Laboratory experiments indicate that the shots are about as effective against the Brazil variant as another that surfaced in the U.K., which has already been shown to be halted by existing vaccines, according to data released Thursday by the University of Oxford. Both inoculations fared a lot less well against another mutant that surfaced in South Africa.

Viruses constantly mutate, and researchers are studying new versions to see what impact they’ll have on global efforts to immunize populations and restore economies, schooling and societies. Companies are already developing booster shots that specifically target the mutant coronaviruses.

The U.K. and Brazil variants rendered the vaccines about three times less effective than against earlier strains, the scientists found. Against the South Africa variant, the researchers found a nine-fold reduction in the level of neutralization by the Astra vaccine and a 7.6-fold reduction for the Pfizer shot.