The influenza vaccines currently being administered in the United States are estimated by the U.S. Centers for Disease Control and Prevention (CDC) to be 42 percent effective in adults. This is an interim estimate for the 2023-2024 “flu season” published in the agency’s Morbidity and Mortality Weekly Report (MMWR) on Feb. 29, 2024.1 2 3
By “effective,” the CDC is referring to reducing the risk of influenza-related hospitalizations or “medically attended influenza virus infection, not actually preventing influenza infections—notably influenza A (H1N1) and B (Victoria lineage).1 In its report, the agency stated:
These findings indicate that the 2023–24 seasonal influenza vaccine is effective at reducing the risk of influenza-associated outpatient visits and hospitalization.1
Less Than 25 Percent of Suspected ILI Cases are Influenza
Most influenza-like-illness (ILI) which occurs during the “flu season” is not actually caused by type A or type B influenza. Studies show that less than 25 percent of suspected ILI cases turn out to be influenza after lab testing. There are many other respiratory infections that look like influenza but are caused by other types of viruses and bacteria.2.4
According to Sascha Ellington, PhD, leader of the CDC’s influenza prevention and control team, the 42 percent influenza vaccine effectiveness rate falls within the “range” that the CDC “typically” sees when the vaccine is a “good match with the [influenza] viruses that are circulating.” This season’s rate thus far lags behind the estimated 54 percent effectiveness rate for the flu shot in 2022-2023, but it is better than the 36 percent rate for 2021-2022.1 2 3 5 6
Hellbound and Down 