Low vaccine effectiveness
As of Jan. 22, results from the U.S. Flu VE network do not show statistically significant evidence that the flu vaccine is effective. Currently, the vaccine is estimated to be 8% effective against preventing influenza A infection (95% confidence interval, –31% to 36%) and 14% effective against preventing A/H3N2 infection (95% CI, –28% to 43%) for people aged 6 months and older.
The network did not have enough data to provide age-specific VE estimates or estimates of effectiveness against influenza B. This could be due to low flu activity relative to prepandemic years, Dr. Flannery said. Of the 2,758 individuals enrolled in the VE flu network this season, just 147 (5%) tested positive for the flu this season. This is the lowest positivity rate observed in the Flu VE network participants with respiratory illness over the past 10 flu seasons, Dr. Grohskopf noted. In comparison, estimates from the 2019 to 2020 season included 4,112 individuals, and 1,060 tested positive for flu.
“We are really at the bare minimum of what we can use for a flu vaccine effectiveness estimate,” Dr. Flannery said about the more recent data. The network was not able to produce any estimates about flu vaccine effectiveness for the 2020-2021 season because of historically low flu activity.
The Department of Defense also presented vaccine efficacy estimates for the 2021–2022 season. The vaccine has been 36% effective (95% CI, 28%-44%) against all strains of the virus, 33% effective against influenza A (95% CI, 24%-41%), 32% effective against A/H3N2 (95% CI, 3%-53%), and 59% effective against influenza B (95% CI, 42%-71%). These results are from a young, healthy adult population, Lieutenant Commander Courtney Gustin, DrPH, MSN, told the panel, and they may not be reflective of efficacy rates across all age groups.
Though these findings suggest there is low to no measurable benefit against influenza A, Dr. Flannery said the CDC still recommends getting the flu vaccine, as it can be protective against other circulating flu strains. “We have been able to demonstrate protection against other H3 [viruses], B viruses, and H1 viruses in the past,” he said. And as these results only show protection against mild disease, “there is still possibility that there’s benefit against more severe disease,” he added. Studies measuring effectiveness against more severe outcomes are not yet available.
A version of this article first appeared on Medscape.com.