By Jenny Strasburg and Joseph Walker

LONDON -- Peer-reviewed data from late-stage human trials of a Covid-19 vaccine developed by the University of Oxford and AstraZeneca PLC reaffirmed the shot's strong safety results and provided some additional evidence that halving the first of two doses of the shot boosts its effectiveness.

AstraZeneca and Oxford said the data gave them confidence to ask the U.K. and other countries for emergency-use authorization of the shot, but said regulators will have to decide which dosing regimen to approve. Last month, AstraZeneca and Oxford said trial data showed the vaccine was between 62% and 90% effective, but that the higher results were observed in a small subset of the wider trial.

This subset received a half-sized initial dose followed by a full second dose. The group was also all under 55 years old, raising questions about whether the lower dosing would also be effective in older subjects.

AstraZeneca and Oxford on Tuesday declined to say whether they would ask regulators to approve the lower dose regimen, leaving unclear whether any initial vaccine approval and rollout would be limited to the less-effective regimen. The 62% effectiveness shown in the majority of the trial participants is still better than what health experts said was necessary to protect people against severe Covid-19.

The publication of the data -- the first full peer-reviewed results from any of the late-stage human trials disclosed so far by the West's three leading vaccine makers -- comes on the day that a vaccine developed by Pfizer Inc. and Germany's BioNTech SE was being administered for the first time outside trials, in the U.K. The country authorized the vaccine for emergency use last week.

It also coincided with a preliminary assessment by the Food and Drug Administration on Tuesday showing the agency was satisfied with the Pfizer shot's effectiveness and safety, paving the way for a potential emergency use authorization in the U.S. in coming days.

The FDA is also reviewing Moderna Inc.'s vaccine, with a verdict expected shortly.

While those two vaccines have raced ahead of AstraZeneca's in the U.S. regulatory process, much of the rest of the world is betting on it. Oxford and AstraZeneca promised to provide many more doses across a wider geographic footprint then either Pfizer or Moderna.

The partners have also promised to provide the shot at cost, making it the vaccine of choice for much of the developing world.

Results from an ongoing U.S. study will likely be needed before a potential approval in the U.S., said Menelas Pangalos, AstraZeneca's head of biopharmaceutical research, citing conversation with regulators at the FDA.

Write to Jenny Strasburg at jenny.strasburg@wsj.com and Joseph Walker at joseph.walker@wsj.com

(END) Dow Jones Newswires

12-08-20 1143ET