LONDON (Reuters) - The United Kingdom reached a bleak milestone in its battle with coronavirus on Wednesday as the official death toll passed 50,000 casting a shadow on the positive news about the effectiveness of a potential vaccine. The death toll is higher than the other worst-affected countries in Europe and the number of people killed by coronavirus is only higher in the United States, Brazil, India and Mexico. Asked about the latest milestone, Prime Minister Boris Johnson said Britain is “not out of the woods yet” even as Pfizer this week announced that early data showed its experimental COVID-19 vaccine was more than 90% effective.