“While seasonal variation exists, it is expected that seniors will continue to benefit from flu vaccines that are stronger than the traditional standard-dose vaccines,” wrote investigators.
High-dose (HD) and adjuvanted influenza vaccines were more effective in preventing flu symptoms and hospitalization in individuals aged 65 years and older during the 2022 and 2023 flu season compared to the standard flu vaccine, according to new research published in Clinical Infectious Diseases.1
Seasonal influenza causes up to 1 billion infections and 3 to 5 million severe illnesses annually worldwide, with the highest burden of severe illness and hospitalization among older adults and those with underlying health conditions.2
Key Takeaways
Although egg-based vaccines have been the historic immunization standard, the CDC’s Advisory Committee on Immunization Practices (ACIP) recommended in 2022 that adults aged 65 and older receive HD, adjuvanted, or recombinant vaccines due to the population’s increased risk. HD vaccines contain a dose 4 times that of standard flu vaccines, while adjuvanted vaccines include an additional ingredient to enhance immune response.
In the first real-world study to examine their effectiveness since ACIP recommendations were established, investigators conducted a retrospective cohort study comparing HD, adjuvanted, and standard-dose (SD) cell-based influenza vaccines, relative to the reference SD egg-based vaccine, in older adults.
“Because VE can vary substantially year-to-year, updated, real-world data on the utilization and effectiveness of the currently licensed vaccines, directly compared to the SD egg-based vaccine, are important considering the recent preferential recommendations, as well as to inform current vaccine recommendations and to guide vaccine development,” wrote investigators.1
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Investigators included adults aged 65 years and older who received an influenza vaccine between August and December 2022. Participants were monitored starting 2 weeks after their initial vaccination until the outcome of interest, death, disenrollment, receipt of an additional dose of influenza vaccine, or end of follow-up in May 2023, whichever came first.
The study included 495,119 adults aged 65 years and older who met the eligibility criteria. Among these, 13.9% received a SD egg-based vaccine, 74.8% received a HD vaccine, 4.7% received an adjuvanted vaccine, and 6.5% received a SD cell-based vaccine.
Primary outcomes were influenza-related medical encounters and polymerase chain reaction (PCR)-confirmed influenza-related hospitalization.
The incidence rate (IR) for influenza-related medical encounters per 1000 person-years in the SD egg-based dose group was 15.6 (95% CI: 14.4, 16.8), whereas IRs in the HD, adjuvanted, and SD cell-based groups were 14.0 (95% CI: 13.5, 14.5), 14.1 (12.2, 16.1), and 17.5 (15.7, 19.5), respectively.
The IR for PCR-confirmed hospitalization per 1000 person-years in the SD egg-based vaccine group was 1.3 (1.0, 1.7), while IRs in the HD, adjuvanted, and SD cell-based groups were 1.1 (1.0, 1.3), 0.6 (0.3, 1.2), and 1.1 (0.7, 1.6), respectively.
Adjusted comparative vaccine effectiveness (cVE) against influenza-related medical encounters in the HD and adjuvanted vaccine groups were 9.1% (0.9, 16.7) and 16.9% (1.7, 29.8), respectively, compared to the SD egg-based group, with non-statistically significant cVE in the SD cell-based vaccine group (−6.3% [−18.3, 6.9]).
Adjusted cVEs against PCR-confirmed hospitalization in the HD and adjuvanted vaccine groups were 25.1% (0.2, 43.8) and 61.6% (18.1, 82.0), respectively, compared to the SD egg-based group, while the adjusted cVE in the SD cell-based group (26.4% [−18.3, 55.7]) was not statistically significant.
Overall, as compared to SD flu vaccine, the vaccine effectiveness of HD and adjuvanted
“Our research showed that there were advantages for older people to receive high-dose or adjuvanted flu vaccines over the standard vaccine,” concluded Jennifer Ku, PhD, MPH, infectious disease epidemiologist with the Kaiser Permanente Southern California department of research and evaluation, in a news release.3 “While seasonal variation exists, it is expected that seniors will continue to benefit from flu vaccines that are stronger than the traditional standard-dose vaccines.”
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