Early data show this season’s flu vaccine has not been effective
Click Here to Manage Email Alerts
Interim estimates published Thursday in MMWR indicate that this season’s influenza vaccine has not been effective.
Based on data from more than 3,600 children and adults, CDC epidemiologist Jessie R. Chung, MPH, and colleagues estimated that the vaccine has been 16% effective against mild or moderate influenza caused by the predominant circulating virus, influenza A(H3N2), with a 95% confidence interval (–16% to 39%) that suggests vaccination “did not significantly reduce the risk of outpatient medically attended illness” caused by H3N2.
Studies to determine the how well the vaccine has prevented more severe outcomes are ongoing, Chung and colleagues said.
It was the first time the CDC has been able to calculate influenza vaccine effectiveness since 2019-2020. Unusually low influenza activity last season — a result of measures taken to fight COVID-19 — meant there were not enough available data to make an estimate.
Influenza activity this season has also been relatively low. It peaked during the last week of 2021 and has remained below the national baseline for the last 5 weeks, according to the CDC’s FluView.
The new report included data from 3,636 children and adults with acute respiratory infections who were enrolled at seven sites in the U.S. Influenza Vaccine Effectiveness Network. Chung and colleagues said “enrollment was insufficient to generate reliable” estimates of vaccine effectiveness by age group or type of vaccine, but noted that the overall estimate was “consistent” with estimates from previous seasons dominated by H3N2 viruses, which are known to evade vaccines and cause relatively severe illness.
They noted that vaccine coverage has been lower this season in some high-risk groups, including pregnant women, infants and preschool-aged children, but also that the rates of influenza-related hospitalizations have been “substantially lower” than in other H3N2-predominant seasons.
Among the participants in the study, 194 (5%) received a positive test for an influenza A virus and none for an influenza B virus. Among the 178 influenza A viruses that were subtyped, only one came back as H1N1. The other 177 were H3N2. Eleven people in the study tested positive for both influenza A and SARS-CoV-2 viruses.
The percentage of patients with an acute respiratory infection who received an influenza vaccine ranged from 31% to 64%. Among participants with a positive influenza test, 41% were vaccinated compared with 50% of those who tested negative.
Chung and colleagues said the estimates “underscore the need for ongoing diagnostic testing for influenza, influenza antiviral treatment and prophylaxis when indicated, and everyday preventive measures” and that the CDC continues to recommend influenza vaccines even when they are less effective “because a growing body of evidence suggests that influenza vaccination can avert serious outcomes, including hospitalization, ICU admission, and death, among persons who are vaccinated but still become infected.”
“In addition, vaccination is likely to prevent illness or serious complications of infection with other influenza viruses that might circulate later in the season, including influenza A(H1N1)pdm09 and B viruses,” they wrote.
[Editor’s note: This story was updated to include information about the vaccine’s effectiveness against more severe outcomes and to better characterize the level of influenza this season.]
Collapse
Read more about
Click Here to Manage Email Alerts