U.K. scientists worry vaccines may not protect against variant found in South Africa.

Both Britain and South Africa have detected new, more transmissible variants of the COVID-19-causing virus in recent weeks that have driven a surge in cases. British Health Secretary Matt Hancock said on Monday he was now very worried about the variant identified in South Africa. Scientists say both the variants from South Africa and the U.K. are associated with a higher viral load, meaning a greater concentration of virus particles in patients’ bodies, possibly contributing to increased transmission.

Simon Clarke, an associate professor in cellular microbiology at the University of Reading, said that while both variants had some new features in common, the one found in South Africa „has a number of additional mutations … which are concerning.” He said these included more extensive alterations to a key part of the virus known as the spike protein — which the virus uses to infect human cells — and „may make the virus less susceptible to the immune response triggered by the vaccines.”

Scientists including BioNTech CEO Ugur Sahin and John Bell, Regius Professor of Medicine at the University of Oxford, have said they are testing the vaccines against the new variants and say they could make any required tweaks in around six weeks. Public Health England said there was currently no evidence to suggest COVID-19 vaccines would not protect against the mutated virus variants. Britain’s Health Ministry did not immediately respond to requests from Reuters for comment.

There are 60 vaccine candidates in trials, including those already being rolled out from AstraZeneca and Oxford, Pfizer-BioNTech, Moderna, Russia’s Sputnik V and China’s Sinopharm. Canada has approved vaccines from Pfizer-BioNTech and Moderna for use.