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Novavax's two-dose COVID-19 vaccine had an overall efficacy of 90%, with 100% efficacy against moderate and severe disease, on Monday.

Data from a phase III trial in the U.S. and Mexico found overall efficacy of 90.4% (95% CI 82.9-94.6) against COVID-19, with 77 cases of laboratory-confirmed SARS-CoV-2 at least 7 days after the second dose in serologically negative adults. Sixty-three of these cases were in the placebo group and 14 in the vaccine group, and the manufacturer said all cases in the vaccine group were defined as "mild" by the trial protocol.

The placebo group had 10 moderate cases of COVID-19 and four severe cases, versus none in the vaccine group, for a vaccine efficacy of 100% (95% CI 87.0-100.0) against moderate to severe disease.

(Pre-Fusion Protein Subunit Vaccine Efficacy Novavax Trial/COVID-19) was a phase III study that enrolled 29,960 adults across 119 sites in the U.S. and Mexico from January 25 to April 30, 2021. Participants were randomized 2:1 to receive two doses of Novavax's recombinant nanoparticle protein-based COVID-19 vaccine or placebo.

The manufacturer noted that sequence data were available for 54 of 77 cases, and that the vaccine met its secondary endpoint of 100% (95% CI 80.8-100) efficacy against strains of the virus not classified as variants of concern or interest. However, only 10 sequenced cases were not variants of concern or interest. Though the release noted the Alpha variant, B.1.1.7, was the "predominant" strain in the U.S., the manufacturer did not give further details about the sequenced variants in their trial beyond "variants of concern or interest."

An exploratory analysis involving sequenced cases of variants of concern or interest -- 38 in the placebo group and six in the vaccine group -- demonstrated a vaccine efficacy of 93.2% (95% CI 83.1-97.1).

The vaccine also was effective among "high-risk" groups, defined as adults older than age 65 with "certain comorbidities" or "life circumstances with frequent COVID-19 exposure," the manufacturer said. There were 62 cases in the placebo group and 13 in the vaccine group, making vaccine efficacy in this population 91.0% (95% CI 83.6-95.0).

Regarding safety, the vaccine was well-tolerated, with serious and severe adverse events comparable between groups, and no single adverse event reported by more than 1% of participants. The most common systemic side effects were fatigue, headache, and muscle pain not lasting more than 2 days.

Novavax said it plans to submit these results to a peer-reviewed publication, and has already applied for emergency use authorization from the FDA.

A trial among adolescents ages 12 to 17 is ongoing, and recently enrolled 2,248 participants, the company noted.

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