AstraZeneca and Oxford University have more work to do to confirm whether their COVID-19 vaccine can be 90% effective, peer-reviewed data published in The Lancet showed Tuesday, potentially slowing its eventual rollout in the fight against the pandemic.

Once seen as the front-runner in the development of a vaccine against the coronavirus, the British team was overtaken by U.S. drugmaker Pfizer, whose shots — with a success rate of around 95% — were administered to British pensioners Tuesday in a world-first hailed as V-Day.

Detailed results from the AstraZeneca/Oxford trials have been eagerly awaited after some scientists criticized a lack of information in their initial announcement last month.