A large, randomized study found no meaningful difference in protection between adjuvanted and high-dose flu vaccines for older adults during the 2023–24 flu season. The findings support the Advisory Committee on Immunization Practices (ACIP) recommendation that either formulation can be used for adults 65 years and older.
For the study, led by a team at Kaiser Permanente Vaccine Study Center and published this week in JAMA Network Open, researchers analyzed health outcomes for 429,595 adults age 65 and older who received either an adjuvanted or high-dose inactivated flu vaccine as part of routine outpatient care.
Both adjuvanted and high-dose flu vaccines boost the immune response more than standard flu shots but in different ways. Adjuvanted vaccines use an additive to create a stronger immune response, while high-dose vaccines use four times the amount of antigen than the standard shot.
1.5% relative vaccine effectiveness
The researchers found that confirmed flu cases occurred at nearly identical rates in both groups. There were 836 influenza cases (3.9 per 1,000 people) in the adjuvanted group and 867 cases (4.0 per 1,000 people) in the high-dose vaccine group.
The relative vaccine effectiveness (rVE) of the adjuvanted vaccine compared with the high-dose version was 1.5% (95% confidence interval [CI], −8.4% to 10.5%), indicating no significant difference in effectiveness but uncertainty in terms of percentage precision.
A similar pattern was seen for more-severe outcomes. The adjuvanted vaccine had a rVE of 9.1% (95% CI, −4.0% to 20.4%) against flu-related hospitalization or emergency department visits, and 1.0% (95% CI, −11.4% to 12.0%) against hospitalization for all-cause community-acquired pneumonia. Again, those differences weren’t statistically significant but also were uncertain.
Older adults hit hardest by flu
The findings align with previous research that has shown adjuvanted and high-dose vaccines offer similar protection against flu-related medical visits in older adults. An estimated 9.4 million to 41 million people get the flu in the United States each year, and older adults bear the greatest burden. Those 65 and older account for roughly 70% to 85% of flu-related deaths and 50% to 70% of hospitalizations.
Beginning in 2022–23, ACIP recommended that adults 65 and older receive either an adjuvanted inactivated flu vaccine or a higher-dose formulation. The current findings add to the body of evidence that adjuvanted and higher-dose formulations may be used interchangeably in older adults.
“These results support the equal inclusion of the adjuvanted and high-dose vaccines in the ACIP preferential recommendation for annual influenza vaccination for adults aged 65 years or older,” the authors write.
