Novavax said Thursday that its vaccine was found to have been 95.6% effective against the original novel coronavirus, and 85.6% effective against the variant first identified in the UK, known as B.1.1.7, based on results from a Phase 3 trial conducted in the UK.

The study included efficacy estimates by strain based on PCR tests performed on variants from 56 Covid-19 cases in the trial.

But the vaccine appeared to be less effective against a variant first identified in South Africa. The shot showed 60% efficacy among those without HIV in a separate Phase 2b study conducted in that country. The efficacy dropped to 49.4% when HIV-positive people were included.

Novavax said it was planning to start development of a vaccine specifically targeting the variant identified in South Africa. The company also said it had already started work on a new version of the vaccine that could be used as a booster shot against emerging strains. It aims to start clinical testing of this in the second quarter of this year.

The company's vaccine, known as NVX-CoV2373, "is the first vaccine to demonstrate not only high clinical efficacy against COVID-19 but also significant clinical efficacy against both the rapidly emerging UK and South Africa variants," Stanley Erck, Novavax president and CEO, said in the announcement. "NVX-CoV2373 has the potential to play an important role in solving this global public health crisis."