Medical professionals are sounding the alarm about an unexpected early winter surge of a potentially lethal respiratory virus currently affecting the United States.

Researchers from Yale University have conducted an independent analysis of hospital records, revealing a nationwide spike in emergency department visits due to respiratory syncytial virus (RSV) as of October 11, the most recent data available.

The increase is particularly concerning among infants under one year old, who are at the greatest risk for severe complications and mortality. Emergency visits for this age group have climbed to 1.2 percent, marking the highest rate among all age categories and a 2.5-fold increase from just a month prior.

While the data from Yale indicates that cases of influenza and Covid-19 are currently low or decreasing, these viruses typically see a rise later in the season.

Epidemiologists Dr. Katelyn Jetelina from Yale University and Hannah Totte from the University of California, Los Angeles, have cautioned that an ‘RSV wave is beginning to take hold.’

In their publication, Your Local Epidemiologist, they stated, “RSV activity is starting to climb, especially among children under four.”

‘This follows a familiar pattern: The virus first hits the youngest children (particularly those under one year) before spreading to adults, often about a month later.’

Doctors are warning over a wave of respiratory infections sweeping the US (stock photo)

Updated CDC data on the spread of RSV, Covid and the flu is currently unavailable because of the government shutdown, now in its 31st day. 

RSV is a particularly dangerous infection for young children because it can cause inflammation that may block their small airways, potentially leading to death.

About 58,000 to 80,000 children under five years old are hospitalized every year due to RSV, and 100 to 500 children die from the infection. 

It is spread via coughs and sneezes, and causes symptoms including a sore throat, runny or stuffy nose and exhaustion. In severe cases, patients can start wheezing.

Doctors treat the infection using over-the-counter painkillers, but may also offer patients devices to help them breathe. Since 2023, vaccines have also been available for older adults, who are also at higher risk, and pregnant women, who can pass on protection from the virus to their children.

Current RSV levels are well below the peak in January last year, where they were behind an estimated 12.8 percent of ED admissions for children under one year and 1.1 percent overall, compared to the current level overall of 0.07 percent.

But experts warn they are rising rapidly and that, because of the lag in data, they are likely already much higher. 

The figures are from Yale University’s PopHIVE platform, which analyzes data from Epic Cosmos, a Wisconsin-based health system that analyzes more than 300million patient records from 1,600 hospitals and 37,900 clinics across all 50 states.

The uptick comes amid the first month of the winter virus season, which generally lasts from October to mid-May, although infections normally do not peak until December to February.

Healthy adults and older children recover from RSV infections often without needing to be hospitalized, doctors say (stock image)

Experts said it was normal for RSV infections to rise first before Covid and flu infections also begin to tick up.

Among children under one year old, they found 1.2 percent of admissions were due to RSV in the week to October 11, the latest available.

This is an increase from 0.9 percent the previous week and 0.4 percent a month ago. 

Among children four years and under, the researchers said an estimated 0.5 percent of ED admissions in the age group were due to the infection in the week to October 11, the latest available.

This is up from 0.4 percent a week earlier and 0.2 percent a month earlier. 

Overall, an estimated 0.07 percent of ED admissions were due to RSV in the week to October 11, the data shows.

This is up from 0.05 percent of admissions the previous week and more 0.02 percent a month earlier.

PopHIVE also tracks data on Google searches related to RSV by state. This does not necessarily mean cases are higher in certain areas, but it provides an approximation of where cases may be rising.

Nationwide, it showed Hawaii had the highest proportion of Google searches related to RSV at 14 percent.

Florida had the second highest, at 10 percent, and Mississippi and Montana had the third highest, at 8.6 percent.

It comes after some counties in California reintroduced mask mandates from the Covid era earlier this month amid concerns over the winter virus season. 

No other counties or states have followed suit to date.

Last year, however, authorities in at least 12 states brough back Covid-era restrictions in the 2024 to 2025 respiratory virus season.  

In California, Massachusetts, New Jersey, New York, Pennsylvania and Wisconsin, face mask restrictions returned in some health care settings. 

In Illinois, Indiana, Minnesota and North Carolina, some officials brought back both mask mandates and visitor restrictions for health care facilities.

Officials in Michigan and South Carolina only brought back visitor restrictions to hospitals.

Last year, the US faced its worst respiratory virus season since the 2017 to 2018 period.

Overall, an estimated 47million people were sickened, while 610,000 were hospitalized and 26,000 died from the flu.