Early flu peak puzzles scientists after last year's reprieve

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The flu is back with case counts that look comparable to those that predate the pandemic, with one important distinction.

Levels of influenza-like illness, used as a measure of the respiratory disease's spread, have been on a decline for three weeks since early January, according to the U.S. Centers for Disease Control and Prevention. That's far earlier than the typical peak in February or March.

"If you've seen one and another, each one is distinctive," said William Schaffner, a Vanderbilt University infectious disease expert. "And this one has caught those of us who watch flu very carefully by surprise."

Overall, for the 2021-22 season appear to be inching back toward prepandemic levels, fueled by hot spots like Missouri, New Mexico and North Dakota. After a historically low season last year, the government estimates there have been at least 2 million flu cases, 20,000 hospitalizations and about 1,200 deaths from the respiratory virus this season.

An increasingly lax approach to COVID-19 control measures like masking, social distancing and hand-washing has allowed the flu to resume its annual pattern of spread around the country, said.

Influenza-like illness—defined as a fever of over 100 degrees with a cough or sore throat— currently accounts for 2.8% of , for the third week in January, down from a high 4.8% in the last week of December. However, it's still above the national baseline of 2.5% for the season.

"It's still early and we're not out of the woods," said Kimberlee Wyche Etheridge, a senior vice president of health equity at the Association of State and Territorial Health Officials. "We have only had a bit of a reprieve."

Now, after the early bump, researchers are trying to determine which path the season will take. Flu rates could still rise again before the season ends, Schaffner said, especially as the influenza B strain generally arrives later in the year than the A strain that's now dominant in the U.S.

"It could go down and stay down, or you could have another increase with influenza B," Schaffner said.

Some states are still battling the first peak. In North Dakota, this season's infections through the third week of January have doubled compared with the same period two years ago. Mid-season data from the state's health department show more than 8,000 laboratory-confirmed cases this season alone, about two-thirds of the state's entire 2019-2020 season.

In New Mexico, reports of influenza-like illness accounted for about 6.2% of visits to hospitals and clinics for the week ending Jan. 22. That's far above the state's baseline of 3.4% for the period, as well as the average national level of 2.5% for this season.

Those rates are raising the pressure on hospitals in New Mexico, where the highly transmissible omicron variant has driven COVID-19 cases to over 480,000 overall, with a quarter of them in January alone.

"COVID clearly dominates the causes of death and number of cases in New Mexico hospitals, but we have an existing huge fire and flu adds more fuel to the fire," said Jagdish Khubchandani, a public health professor at the New Mexico State University.

New Mexico's Lovelace Health System is one of many health care organizations already overwhelmed by omicron that's now taking on flu. The network of five hospitals and 24 health clinics has also been dealing with labor shortages, as some staff have fallen ill and others have left the state, according to Chief Medical Officer Vesta Sandoval. Despite the issues, she said, the hospital system has continued to manage.

"Typically our worst flu months for our system kind of historically have always been in January and February," said Sandoval. "I do expect that we probably will still see more cases."

While flu vaccines can greatly reduce the risk of hospitalizations or deaths, immunization rates have slowed in comparison to previous seasons. Adult flu-shot coverage estimates by the CDC as of Dec. 31 remain at 40% nationally. For the 2020-21 season, flu vaccination among adults was about 50%, up about 2 percentage points from the year earlier. The CDC also reported a 10% decline in doses distributed as of Jan 31., compared with the same period last year.

Experts say vaccine fatigue may play a role in the decline of influenza shots. For many, the could represent their fourth shot within a year, after two COVID-19 immunizations and a booster. Misinformation is another possible factor.

"There's been so much misinformation about vaccines that some people aren't comfortable," said Wyche Etheridge, who sees flu patients as a physician at Meharry Medical College in Tennessee.

Flu vaccine efficacy may also be an issue. Unlike some highly effective standard childhood immunizations, flu shots often give only partial protection against the illness, leading some people to focus on their potential risks, rather than benefits.

It's still too early to say how effective vaccines are against the dominant flu strain, according to the CDC. While preliminary data suggests a reduced protection against the circulating H3N2 strain, vaccination is still likely to offer at least a partial shield, including against serious flu illness and death, Michael Jhung, an epidemiologist with CDC's Influenza Division, said in an email.

That lack of certainty is just one of many reasons this year's flu has been so difficult to analyze.

"It's very dangerous to try to predict what flu is going to do," Vanderbilt's Schaffner said.

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