BRUSSELS—Demand for the COVID-19 vaccine produced by U.S. biotech firm Novavax has been underwhelming in the European Union’s main countries in the early rollout, so far undermining hopes that it could convince vaccine sceptics to get a shot. Over 85% of adults in the 27-country EU bloc have received at least one dose and nearly two-thirds of them have also had a booster, but tens of millions remain unvaccinated. The Novavax vaccine, the latest to receive the EU regulators’ approval under the trade name of Nuvaxovid, was expected to persuade some sceptics because it is based on a more conventional technology than the other four vaccines authorised so far in the EU. But initial data on Nuvaxovid’s rollout show that it has had a small impact on vaccination campaigns. In Germany, the EU’s most populous country and home to more than 10 million unvaccinated adults, only about 38,000 Novavax doses …