World faces around 4,000 Covid-19 mutated viruses that spread swiftly

The world faces around 4,000 variants of the virus that causes Covid-19, prompting a race to improve vaccines, Britain said on Thursday, as researchers began to explore mixing doses of the Pfizer and AstraZeneca shots.

Thousands of variants have been documented as the virus mutates, including the so-called British, South African and Brazilian variants, which appear to spread more swiftly than others...