The Projected Impact of Immunization This Influenza Season

As the United States enters influenza season, there has been a huge push to encourage members of the public to protect themselves and their loved ones by getting vaccinated. But with last year's flu vaccination coverage estimated to be just 44.6% for adults and 55.4% among children over 6 months, questions remain about how effective vaccination efforts will be at reducing flu-related deaths and hospitalizations. The Flu Scenario Modeling Hub (SMH) convenes modeling teams to generate long-term projections under various scenarios that can guide immunization decision-making and provide situational awareness across different axes of future uncertainty. In the most recent round of projections, SMH combined projections from nine teams representing institutions across the country, with several IVAC faculty members working to coordinate the project.

Earlier this month, the Flu Scenario Modeling Hub released its first round of projections for the 2024-25 flu season, covering the time period from August 11, 2024, to June 7, 2025. These ensemble projections pool contributions from nine teams and consider six scenarios across two axes of uncertainty: vaccination coverage 20% higher, 20% lower, or similar to the 2022-23 reference season, and seasons dominated by one of two different subtypes (influenza A/H3N2 or A/H1N1). 

Although the dominant subtype is expected to drive differences in hospitalizations and deaths in the upcoming flu season, immunization can have a substantial impact. If we could achieve national vaccination coverage of approximately 58%, a 20% relative increase from the 49% coverage during the 2022-23 season, we expect a reduction in influenza-related hospitalizations of 11% or 12%, depending on the dominant subtype; that represents an estimated 25,000 to 32,000 hospitalizations averted, which would significantly ease the burden on already overloaded health systems. Conversely, a 20% drop in vaccination (approximately 39% national coverage) is projected to generate an 11–12% increase in hospitalizations. In addition to hospitalizations averted, higher vaccination coverage is projected to result in 9–14% fewer influenza-related deaths, while lower vaccination coverage is projected to result in 9–13% more deaths, depending on the scenario; this is equal to 1,400–1,900 deaths that could be averted with higher vaccination coverage.

“Despite the bad rap the influenza vaccine gets sometimes, we see with our results how impactful it is and can be,” said Dr. Shaun Truelove, an Associate Scientist at the Johns Hopkins Bloomberg School of Public Health and an IVAC faculty member. “With just small increases in vaccination coverage–only 9% more of the population–we avert thousands of hospitalizations and deaths. And this does not even account for all the milder illnesses averted, including kids home from school and missed work. Get your flu shot!”