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Novavax's COVID-19 vaccine candidate appeared to show comparable efficacy when co-administered with the influenza vaccine in a substudy from one of its clinical trials, researchers found.

A post hoc analysis of a small subgroup of participants, ages 18 to 64, who received both vaccines found COVID vaccine efficacy was 87.5%, albeit with a wide confidence interval (95% CI -0.2 to 98.4), while efficacy in the entire per-protocol population of that age group in the main study was 89.8% (95% CI 79.7-95.5), reported Paul Heath, PhD, of University of London, and colleagues.

While there was no change in influenza vaccination antibody response, there was a reduction in antibody responses to the COVID-19 vaccine in the substudy group, although it did not appear to be clinically meaningful, they wrote in a pre-print study on

However, local reactogenicity was either mild or largely absent among those who received both vaccines, and systemic reactogenicity events were similar across groups, they noted.

"This is the first study to demonstrate the safety, immunogenicity, and efficacy profile of a COVID-19 vaccine when co-administered with a seasonal influenza vaccine," the authors wrote.

They noted that immune interference and safety are concerns when vaccines are co-administered. Notably, the CDC Advisory Committee on Immunization Practices (ACIP) said that COVID vaccines may be co-administered, though it is unknown whether reactogenicity is increased with co-administration.

The authors reported on data from Novavax's phase III trial in the U.K. from Sept. 28 to Nov. 28, 2020, where two doses of vaccine or placebo were administered 21 days apart. Participants included 15,139 non-pregnant adults, ages 18 to 84, who were either healthy or had stable chronic medical conditions.

Around the first 400 participants were then invited to participate in a substudy with co-administration of the influenza vaccine. They were eligible if they had not already received an influenza vaccine for the 2020-2021 flu season and had no history of allergy or severe reaction to influenza vaccines.

Substudy participants were randomized 1:1 to receive either two doses of Novavax's COVID-19 vaccine or saline placebo 21 days apart, although all participants received influenza vaccine with their first dose. Those younger than age 65 received influenza quadrivalent cellular vaccine (Flucelvax), while older adults received adjuvanted trivalent vaccine (Fluad).

Serum samples were collected at baseline, at day 21 in the influenza substudy, and at day 35 for all trial participants. Molecular assays were performed at baseline and on day 35 from 900 participants in the immunogenicity cohort of the main study and those in the influenza substudy.

A 7-day electronic diary collected local and systemic adverse events from influenza substudy participants, along with 2,000 participants in the reactogenicity cohort, though substudy participants were told to record local reactogenicity for the COVID injection site only.

There were 431 participants in the substudy; 217 who received COVID vaccine and 214 who received placebo. Median age of participants was 39, about 57% were men, three-quarters were white, and 27% reported at least one comorbid condition. There were 402 participants who received the quadrivalent vaccine and 29 who received the trivalent vaccine.

The authors reported the substudy group was younger, more racially diverse, and had fewer comorbid conditions than main study participants.

Among 360 substudy participants included in the per-protocol main analysis, there was one case of symptomatic COVID-19 in the vaccine group and eight in the placebo group. The authors noted there were too few cases among adults, ages 65 and older, to calculate vaccine efficacy in the per-protocol population.

Examining safety, 60% of substudy participants reported any systemic reaction compared with 45.7% of those who only received only the COVID-19 vaccine. The most commonly reported systemic events were muscle pain (28.3%) and fatigue (21.4%) in the substudy, which occurred in a lower proportion of COVID-19 vaccine-only recipients (27.7% and 19.4%, respectively).

Less than 1% of both the substudy group and the COVID-only group reported severe adverse events. There were no reports of anaphylaxis or death within the substudy, the authors said.

Limitations to the data include the small sample size, the few participants age 65 and older, and the open-label design in administering the influenza vaccine.

The authors urged additional study of co-administering influenza and COVID-19 vaccines, especially in the older population. They indicated on their manuscript that they have submitted this data to The Lancet for peer review.

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