Novavax is a protein-based COVID-19 vaccine that uses a traditional approach. This article explores Novavax’s efficacy findings from its clinical trials.
Understanding Vaccine Efficacy
Vaccine efficacy refers to the percentage reduction in disease risk among vaccinated individuals compared to unvaccinated individuals within a controlled clinical trial setting. It measures how well a vaccine prevents symptomatic illness, severe disease, or death under ideal research conditions. This metric is distinct from “vaccine effectiveness,” which pertains to real-world performance after a vaccine is deployed. Efficacy is calculated by comparing disease cases in the vaccinated group to cases in the placebo group during a clinical trial. If the vaccine group has significantly fewer cases, this difference is expressed as a percentage.
Novavax Efficacy Findings
Novavax’s primary efficacy results come from pivotal Phase 3 clinical trials in adult populations. In a trial with approximately 29,960 participants across the United States and Mexico, the vaccine demonstrated an overall efficacy of 90.4% against symptomatic COVID-19. This study, known as PREVENT-19, collected data when certain variants were circulating.
The vaccine also showed 100% efficacy in preventing moderate-to-severe disease and hospitalizations in these trials. All cases observed in the vaccine group were mild, while the placebo group experienced moderate and severe cases. A separate Phase 3 trial in the United Kingdom, involving 15,000 subjects, reported an efficacy of 89.3% against symptomatic COVID-19. These initial trials were conducted against original SARS-CoV-2 strains and prevalent variants like Alpha.
Efficacy Against Variants and Real-World Effectiveness
Novavax’s efficacy against specific SARS-CoV-2 variants has been assessed in subsequent studies. The vaccine demonstrated high efficacy against the Alpha (B.1.1.7) variant. In a pediatric expansion of the PREVENT-19 trial, it showed approximately 82% efficacy against the Delta variant in adolescents aged 12 to 17 years when Delta was the predominant strain.
While initial studies were conducted before widespread Omicron circulation, more recent data suggest Novavax generates broad immune responses against Omicron sub-variants. A two-dose regimen showed reduced neutralizing antibody titers against Omicron BA.1 and BA.4/BA.5 compared to the ancestral variant. A third dose significantly increased these antibodies, with responses comparable to some mRNA vaccines.
An Italian study during the 2022 Omicron wave estimated Novavax’s effectiveness against symptomatic infection at around 50% for fully vaccinated individuals. Protection against any notified infection decreased over four months, from 41% to 28%, but protection against symptomatic COVID-19 remained stable. Preliminary real-world results from a 2024-2025 study comparing Novavax’s JN.1-adapted vaccine to an mRNA vaccine showed fewer and less severe reported symptoms for Novavax recipients.