Vaccination confers direct, indirect benefits
For the first study, published
last week in JAMA Network Open, University of Pittsburgh
researchers simulated different levels of flu virus transmissibility, vaccine
effectiveness (VE), and vaccine uptake among the 1.2 million residents of
Allegheny County, Pennsylvania, in 2010 to estimate the burden of flu and direct
and indirect benefits of vaccination under different conditions.
The team noted that case numbers vary widely even within an
influenza subtype in different seasons, likely due to factors such as immunity
from previous infections, antigenic drift, and vaccine protection in the
current season, which depends on match to circulating strains and vaccine
uptake.
The investigators ran agent-based simulations using the Susceptible, Exposed, Infectious, and Recovered model and overall vaccine uptakes of 22% to 71% from August 2022 to May 2023. The median participant age was 40.6 years, and 51% were female.
"Vaccination also provides indirect benefit,
effectively shielding unvaccinated portions of the population from infection by
decreasing the number of exposures unvaccinated individuals receive," the
study authors wrote. "Both direct and indirect benefits decrease the
number of infections required to reduce the effective reproduction rate [number
of people an infected person will transmit the virus to] to less than 1 and
halt transmission."
The flu burden averted by vaccination varied based on strain transmissibility and VE. Modeling based on a VE of 40% suggested that vaccination prevented 32.9% to 41.5% of symptomatic and asymptomatic flu infections.
While the benefit to vaccinees was always greater,
immunization also indirectly benefitted the unvaccinated. But at the highest
transmissibility levels, like that may be seen in pandemic flu, the indirect
benefit declined and eventually disappeared.
The reduction in cases was marginal in low vaccine–uptake
scenarios in which VE was low, even when transmission was low overall. When VE
was higher (60%), cases could be almost eliminated when only about 56% of people were vaccinated in the
lowest-transmission scenarios.
When spread was greater, increased vaccine uptake resulted
in less case reduction. Vaccine coverage of about 61% when the reproduction rate without
vaccination was about 1.98 still halved cases when VE was 60%.
"These findings suggest that influenza vaccination
benefits both the vaccinated and unvaccinated segments of the population and
that vaccine effectiveness estimates in those groups may underestimate the
actual effectiveness of vaccination," the authors wrote.
"Even when both vaccine effectiveness and vaccine
uptake were low, vaccination showed marked reductions in disease burden for
transmission levels characteristic of seasonal influenza," they added.
"However, when the level of transmission was very high, even a highly
effective vaccine did not protect unvaccinated individuals. These findings
underscore the importance of vaccination in disease prevention and control and
show that indirect benefits are limited in high transmission situations."
Uptake highest in older adults, although lower than
before
The second study, published
last week in PLOS Global Health, consisted of two cross-sectional
surveys that evaluated flu vaccine coverage among US adults from May 2020 to
October 2024.
Researchers at Southwestern Medical Center in Dallas recruited a census-matched national sample to determine whether participants had received or planned to receive the flu vaccine. They found that overall flu vaccine uptake fell over the study period, with significant drops in groups who traditionally have had high coverage.
"Between 2010 and 2024, influenza is estimated to have
caused 9.3 million–41 million illnesses, 120,000–710,000 hospitalizations, and
6,300–52,000 deaths annually," in the United States, they wrote. "The
Centers for Disease Control and Prevention (CDC) recommends the influenza
vaccine for all people over the age of 6 months who do not have
contraindications."
In both 2020 and 2024, coverage was lowest among people aged
18 to 25 years (47% and 50%, respectively). In both years, uptake was highest
among people older than 55. But relative to 2022, uptake in 2024 fell among
participants who were older (11% decrease), men (13%), White (7%), non-Hispanic
(5%), and more educated (16%).
"Changes in attitudes and intentions towards
vaccination during and since the COVID-19 pandemic have been associated with
several factors, including changes in perceived risk and the rise of
vaccine-related mis- and disinformation," the researchers wrote.
"Targeted behavioral interventions can be used to shift
attitudes, intentions, and eventually, behaviors, towards health-seeking
behaviors like vaccination," they concluded. "We must target these
demographics with evidence-based behavioral interventions to improve uptake of
influenza vaccination."
