Cases of influenza in the United States are rising, driven by a new strain that public health officials worry current vaccines may not protect against as effectively.

Health officials and researchers say that although the flu season has not reached its peak, the spike in cases is not historically unusual - and they stress vaccines probably still offer protection against the worst effects of the strain.

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The number of hospitalizations, emergency room and outpatient visits, and deaths associated with the flu have shot up, according to the Centers for Disease Control and Prevention, although the U.S. isn’t likely to reach peak until early in the year, possibly February.

“It seems to be a bit of a swift increase, but it’s not atypical,” said Jennifer Nuzzo, an epidemiologist and director of the Pandemic Center at the Brown University School of Public Health. “We often see flu seasons come in sort of fast and furious.”

Winter creates ideal conditions for airborne viruses such as flu, covid and RSV to circulate as people gather indoors for long stretches and travel for the holidays. The Northeast is already feeling the surge. On the day after Christmas, New York health officials said the state experienced the highest number of flu cases in a single week since they began tracking the data in 2004.

Since the coronavirus pandemic began in the United States in 2020, winter virus seasons have been marked by waves of covid infections. But covid in more recent years has become less prevalent during the winter months, researchers say, even if comparing seasons isn’t perfect because fewer patients, doctors and hospitals test for the coronavirus than in years past.

The coronavirus does not follow a seasonal pattern, but public health officials urge vaccinations and testing in advance of colder months.

Levels of coronavirus tracked in wastewater are lower than during the peak summer wave, said Marlene Wolfe, co-principal investigator for WastewaterSCAN, a private initiative that tracks municipal wastewater data.

“There’s plenty of people who are shedding covid into sewer systems when they’re sick, but we aren’t seeing really high levels,” said Wolfe, also an assistant professor of environmental health at Emory University’s Rollins School of Public Health.

For the flu, however, the trajectory of cases is expected to keep rising after a sharp uptick in recent weeks.

It remains unclear whether this winter flu season will rival last year’s, which was considered particularly severe, Nuzzo said. During last year’s flu season, the CDC reported 288 pediatric-influenza-associated deaths, the highest number since the 2009-2010 H1N1 swine flu pandemic.

She said severe flu seasons tend to oscillate, “and that’s probably due to the residual immunity from a previous year.”

The CDC estimates there have been at least 4.6 million flu-related illnesses, at least 49,000 hospitalizations and 1,900 deaths this season, which started in late September and may run through March or April.

The wrinkle is a new strain of flu. That’s not unusual. The virus constantly mutates through a process known as antigenic drift - the reason vaccine makers reformulate shots each year. The new strain is a version of what is known as H3N2, a type of influenza A, and it’s quickly become the most common one in the U.S., according to the CDC.

The strain has been tied to early and severe flu outbreaks in countries such as Japan, Canada and Britain. Alicia Budd, a CDC epidemiologist who tracks the spread of flu in the U.S., said that while the strain is now common here, flu levels in the U.S. remain within the expected range.

“We’re not seeing that increase, that early season that some other countries have seen,” Budd said.

The H3N2 strain is typically associated with more flu hospitalizations and deaths in older people. The latest strain popped up during the summer, after U.S. drugmakers finalized their flu shots for the season. Experts worry that means the vaccines could be less effective, leading to more hospitalizations and taxing health systems that must deal with other winter viruses.

In Britain, preliminary data has shown vaccines in that country provide strong protection against the new variant, although shots differ from those in the U.S., experts said.

According to CDC estimates, about 40 percent of U.S. adults have received their flu vaccine, with the number slightly lower for children.

Nuzzo, of Brown University, stressed that getting vaccinated remains important, even with the variation. “The flu vaccine may not prevent you from getting flu, but it certainly will keep you out of the hospital and may just reduce the severity of your symptoms or how long you’re sick for,” Nuzzo said.

In Jacksonville, Florida, pediatrician Pamela Lindor of Bluebird Kids Health has already seen an uptick in children who reported sudden onset of fever, headaches, fatigue and cough while at school. If they are diagnosed within a day or two, she prescribes them antiviral medication.

Federal health officials under President Donald Trump are casting doubt on childhood vaccine recommendations, suggesting kids receive far too many shots compared with other countries. In early December, Trump ordered a review of the vaccine schedule, questioning the practice of giving children annual flu shots.

For years, Lindor said, parents she has encountered have resisted yearly flu shots for their children, even if they agree with other vaccines required by schools.

“It’s frustrating, in a way, because even though we do see quite a bit of flu this time of year, it could be a lot less if people were more compliant with the flu vaccine,” Lindor said.

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