Vaccinated people aren’t the only ones who benefit from the flu shot, study finds

University of Pittsburgh School of Public Health scientists report that seasonal influenza vaccination prevented 32.9% to 41.5% of infections in a simulated population, extending protection even to some unvaccinated residents.

Influenza seasons vary widely, with U.S. symptomatic cases estimated at 9 million to more than 40 million between 2010 and 2024. Vaccination is the safest and most effective way to prevent the disease, though not everyone can or will be vaccinated in a given population.

Unvaccinated individuals receive indirect benefits from living in a highly vaccinated population as there is less risk of acquiring a pathogen from a herd of unsuitable hosts. Herd immunity is particularly important for those who cannot be vaccinated, such as infants under 6 months or those with specific medical conditions or vaccine allergies.

In the study, “Estimated Burden of Influenza and Direct and Indirect Benefits of Influenza Vaccination,” published in JAMA Network Open, researchers implemented an agent-based model to estimate the influenza case burden, both direct and indirect, averted by vaccination.

A digital cohort of 1,218,695 computer-simulated individuals called “agents” was statistically matched to the 2010 Allegheny County, Pennsylvania, census demographics. Researchers used a simulation platform called the Framework for Reconstructing Epidemiologic Dynamics (FRED). Agents in FRED models have demographic attributes, household locations, and simulated schools and workplaces.

Simulations ran from August 15, 2022, to May 31, 2023. Agents were vaccinated beginning in September at age-specific uptake rates that yielded 51% overall, with scenario analyses spanning 22% to 71%. Modelers varied vaccine effectiveness from 40% to 60% in most runs, with select runs extending 30% to 70%.

To represent how easily the virus might spread, they used a measure called the effective reproductive number (Rt). Rt value describes the average number of new infections caused by one infected person at a given time in the outbreak with a higher Rt meaning faster spread. The team tested scenarios ranging from modest transmission, with Rt of 1.33, to extreme levels above 5.0, more consistent with known pandemic conditions.

Results showed mean influenza burden averted of 32.9% in the high seasonal transmission scenario (Rt = 1.88) and 41.5% in the low transmission scenario (Rt = 1.43) when vaccine effectiveness was set at 40%. With higher effectiveness and low transmission, reductions reached 70.3%.

Direct benefit to vaccinated agents exceeded indirect benefit in every seasonal scenario. The attack-rate ratio of unvaccinated to vaccinated ranged from 1.43 to 1.73. In simulations where Rt rose above 3.92, indirect benefit essentially disappeared while vaccinated agents still experienced 52.6% to 61.0% fewer infections.

The authors conclude that influenza vaccination delivers substantial community protection during typical seasons. Only vaccinated individuals remain shielded when transmission climbs to pandemic levels.

© 2025 Science X Network

University of Pittsburgh School of Public Health scientists report that seasonal influenza vaccination prevented 32.9% to 41.5% of infections in a simulated population, extending protection even to some unvaccinated residents.

Influenza seasons vary widely, with U.S. symptomatic cases estimated at 9 million to more than 40 million between 2010 and 2024. Vaccination is the safest and most effective way to prevent the disease, though not everyone can or will be vaccinated in a given population.

Unvaccinated individuals receive indirect benefits from living in a highly vaccinated population as there is less risk of acquiring a pathogen from a herd of unsuitable hosts. Herd immunity is particularly important for those who cannot be vaccinated, such as infants under 6 months or those with specific medical conditions or vaccine allergies.

In the study, “Estimated Burden of Influenza and Direct and Indirect Benefits of Influenza Vaccination,” published in JAMA Network Open, researchers implemented an agent-based model to estimate the influenza case burden, both direct and indirect, averted by vaccination.

A digital cohort of 1,218,695 computer-simulated individuals called “agents” was statistically matched to the 2010 Allegheny County, Pennsylvania, census demographics. Researchers used a simulation platform called the Framework for Reconstructing Epidemiologic Dynamics (FRED). Agents in FRED models have demographic attributes, household locations, and simulated schools and workplaces.

Simulations ran from August 15, 2022, to May 31, 2023. Agents were vaccinated beginning in September at age-specific uptake rates that yielded 51% overall, with scenario analyses spanning 22% to 71%. Modelers varied vaccine effectiveness from 40% to 60% in most runs, with select runs extending 30% to 70%.

To represent how easily the virus might spread, they used a measure called the effective reproductive number (Rt). Rt value describes the average number of new infections caused by one infected person at a given time in the outbreak with a higher Rt meaning faster spread. The team tested scenarios ranging from modest transmission, with Rt of 1.33, to extreme levels above 5.0, more consistent with known pandemic conditions.

Results showed mean influenza burden averted of 32.9% in the high seasonal transmission scenario (Rt = 1.88) and 41.5% in the low transmission scenario (Rt = 1.43) when vaccine effectiveness was set at 40%. With higher effectiveness and low transmission, reductions reached 70.3%.

Direct benefit to vaccinated agents exceeded indirect benefit in every seasonal scenario. The attack-rate ratio of unvaccinated to vaccinated ranged from 1.43 to 1.73. In simulations where Rt rose above 3.92, indirect benefit essentially disappeared while vaccinated agents still experienced 52.6% to 61.0% fewer infections.

The authors conclude that influenza vaccination delivers substantial community protection during typical seasons. Only vaccinated individuals remain shielded when transmission climbs to pandemic levels.

© 2025 Science X Network