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A high-dose recombinant influenza vaccine (Flublok Quadrivalent) was more protective than an egg-based standard-dose influenza vaccine in adults, according to results of a cluster-randomized, .

Among adults ages 50 to 64, the high-dose vaccine was 15.3% more effective in preventing influenza than the standard-dose vaccine (95% CI 5.9-23.8, P=0.002) and 15.7% more effective against influenza A (95% CI 6.0-24.5, P=0.002), reported Nicola Klein, MD, PhD, from the Kaiser Permanente Vaccine Study Center in Oakland, California, and colleagues in the .

Although the relative benefit of the high-dose vaccine appears to be modest, "reducing breakthrough influenza cases by 15% would provide a substantial public health benefit, especially during more severe influenza seasons," Klein told MedPage Today in an email.

Georges Benjamin, MD, executive director of the American Public Health Association in Washington, D.C., agreed. "The high-dose vaccine increased protection by about 15% overall," he told MedPage Today in an interview. "When you think about the fact that we don't usually get more than 40% to 50%, maybe 60% efficacy in a good year to begin with, that's a nice boost. The study authors convinced me, that for this age range, it does provide better protection than the standard dose," he said.

Researchers did not find that the recombinant vaccine was significantly more protective than the standard-dose vaccine against influenza-related hospitalization. However, "a post hoc analysis combining hospitalization for PCR-confirmed influenza and hospitalization for community-acquired pneumonia yielded a relative vaccine effectiveness of 19.7% (95% CI 2.8-33.7)," the authors wrote.

The Advisory Committee on Immunization Practices (ACIP) high-dose influenza vaccines for adults ages 65 and older. However, for people younger than 65, ACIP does not preferentially recommend any age-appropriate influenza vaccine over another. Recommended options for this age group include inactivated influenza vaccine, recombinant influenza vaccine, or live attenuated influenza vaccine.

The high-dose recombinant influenza vaccine evaluated in this trial is , and is three times the dose of standard-dose influenza vaccines.

Participants included all members of a large U.S.-based integrated healthcare delivery system over seven geographic regions. The study population included 1,630,328 people from the ages of 18 and 64 who were vaccinated for influenza in the healthcare system -- 632,962 received the high-dose recombinant vaccine and 997,366 received the standard-dose vaccine.

Although the study was observational, it had a unique cluster-randomized design "intended to emulate a randomized trial," the authors wrote. Facilities in each region were assigned to Block A or Block B to even out differences in facility size. To achieve balance between the two vaccine groups, facilities alternated on a weekly basis between administering the high-dose recombinant vaccine and the standard dose vaccine. Patients were unaware of which vaccine they received.

During the study period of the 2018-2019 and 2019-2020 influenza seasons, 1,386 cases of PCR-confirmed influenza were diagnosed in the recombinant-vaccine group and 2,435 cases in the standard-dose group. Among those who were ages 50 to 64, 559 participants (2 cases per 1000) tested positive for influenza in the recombinant-vaccine group and 925 participants (2.34 cases per 1000) tested positive in the standard-dose group.

The study data were limited to two influenza seasons and relative vaccine effectiveness may vary across seasons, the researchers acknowledged. In addition, participants included in the study might not be representative of other U.S. populations.

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