Some of the participants were given a placebo and some were administered two doses of the Novavax vaccine 21 days apart.

The company said the vaccine was "generally well-tolerated" and common side effects included pain at the injection site, lasting less than three days, and fatigue, headache and muscle pain, lasting less than two days.

There were 77 Covid-19 cases that emerged among participants during the study, according to Novavax.

There were 10 moderate and four severe cases in the placebo group, demonstrating a vaccine efficacy of 100% against moderate or severe disease, according to Novavax.

Most cases were in "high-risk" people, such as those older than 65, with certain comorbidities or having frequent exposure to Covid-19.