How Bad Will This Winter Be for Flu, COVID, RSV, and Measles?

As of mid-December, flu, COVID, and RSV activity are relatively low throughout most of the U.S., but rates of all three are climbing just as many Americans travel and gather for the holidays.

And, for the first time in decades, measles outbreaks in parts of the country are adding even more questions for families about how to stay informed and safe.

During a December 16 media briefing, Andrew Pekosz, PhD, professor of Molecular Microbiology and Immunology, and William Moss, MD, professor of Epidemiology and executive director of the International Vaccines Access Center, explained the current status of flu, COVID-19, RSV, and measles in the U.S. and the trends they see heading into the annual peak for respiratory virus transmission.

Influenza

Flu in the U.S. is following typical seasonal patterns. Cases began picking up around Thanksgiving and have been increasing weekly since then. This growth is expected to continue into the new year, says Pekosz.

Which flu viruses are circulating right now?

There are two types of flu viruses—influenza A and influenza B—that are responsible for seasonal outbreaks. Influenza A has two subtypes, H1N1 and H3N2, and influenza B has two lineages, Victoria and Yamagata. Those subtypes and lineages can then be further classified into clades and subclades.

The vast majority of cases since the end of September have been flu A, according to the latest CDC data. And of flu A cases, most of those sequenced are H3N2, subclade K.

What is subclade K?

Subclade K—a relatively new strain of H3N2 that was identified this year—has driven an earlier start to the influenza season and higher-than-usual case rates in the UK, Japan, and Canada, says Pekosz.

The U.S.’s flu season did not start early like it did in those countries, but subclade K is already showing to be the dominant virus circulating here. “We're seeing subclade K everywhere we see influenza,” Pekosz says, because subclade K’s mutations allow it to evade existing immunity in the population. It is less recognizable by your body's immune response, making you more susceptible to infection.

Public health researchers are still working to understand whether subclade K is more likely than other flu viruses to cause severe illness. “There have been reports that the disease is particularly strong in children, and certainly there are reports of the elderly having higher hospitalization rates,” Pekosz explains. “Those are two things we expect with any influenza season surge." It’s not clear yet, he says, whether the high rates of illness and hospitalization are proportional to the higher number of total cases, or whether subclade K is more likely to cause severe symptoms.

What is clear is that "influenza cases [are] increasing across the country, and that's a trend that we're sure will continue into the new year,” Pekosz says.

How bad will this flu season be?

“It's very difficult to predict a flu season, but right now, we're at the beginning of what will probably be a pretty severe influenza season, driven primarily by this subclade K virus,” Pekosz says.

As of mid-December, cases are rising throughout the U.S., but there is still a fair amount of variation from state to state. Louisiana, Colorado, and New York, for example, are seeing high numbers of cases, while case rates in other states are low for now—but increasing.

Is this year’s flu vaccine effective against the flu viruses circulating this year?

Yes, this year’s flu vaccine provides effective protection against severe illness and hospitalization and is recommended for everyone six months and older. The 2025–26 flu vaccine is trivalent, meaning it’s designed to protect against three flu viruses: H1N1, H3N2, and influenza B.

According to Pekosz, the vaccine appears to be a good match to the H1N1 and influenza B viruses circulating in the U.S. and should provide partial protection against subclade K, which “has mutations that may allow it to evade some, but not all, of the influenza vaccine-induced protection,” he says.

The CDC reports that “early estimates of 2025–26 influenza vaccine effectiveness in England against influenza-associated hospitalization remained within expected ranges of 70%–75% for children and 30%–40% for adults, suggesting that influenza vaccination remains an effective tool in preventing influenza-related hospitalizations this season.”

Is it too late to get a flu shot?

There is still time to get a flu shot if you haven’t yet, Pekosz says. It takes around 10-–14 days from getting the vaccine to build an immune response to help fight infection or reduce disease severity, and as of mid-December, “we’re really at the beginning of the influenza season here in the U.S.," he says. “It’s certainly not too late, especially as we’re thinking about holiday gatherings and travel.”

COVID-19

COVID is circulating in the U.S., but case rates are relatively low, especially compared to previous winters. “We had a small spike of cases in the later summer, early fall, but nowhere near what we’ve seen in prior years,” Pekosz says. New variants continue to emerge, but recent ones are “not particularly worrisome,” he adds, noting that he’s closely monitoring variants that are circulating globally.

Is the COVID vaccine safe?

Yes, “it’s very clear that the COVID-19 and influenza vaccines are safe,” says Pekosz. And they’re effective at what they’re intended to do: prevent severe disease and hospitalization.

“Vaccines are our best line and our first line of protection from these respiratory viruses that are causing significant amount of disease in our population,” says Pekosz. ““They certainly are very good in terms of stemming severe disease, particularly in at-risk populations.”

RSV

Respiratory syncytial virus affects people of all ages, but infants, young children, and people over age 65 are at the highest risk of severe disease. RSV is the leading cause of infant hospitalization in the U.S.

Heading into this winter, RSV activity in the U.S. is low, which Pekosz attributes in part to the U.S. having had strong RSV seasons the past two winters, and to the new RSV immunizations and treatments that have become available in recent years.

Despite currently low levels of viral activity, RSV test positivity and hospitalizations are trending upward in most of the U.S., according to CDC data.

Preventing and Treating Respiratory Illnesses

Everyone can take actions to limit the spread and impact of respiratory infections this winter, says Pekosz. These include:

• Getting vaccinated against flu and COVID, as well as RSV for eligible groups.
• Staying home when you have a fever or other symptoms.
• Minimizing time spent in crowded indoor settings.
• Wearing a well-fitting mask (N95 or KN95), especially if you are recovering from a recent illness or are at high risk for severe complications.

COVID and flu infections share many of the same symptoms, including sore throat, congestion, runny nose, fever and chills, muscle aches, and fatigue. If you feel symptoms coming on, it's important to test to know what you’re sick with, so that you can treat it quickly and effectively.

Combined at-home tests for flu and COVID-19 are available, as are antiviral treatments for both. Pekosz encourages people—especially those at high risk for severe flu or COVID illness—to be prepared with at-home tests and knowledge of how to get prescription antivirals from your local pharmacy. “The earlier you take those drugs, the more likely you'll have a positive impact in terms of shortening the disease and stopping its progression to a serious nature,” he says.

Measles

As of the year 2000, measles was considered eliminated in the U.S., thanks primarily to high vaccination rates. Unfortunately, this year measles has resurged, starting in January with an outbreak in the Southwestern U.S. and surpassing 1999’s case count as of July. The last time the U.S. recorded more than 2,000 cases of measles was in 1992.

“It's very important that we track measles because it can have serious outcomes, including hospitalization and, more rarely, death, and we have safe and effective vaccine,” says Moss. Because measles is so contagious, outbreaks often signal weakening immunization systems, he adds.

Where is measles currently spreading in the U.S.?

Heading into winter, an outbreak in South Carolina continues to grow—more than 130 cases have been reported as of December 14, and more are expected, says Moss. In the past two weeks, Arizona, Utah, and New York have also added at least ten new measles cases each to their states’ 2025 totals: 190, 125, and 45, respectively, as of December 16.

Who has been most affected in these measles outbreaks?

According to CDC data, 11% of measles cases in the U.S. this year have resulted in hospitalization, and three people have died. The vast majority (93%) of confirmed cases have been in people who are unvaccinated or whose vaccination status is unknown. The same data show that about a quarter of cases (26%) have occurred in children under five, and about a third (32%) have been in adults 20 years and older.

Learn more about measles, how it spreads, and why vaccination is essential

“This reflects the fact that we've had this longstanding under-vaccination in some communities here in the United States where these outbreaks are occurring,” Moss says.

Should adults get their measles antibody levels tested to make sure they’re still protected?

Most people who have the recommended two doses of measles vaccine do not need to get their antibody levels checked, says Moss. This test, called a measles titer test, is typically only recommended in specific circumstances, including for pregnant women, health care providers, and in some health care settings.

Individuals who are uncertain of their measles vaccination status should get vaccinated, rather than have their antibody levels tested, Moss says. “This is what the CDC currently recommends across a number of vaccine-preventable diseases.” People who have been fully vaccinated against measles do not need to be revaccinated, per the CDC.