LONDON, U.K.—All the usual caveats apply: don’t go out and celebrate, don’t let your guard down, it’s still going to be a long haul. This winter will be “hard,” warned Uğur Şahin, co-founder and CEO of BioNTech, the German company that announced the first effective COVID-19 vaccine recently. It can’t be rolled out fast enough to reduce infections much in the current wave, he said.