Boris Johnson said on Monday Britain was in “a race against time” to rollout COVID-19 vaccines as deaths hit record highs and hospitals run out of oxygen, with his top medical adviser saying the pandemic’s worst weeks were imminent.A new, more transmissible variant of the disease is now surging through the population, with one in 20 people in parts of London now infected, threatening to overwhelm the health service with hospitals overloaded with patients.The death toll in the United Kingdom has been soaring and now stands in excess of 81,000 — the world’s fifth-highest toll — while more than 3 million people have tested positive.