RSV is still spreading, prompting states to extend the immunization period

Respiratory syncytial virus is continuing to spread later into the spring than usual, driving most states to extend the window for RSV immunizations for eligible infants and toddlers.

RSV is a common respiratory virus that typically causes a mild illness like a cold. But it can cause serious illness for young children. The RSV season usually starts in the fall, peaks in the winter and continues into spring. Immunization is recommended through the end of March in most states, but this year, nearly all of those states have extended the immunization period through the end of April.

For the third week of March, federal data shows that 7.5% of tests were positive for RSV — significantly higher than the 5% test positivity rate at this time last year and even lower rates from the few years before that.

“RSV peaked later than usual this year, and many areas of the country are continuing to see [emergency department] visits and hospitalizations linger into spring. Given this ongoing transmission, health departments are looking at their local data, and many are recommending that providers continue monoclonal antibody administration into April,” Dr. Susan Kansagra, chief medical officer for the Association of State and Territorial Health Officials, said in an email.

“This is a really important, data-driven recommendation because RSV is the leading cause of infant hospitalization, and monoclonal antibodies can significantly decrease that risk.”

Two to three out of every 100 infants younger than 3 months are hospitalized with RSV in the US every year, according to the US Centers for Disease Control and Prevention. Already this season, tens of thousands of children have been hospitalized with RSV.

To prevent infection in infants and some young children, the American Academy of Pediatrics recommends immunization with monoclonal antibodies. There is also a vaccine available during pregnancy. The antibodies offer more passive immunity than vaccines, but they do offer effective protection: A CDC study published last year found that RSV-associated hospitalization rates among infants up to 7 months old during the 2024-25 season were lower than in previous seasons, when those immunizations weren’t available.

This is only the third season that monoclonal antibodies have been available to protect against RSV. Dr. Michelle Fiscus, a pediatrician and chief medical officer for the Association of Immunization Managers, said she considers these RSV immunizations to be “game-changers” – and she doesn’t use that phrase lightly.

“I’ve waited my 30 years in pediatrics to be able to prevent RSV disease in babies,” she said. “We should be doing everything we can, as pediatricians and as those of us who work in public health, to make sure that the public understands how important it is to protect every baby from RSV disease.”

After the CDC issued a sweeping overhaul of its childhood immunization schedule this year to recommend fewer vaccines for most American children, the US Department of Health and Human Services said its recommendations for immunizations against RSV remain unchanged: Infants born to mothers who did not receive the vaccine should get one dose of the antibody shot.

But US health regulators have also launched inquiries into the RSV shots, despite their documented safety and efficacy.

In mid-March, it was the CDC that encouraged states to assess their local RSV data.

The Association of Immunization Managers coordinated with the 66 federally funded immunization programs in the US — a collection of states, territories and major cities — to understand how they’re responding to ongoing virus circulation. As of Wednesday, 48 jurisdictions had extended their RSV season through at least April 30, according to data collected by the association.

Formally, the extension allows states and other jurisdictions to order immunizations through the federal government’s Vaccines for Children program for an additional month, Fiscus said. But the move also generally encourages providers to continue to administer RSV immunization as activity stays elevated.

“Our hope is that by extending the administration period for RSV immunizations, we can continue to keep babies out of the hospital while disease activity is still high,” Dr. Tao Sheng Kwan-Gett, the Washington state health officer, said in a statement last month. In Washington, RSV activity started to increase in December, more than a month later than previous seasons.

Ten jurisdictions — including Florida, Hawaii and Oregon — have year-round RSV seasons or epidemiological data that does not support the need to extend the immunization period. Louisiana and Washington, D.C., have decided against extending the immunization period, while Missouri and Virginia will consider orders from providers on a case-by-case basis.

Experts don’t know exactly why this RSV season has shifted. As with other viruses, a few different factors could be involved: environmental, biological and behavioral. There may have been some variation in the timing of when people decided to visit their doctors for immunization, for example, or climate patterns may have changed when people were spending more time indoors, where there is increased risk for spread.

“The bottom line is, it’s important that plans to respond to these virus viruses should be tied to actual disease trends, rather than a calendar on the wall,” said Dr. Jennifer Nuzzo, an epidemiologist and director of the Pandemic Center at the Brown University School of Public Health.

“This is why it’s so important that we have real-time surveillance to monitor diseases in our communities,” she said. “It’s important that RSV monoclonal antibodies be available for as long as the disease trends indicate the virus is circulating.”

This may be the first time an immunization period has been extended in this way, Fiscus said – but most other viruses aren’t seasonal like RSV.

An extra month of immunization to protect against RSV will make a difference, she said.

“Ten thousand babies are born each day,” Fiscus said. “We are doing everything we can to protect those 10,000 babies a day. That’s a lot of RSV disease and a lot of hospitalizations [to avoid] if we still have RSV circulating.”

CNN’s Jen Christensen contributed to this report.