| The U.K. now has a second Covid-19 vaccine to tackle a worsening outbreak of the pandemic, after regulators cleared a shot from AstraZeneca Plc and the University of Oxford. With hundreds of thousands of immunizations already in progress, using a vaccine from Pfizer Inc. and BioNTech SE, Britain is building on its status as a trailblazer in the global fight against the virus. Yet the way in which it's doing so is also raising questions. On the Astra-Oxford vaccine, the U.K. is moving well ahead of other western countries, including the U.S. and the European Union. It's given the green light based on clinical trials that involved a relatively small number of participants and that were marked by a dosing error. The mistake turned out to be fortuitous, turning up a regimen that showed better results than the prescribed dosage. In approving the vaccine, however, U.K. regulators opted for the conventional formula of two full shots, rather than the accidental version -- a half shot followed by a full one later. Instead, they added a new twist, spreading the interval between the two injections to as much as 12 weeks. This is an effort to get as many initial shots into vulnerable people's arms as quickly as possible, while supplies are still limited. The vaccine shows substantial effectiveness after a single dose, officials said, so it's better to maximize the breadth of coverage. With new Covid infections running at record levels and many U.K. hospitals swamped with patients, there's no time to lose. Still, the vaccine rollout is a delicate balancing act, pitting the need for speed against the imperative of upholding confidence. The changing narrative around the Astra-Oxford shot could test that equilibrium. U.S. regulators are waiting for more data before assessing the vaccine, which has some advantages over other coronavirus vaccines that have been approved: It's cheaper and it doesn't require deep-freezing.
Those attributes will make it easier to roll out, not just in the U.K. but potentially to developing countries that have signed deals for hundreds of millions of doses. Britain will be a leader in vaccinating its citizens, but the stakes are high if its home-grown shot stumbles.--Eric Pfanner | 
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