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  • Seasonal influenza activity remains relatively low in the U.S.
  • However, cases in some areas of the country are rising.
  • Experts aren’t sure why cases are rising so late in the season but say the end of mask mandates may play a role.

This flu season has been mild, again, despite flu experts’ warnings that influenza could potentially make a severe comeback after lying dormant for most of 2020 and 2021.

The flu has caused at least 3.5 million flu illnesses, 34,000 hospitalizations, and 2,000 deaths in the United States this season — a stark increase from last year’s record-low levels but still a much lower burden than pre-pandemic flu seasons.

Even though we’re at the tail end of a mild flu season, cases are still increasing in some parts of the country, like the northeast, northwest, and certain south-central regions.

Lynnette Brammer, Team Lead of the Domestic Influenza Surveillance Team in the Influenza Division at the Centers for Disease Control and Prevention (CDC), says we typically don’t see flu activity pick up this late in the season.

“In the last 38 seasons, flu activity has only peaked 6 times during the month of March, so it is unusual, but not unprecedented for flu activity to be elevated at this time,” Brammer told Healthline.

Despite the recent uptick in cases, flu experts expect this year’s flu season to simmer down within the next few weeks.

It’s unclear why the flu is increasing this late in the season.

Some public health experts suspect the recent relaxation of mitigation measures, like face masks, has contributed to the recent increase.

“Most of us have thought that this last end-of-season smoldering, slightly increasing number of cases here and there is probably due to people taking off their masks, going out to group activities, going indoors to restaurants, worship services, concerts, starting to go back in-person in business — all of those kinds of activities are still presenting some opportunities for this respiratory virus to be spread,” Dr. William Schaffner, the medical director of the National Foundation for Infectious Diseases, said.

Throughout the 2021-2022 flu season, most flu cases were caused by the H3N2 strain, which is typically more severe in older adults.

At this point in the season, we typically see an increase in a different influenza strain, but H3N2 has remained dominant.

Another issue is that after the flu shot was developed last summer, H3N2 has evolved and mutated, decreasing the efficacy of this year’s flu shot.

A preliminary report published by the CDC mid-March suggested that the shot reduced a person’s risk of contracting a mild case of influenza by just 16 percent.

According to Schaffner, the shot offered very little protection this year — it was off target.

Last year, infectious disease experts warned that this year’s flu season could be severe since the population essentially lost a year of immunity due to the abnormally mild flu season recorded in 2020 and 2021.

However, while more flu was going around this season than the 2020-2021 flu season, the burden of flu — transmission, hospitalizations, and deaths — has been significantly lower than in pre-pandemic flu seasons.

This season, influenza picked up between November and January — then, cases plummeted.

“That still has all of us puzzled. We anticipated that we would have a moderately severe flu season, but all of a sudden, it stopped in its tracks,” Schaffner said.

During the week ending March 26, 2022, the hospitalization rate in the CDC’s FluSurv-NET system was 7.2 per 100,000 population, which was higher than the rate for the entire 2020-2021 season, says Brammer, but lower than the hospitalization rate recorded at the same time during the four seasons preceding the COVID-19 pandemic.

The hospitalization rate ranged from 52.5 to 96.1 per 100,000 during the 2016-17 through 2019-20 seasons, says Brammer.

Preliminary flu burden estimates suggest 3.5 – 5.8 million people have contracted the flu, between 34,000 – 69,000 people were hospitalized with flu, and around 2,000 – 5,800 people have died from flu.

The CDC estimates that between 2010 and 2020, before COVID hit, the flu caused between 9 to 41 million illnesses, 140,000 to 710,000 hospitalizations, and 12,000 to 52,000 deaths each season, according to Brammer.

Schaffner expects flu activity to dissipate during April, as it typically does. But it’s difficult to predict how a flu season will play out.

“So, at this point, we’ve had two mild flu seasons. It remains to be seen what this might mean for the upcoming 2022-2023 flu season,” Brammer said.

This flu season has been mild, despite flu experts’ warnings that influenza could potentially make a severe comeback after the record-low flu activity in 2020 and 2021. Still, cases have been increasing in some parts of the country, like the northeast, northwest, and certain south-central regions. Still, experts expect this late-in-season flu activity to dissipate in late April.