Durham, NC—With influenza season beginning in the next few months, pharmacists can expect to be questioned about the adjuvanted inactivated influenza vaccine (aIIV) and the high-dose inactivated influenza vaccine (HD-IIV). Older adults will ask which is better.

A new study in Immunity and Ageing provided an answer: Although there are some differences in effectiveness, they are not that significant.

Both are FDA-approved for adults aged older than 65 years. Duke University School of Medicine–led researchers compared serum hemagglutination inhibition antibody titers for the A(H3N2) and A(H1N1)pdm09 and B strains after trivalent aIIV3 and trivalent HD-IIV3 in an older adult population.

Overall, 342 participants received aIIV3 and 338 participants received HD-IIV3. “The proportion of participants that seroconverted to A(H3N2) vaccine strains after allV3 (112 participants [32.8%]) was inferior to the proportion of participants that seroconverted after HD-IIV3 (130 participants [38.5%]) at day 29 after vaccination (difference, –5.8%; 95%CI, –12.9% to 1.4%),” according to the researchers.

No meaningful differences between the vaccine groups in percent seroconversion to A(H1N1)pdm09 or B vaccine strains, in percent seropositivity for any of the strains or in postvaccination geometric mean HI antibody titers (GMTs) for the A(H1N1)pdm09 strain were identified, however.

The GMTs for the postvaccination A(H3N2) and B strains were higher after HD-IIV than after aIIV3, the authors pointed out, adding, “Overall immune responses were similar after aIIV3 and HD-IIV3. For the primary outcome, the aIIV3 seroconversion rate for H3N2 did not meet noninferiority criteria compared with HD-IIV3, but the HD-IIV3 seroconversion rate was not statistically superior to the aIIV3 seroconversion rate.”

The CDC makes a preferential recommendation for three flu vaccines for adults aged 65 years and older. Those include:

• Fluzone High-Dose Quadrivalent inactivated flu vaccine
• FluBlok Quadrivalent recombinant flu vaccine
• Fluad Quadrivalent adjuvanted inactivated flu vaccine.

“These vaccines are preferred for people 65 and older because a review of available studies suggests that, in this age group, these vaccines are potentially more effective than standard dose, unadjuvanted flu vaccines,” according to the CDC, which explained that Fluzone High-Dose Quadrivalent vaccine contains four times the amount of antigen as a regular flu shot; Fluad Quadrivalent vaccine is a standard-dose flu vaccine with an adjuvant added to help create a stronger immune response; and Flublok Quadrivalent vaccine is a recombinant protein flu vaccine approved for use in people aged 18 years and older, which contains three times the amount of antigen compared with a regular flu shot.

The quadrivalent formulation of cell- or recombinant-based influenza vaccines in the United States for the 2023-2024 influenza season contains the following:

• An A/Victoria/4897/2022 (H1N1)pdm09-like virus
• An A/Darwin/9/2021 (H3N2)-like virus
• A B/Austria/1359417/2021-like virus (B/Victoria lineage)
• A B/Phuket/3073/2013-like virus (B/Yamagata lineage).

The content contained in this article is for informational purposes only. The content is not intended to be a substitute for professional advice. Reliance on any information provided in this article is solely at your own risk.

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