The head of Britain’s vaccination deployment effort says the world faces around 4,000 variants of the virus that causes COVID-19. As a result of new mutated versions, vaccine manufacturers are looking for ways to improve the shots to resist the virus variants, Vaccine Deployment Minister Nadhim Zahawi said Thursday. British researchers plan to test a mix of two vaccines, the Pfizer-BioNTech and Oxford-AstraZeneca injections, to see if the two together can create stronger immunity. Experts say thousands of individual changes arise as the virus mutates and develops into new variants over time. However, only a small number of mutations are likely to change the virus in an important way, the British Medical Journal reports.