the virus is still around.”