Reports from Britain and South Africa of new coronavirus strains that seem to spread more easily are causing alarm, but virus experts say it’s unclear if that’s the case or whether they pose any concern for vaccines or cause more severe disease.

Viruses naturally evolve as they move through the population, some more than others. It’s one reason we need a fresh flu shot each year.

New variants, or strains, of the virus that causes COVID-19 have been seen almost since it was first detected in China nearly a year ago.

On Saturday, Prime Minister Boris Johnson announced new restrictions because of the new strain. Several European Union countries and Canada were banning or limiting some flights from the U.K. to try to limit any spread.