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The adjuvanted trivalent flu vaccine (Fluad) was comparable with high-dose trivalent flu vaccine at preventing influenza-related medical visits among older adults at high risk of influenza complications in the last two flu seasons, a researcher said.

Among adults, ages 65 and older, with at least one high risk medical condition, such as a respiratory or cardiovascular condition, there was no difference in adjusted relative vaccine effectiveness between the M59-adjuvanted trivalent flu vaccine and the high-dose trivalent flu vaccine during the 2017-2018 and 2018-2019 flu seasons, reported Constantina Boikos, PhD, of Seqirus in Kirkland, Quebec, Canada.

However, there was a significant difference in effectiveness between the adjuvanted trivalent flu vaccine and standard dose egg-derived quadrivalent influenza vaccines throughout the study period in this population.

In a late-breaking presentation at the virtual IDWeek, Boikos noted this population is at increased risk of influenza-related morbidity and mortality, but that standard, egg-derived vaccines have proven to be less effective in these high-risk adults, due to immunosenescence, or the gradual deterioration of the immune system caused by natural age.

"Multiple studies have evaluated vaccine effectiveness in the general population, but there remains a paucity of data for" the adjuvanted trivalent flu vaccine "for individuals at high risk of influenza with high-risk medical conditions in the U.S."

The CDC noted that to date, there have been comparing adjuvanted trivalent flu vaccine with high-dose flu vaccine.

The authors examined data from a retrospective cohort study looking at de-identified patient-level electronic health records linked with pharmacy and medical claims to determine exposure. Individuals were vaccinated from August 1-February 28, and assessed for the primary outcome up through May 19 in the 2017-2018 and 2018-2019 flu seasons. Influenza-related medical encounters were the primary outcome, as defined by ICD-10 codes. Estimates were not pooled across flu seasons, Boikos added.

Patients had to have at least one medical condition, though diabetes, respiratory conditions, cardiovascular conditions, cerebrovascular conditions, and cancer were the most common. Mean age was 75-76, about 55%-56% were women, and 59%-67% were white. Boikos noted that demographics were comparable between groups and flu seasons.

Comparing adjuvanted trivalent flu vaccine with high-dose trivalent flu vaccine, point estimates for 2017-2018 and 2018-2019, there was a non-significant difference between groups (-0.8 and 2.7 point estimates, respectively), and this was generally consistent across medical groups.

Point estimates were much larger in favor of adjuvanted trivalent flu vaccine versus standard quadrivalent vaccine in both years (7.1 in 2017-2018 and 20.4 in 2018-2019). In 2017-2018, Boikos noted the difference was likely driven by subcohorts of medical conditions with large sample sizes, such as diabetes.

She added the more pronounced difference between the two groups in 2018-2019 was "likely related to different circulation of influenza subtypes" in each season. The U.S. also had a high severity flu season in 2017-2018.

Boikos said these findings support the use of adjuvanted trivalent flu vaccine as an enhanced vaccine in this population "known to be at high risk from influenza and its complications that is often prioritized for immunization," she noted.

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