Early estimates of flu vaccine effectiveness for 2024-2025

How good has the flu vaccine been so far?

The CDC published early estimates of influenza vaccine effectiveness (VE) in their Morbidity and Mortality Weekly Report which you can find here. This post will summarize the key findings and what it means.

The CDC collected vaccine effectiveness data for the 2024-2025 influenza season and made these estimates based off of four US Vaccine Effectiveness networks from October 2024 through February 2025.

Keep in mind that vaccine effectiveness is essentially measuring how well a vaccine works in the real world. If the effectiveness is 80%, that does not mean that it only works 80% of the time. It means that in a vaccinated population, 80% fewer people will get sick when then come into contact with that particular germ.

If the uptake of vaccination in a population increases, then the effectiveness may improve because of the concept of herd immunity. This means that people are less likely to come into contact with the germ if more people are vaccinated against it. As an example, a study published in Clinical Infectious Diseases by Hughes et al demonstrated that even small increases in vaccine coverage can lead to significant reductions in influenza-associated illnesses and hospitalizations. This study reported that a 5% increase in vaccine coverage could prevent 785,000 additional illnesses and 11,000 hospitalizations during a single influenza season.

Back to the study…

For children and adolescents, the flu VE was 32-60% effective against influenza infections requiring outpatient clinic doctor visits and 63-78% effective against influenza related hospitalizations.

For adults, the flu VE was 36-54% effective against influenza infections requiring outpatient clinic doctor visits and 41-55% effective against influenza related hospitalizations.

The influenza vaccine is difficult to make in a way that matches the circulating influenza strain. That is because there is some guess work in trying to predict what future strains will circulate since the virus mutates regularly and it takes time to make vaccines. Despite this, the vaccine is clearly highly effective at reducing the risk of severe disease. It truly takes influenza from being “wild to mild.”

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