LONDON (AP) — Britain has authorized a COVID-19 vaccine for use, greenlighting the first shot backed up by rigorous scientific review. The first vaccinations are expected within days — a major step toward eventually ending the pandemic. The go-ahead Wednesday for the vaccine from American drugmaker Pfizer and Germany’s BioNTech comes as the virus surges again in the United States and Europe. Officials cautioned that several tough months still lie ahead, even in Britain, given the scale of the operation needed to vaccinate large swaths of the population. Britain’s Medicines and Healthcare Products Regulatory Agency recommended the vaccine for emergency use after it reviewed data, including testing that showed it was 95% effective.