We should be doing everything we can to protect every baby from RSV disease.
Each spring, public health systems quietly recalibrate their assumptions about when danger passes — and this year, RSV has refused to cooperate with the calendar. Across nearly every American state, officials have extended infant immunization windows into April, responding to a virus that is circulating later and more intensely than it has in recent memory. For the tens of thousands of infants already hospitalized this season, and the ten thousand born each day who remain vulnerable, the extension is not a bureaucratic adjustment but a meaningful act of protection. The season is a reminder that nature does not observe the schedules we set for it.
- RSV test positivity hit 7.5% in mid-March — well above last year's 5% — signaling a virus that peaked late and refuses to retreat on schedule.
- Emergency departments across the country are still filling with infant RSV cases at a point in the year when hospitalizations should be declining.
- 48 of 66 federally funded immunization programs have extended their RSV protection windows through April, unlocking additional monoclonal antibody doses for the most vulnerable newborns.
- Monoclonal antibodies — only in their third season of use — have already shown measurable reductions in infant hospitalizations, giving pediatricians a tool they lacked for decades.
- Health officials remain uncertain whether the seasonal shift stems from climate, viral mutation, or changing human behavior, but the lesson is clear: fixed seasonal assumptions can no longer be trusted.
Respiratory syncytial virus is lingering in American communities well past its usual exit, arriving later this winter and refusing to fade as spring takes hold. The shift has prompted nearly every state to extend infant immunization campaigns into April — a window that would normally have closed by the end of March.
For most people, RSV feels like a common cold. For infants under three months old, it can escalate quickly. Two to three of every hundred babies that young are hospitalized with the virus each year, and this season tens of thousands of children have already been admitted — making RSV the leading cause of infant hospitalization in the United States. Federal data from mid-March showed a 7.5% test positivity rate, a notable rise from 5% at the same point last year.
The response has been swift. As of early April, 48 of 66 federally funded immunization programs had extended their RSV periods, allowing states to order additional doses of monoclonal antibodies through the federal Vaccines for Children program. A handful of jurisdictions — including Florida, Hawaii, and Oregon — determined local data did not justify an extension, while Missouri and Virginia are handling requests individually.
Monoclonal antibodies are still a relatively new tool, now in only their third season of availability. Early evidence is encouraging: a CDC study found RSV-related infant hospitalizations were lower in 2024-25 than in years before these immunizations existed. A maternal vaccine offering newborn protection is also available, and for pediatricians who spent decades watching infants suffer with no preventive option, both feel like genuine turning points.
Why the season has shifted remains uncertain — environmental changes, viral variation, and behavioral factors have all been suggested. What is clearer is the broader implication: seasonal patterns once treated as fixed are proving flexible, and the real-time surveillance systems that detected this shift are themselves being tested. With ten thousand infants born each day in the United States, thirty additional days of eligibility translates into thousands of doses — and potentially thousands of hospitalizations prevented.
Respiratory syncytial virus is lingering in American communities well into spring this year, arriving later than expected and staying longer than health officials have seen in recent memory. The shift has prompted nearly every state to extend the window for protecting infants against the virus, pushing immunization campaigns into April when they would normally have wound down by the end of March.
RSV is ordinarily a mild illness—something that feels like a cold to most people who catch it. But for infants, particularly those under three months old, it can turn serious fast. The virus sends two to three of every 100 babies that young to the hospital each year. This season, tens of thousands of children have already been hospitalized with RSV, making it the leading cause of infant hospitalization in the United States. The numbers underscore why public health officials are treating this extended season with urgency.
Federal data from mid-March showed that 7.5 percent of respiratory tests came back positive for RSV—a significant jump from the 5 percent positivity rate at the same point last year, and well above the rates from the years before that. The virus peaked later than usual this winter and has refused to fade on schedule. Emergency departments across the country continue to see RSV patients, and hospitalizations that should have been declining are instead holding steady or climbing in many regions.
The response has been swift. As of early April, 48 of the 66 federally funded immunization programs across states, territories, and major cities had extended their RSV immunization periods through at least the end of April. The extension is more than symbolic—it allows states to order additional doses of monoclonal antibodies through the federal Vaccines for Children program and signals to pediatricians and other providers that they should continue administering the shots while the virus remains active. Ten jurisdictions, including Florida, Hawaii, and Oregon, determined that their local data did not warrant an extension, while Louisiana and Washington, D.C., decided against extending the period. Missouri and Virginia are handling requests on a case-by-case basis.
Monoclonal antibodies represent a relatively new tool in the fight against RSV. This is only the third season they have been available to protect infants and some young children, and early evidence suggests they work. A CDC study from last year found that RSV-related hospitalizations among infants up to seven months old during the 2024-25 season were lower than in previous years, when these immunizations did not exist. There is also a vaccine available for pregnant women, which provides protection to their newborns. For pediatricians who have spent decades watching infants suffer from RSV with no preventive option available, the arrival of these tools feels transformative.
Why this season has shifted remains unclear. Experts point to a mix of possible factors—environmental changes, biological variation in the virus itself, or shifts in human behavior. The timing of when people sought out immunizations may have changed. Climate patterns might have altered when people spent more time indoors, where respiratory viruses spread more readily. The precise cause matters less than the lesson: seasonal patterns that seemed fixed are proving flexible, and public health responses need to be as well.
Ten thousand infants are born each day in the United States. Each of those babies is now eligible for RSV protection through April in most states, rather than having that window close at the end of March. The extension may seem modest—just thirty additional days—but across a population of newborns, it represents thousands of doses that would not otherwise be administered. If RSV continues circulating at current levels, those extra immunizations could prevent a significant number of hospitalizations. The real-time surveillance systems that allowed health officials to detect this shift and respond to it are themselves relatively new, and this season is testing whether they can keep pace with a virus that refuses to follow the calendar.
Notable Quotes
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.— Dr. Susan Kansagra, chief medical officer for the Association of State and Territorial Health Officials
It's important that plans to respond to these viruses should be tied to actual disease trends, rather than a calendar on the wall.— Dr. Jennifer Nuzzo, epidemiologist and director of the Pandemic Center at Brown University School of Public Health
The Hearth Conversation Another angle on the story
Why is RSV spreading so much later this year? Is the virus itself changing?
We don't know for certain. It could be environmental—maybe people are spending more time indoors at different times than they used to. It could be behavioral, or biological variation in the virus. The honest answer is that experts are still trying to figure it out.
So states are just extending the immunization window and hoping it helps?
It's more deliberate than that. They're looking at their actual disease data—the test positivity rates, the hospitalizations happening right now—and making decisions based on what they're seeing, not on a calendar. That's the shift here.
How new is this monoclonal antibody approach?
Only the third season it's been available. For pediatricians who've been practicing for thirty years, this is the first time they've had a tool to actually prevent RSV in babies. Before this, you could only treat it after infection.
Are there any concerns about the shots themselves?
They have documented safety and efficacy. But regulators have launched inquiries into them anyway, despite that track record. It's worth noting that this happened after the CDC made broader changes to childhood immunization schedules.
What happens if RSV keeps circulating into May?
That's the question. If the pattern continues, we might see further extensions. But more importantly, it suggests that seasonal virus patterns we thought were fixed might not be anymore. Public health needs to stay flexible and data-driven.
Does this affect how we should think about other seasonal viruses?
Potentially. If RSV's timing is shifting, flu and other respiratory viruses might too. The lesson is that we need real-time surveillance systems that can detect these changes and allow us to respond quickly, rather than relying on historical patterns.