The U.S. is in the midst of its first “high severity” flu season in seven years.
Since October, there have been at least 37 million infections, 480,000 hospitalizations and 21,000 deaths, the Centers for Disease Control and Prevention said Friday. As of last week, 98 children have died from influenza, the CDC reported. In some states, flu surges have been so bad that schools and entire districts have had to close down.
The U.S. has also seen the highest rate of doctors visits for flu-like illnesses overall — which include influenza, Covid and respiratory syncytial virus (RSV) — in 15 years.
While flu activity has slowed in recent weeks, experts warn that the season isn’t over yet.
“We may have months of disease and illness in front of us,” said Dr. Susan Donelan, an infectious disease doctor and medical director of health care epidemiology at Stony Brook Medicine in New York.
Although experts don’t know exactly why the flu seems so bad this year, they say several factors may be to blame.
The circulating strains are more severe
Two types of influenza A viruses — H1N1 and H3N2 — have dominated this year’s flu season, according to the CDC. Some experts, including Donelan, say these strains tend to spread more quickly and cause more severe disease, which could explain this season’s intensity so far.
H3N2, specifically, is known for mutating more frequently than other influenza strains, which can “make it more difficult for our bodies to recognize that particular virus even if we’ve been in contact with it or immunized against it before,” said Taj Azarian, an epidemiologist and assistant professor of genomics at the University of Central Florida. “As a result, we can be more susceptible to getting sick.”
By default, those mutations could also make flu vaccines — which are formulated months before the beginning of the season they’re intended for — less effective, said Robert Bednarczyk, an epidemiologist and associate professor of global health at Emory University in Atlanta.
However, data released Thursday by the CDC suggests this season’s vaccine’s effectiveness is on par with previous years.
Early estimates from this season found that children and teens who were vaccinated and later got the flu were 63%-78% less likely to be hospitalized; vaccinated adults were 41%-55% less likely to be hospitalized because of the flu, according to the CDC report.
The vaccine is “overall a very good match” this year because it protects against all three strains that are circulating, said Dr. Inessa Gendlina, an infectious disease doctor with the Montefiore Health System in New York.
Flu vaccination rates continue to drop
Flu vaccines reduce the odds of severe illnesses, hospitalizations and deaths — yet vaccination rates have fallen since the Covid pandemic, according to the CDC. This alarming trend, experts say, could help explain why this flu season is so severe.
While vaccination rates among adults are similar compared to this time last year — around 45% — CDC data shows, the difference among children “is most striking,” Azarian said.
As of Feb. 15, about 46% of kids had received a flu shot; about 51% were vaccinated by this time last year. Overall, flu vaccination coverage dropped 8 percentage points from about 63% to 55% from the pre-pandemic 2019-2020 season to the 2023-2024 season.
“That’s a pretty significant drop,” Azarian said.
We may have lost some immunity to the flu over the years
The steps we took to prevent Covid infection, such as wearing masks and social distancing, may have weakened our immunity to the flu — another potential reason influenza is hitting us so hard this year, Gendlina said.
“We’ve been masking quite a bit in the last five years, which prevents influenza spread. So we don’t have as much immunological memory going from year to year as we’ve had in the past,” she said. “Not having that memory isn’t in our favor in terms of how this influenza season pans out.”
Now that fewer people are wearing masks, respiratory viruses like influenza are free to spread, Gendlina said.
It’s not too late to get a flu shot
Although flu activity is decreasing, experts say there’s no telling when this flu season might end or whether there will be another surge in cases. That means there’s plenty of time to get a flu shot if you haven’t yet, Donelan said.
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In fact, national wastewater samples are showing a “steady increase” in influenza B concentrations, according to Amanda Bidwell, the scientific program manager at WastewaterSCAN. It’s too early to say if we’ve hit a peak with this strain, she added.
Gendlina said the threat of a new wave is reason enough to get vaccinated if you haven’t already.
Even if you’ve already had the flu this year, it’s still a good idea to get vaccinated if you haven’t gotten the shot, Donelan said. Because there are three flu viruses spreading right now, you can still get sick with either of the other two.
Donelan said people should talk to their doctors about their personal risks of a severe infection. In some cases, she said, people might be able to get a prescription for the antiviral medication Tamiflu ahead of time so they have it if they do catch the virus. (Tamiflu must be taken within two days of feeling sick.)
In the meantime, stay home when ill, wash your hands frequently, and wear a mask if you want to protect yourself and others when in crowded or poorly ventilated indoor spaces, Azarian said.
Erika Edwards contributed.
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