B117 variant of coronavirus may have begun to circulate in US in October Researchers examined 2 million positive cases from US in recent months They found more instances of virus that was missing signal from genetic code The 'S gene dropout' is similar to the mutation found in B117 variant from the UK New strain of virus was first announced by UK officials in mid-December Research shows that the mutation had been circulating in UK since September New variant is thought to be much more contagious, according to experts Experts warn it could fuel case surge in US unless more people are vaccinated