Flu Vaccine Worked Well in Season that Faded Fast, CDC Says

February 23, 2023

NEW YORK — Early estimates suggest the flu vaccine performed well in a U.S. winter flu season that has already dissipated.

The vaccines were more than 40% effective in preventing adults from getting sick enough from the flu that they had to go to a doctor’s office, clinic or hospital, health officials said during a Centers for Disease Control and Prevention vaccines meeting Wednesday. Officials generally are pleased if a flu vaccine is 40% to 60% effective.

One reason is the vaccine was a good match against the strains that spread over the fall and winter, officials say.

But one expert at the meeting was underwhelmed and said it points out the need for better flu vaccines. “It is still disappointing” that the vaccine was a good match and yet effectiveness was still modest, said Dr. Sarah Long of Drexel University.

Annual flu vaccines are recommended for everyone 6 months and older in the U.S. About half of eligible kids and just under half of adults got flu shots in the last several months, according to CDC data. Vaccination rates were up compared with 2021-2022, but below what they were before the COVID-19 pandemic hit in 2020, said the CDC’s Brendan Flannery.

Initially, it looked like it might be a bad flu season. The virus took off in early November as COVID-19 and another respiratory virus, RSV, roiled emergency departments. Among kids, flu-related hospitalization rates in November and December were as high as any seen in recent years, Flannery said. At least 111 flu deaths have been reported in children, the most since the 199 reported in the 2019-2020 season.

The dominant flu strain was the kind typically associated with higher rates of hospitalizations and deaths, particularly among older people. In some years, the vaccines were virtually ineffective against that strain in people 65 and older. But this season’s vaccine has done unusually well, with the best results seen in at least 10 years, said Flannery, who is responsible for the CDC’s flu vaccine effectiveness data.

Flu also apparently made a very early exit, with the virus declining since the end of November. Some pockets of high flu activity have persisted this month, including in New Mexico and New York City. But for the vast majority of the country, it’s low.

It’s not clear exactly why the wave crested so early, but flu seasons have been unusually mild or otherwise strange since the COVID-19 pandemic hit in early 2020, Flannery said. CDC officials also caution that flu season might not really be over — late winter or spring second surges have occurred in the past.

The CDC uses several systems to track flu vaccines. One is a network of hospitals that offer information on how well the vaccine prevented flu-related illnesses bad enough to require admission to a hospital. Another is a collection of urgent care clinics and hospital emergency departments, which produce estimates of how well the vaccine worked against in preventing those kinds of medical visits.

Among the findings:

—The vaccine was 44% effective in preventing adult lab-confirmed flu visits to urgent care clinics and hospital emergency rooms, and 39% effective for seniors age 65 and older.

—It was 43% effective against flu-related hospitalizations of all adults, and 35% against flu hospitalizations of seniors.

—In kids, the vaccine was 68% effective in preventing illnesses severe enough to require hospitalization, and 42% effective for pediatric visits to the emergency department.



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