LONDON — The first studies of Britain’s mass inoculation program showed strong evidence Monday that the coronavirus vaccines were working as intended, offering among the clearest signs yet that the vaccines slash the rate of COVID-19 hospital admissions and may be reducing transmission of the virus.
A single dose of either the AstraZeneca vaccine or the one made by Pfizer could avert most coronavirus-related hospitalizations, the British studies found, though researchers said it was too early to give precise estimates of the effect.
The findings on the AstraZeneca shot, the first to emerge outside of clinical trials, represented the strongest signal yet of the effectiveness of a vaccine that much of the world is relying on to end the pandemic.
And separate studies of the Pfizer vaccine offered tantalizing new evidence that a single shot may be reducing the spread of the virus, showing that it prevents not only symptomatic cases of COVID-19 but also asymptomatic infections.
The studies released Monday showed both vaccines were effective against the more infectious coronavirus variant that has taken hold in Britain and spread around the world.
“Both of these are working spectacularly well,” said Aziz Sheikh, a professor at the University of Edinburgh who helped run a study of Scottish vaccinations.
Still, the findings contained some cautionary signs. And even as British lawmakers cited the strength of the vaccines in announcing a gradual loosening of lockdown restrictions, government scientists warned that many more people needed to be injected to prevent cases from spreading into vulnerable, vaccinated groups and occasionally causing serious disease and death.
Britain has decided to delay giving people second doses of the Pfizer and AstraZeneca vaccines until up to three months after their first doses, opting to offer more people the partial protection of a single shot.
The trade-offs involved in that strategy were not entirely clear from the evidence released Monday, but government scientists said the sharply reduced rates of hospitalization justified the strategy.
But the findings also suggested that people became better protected from the coronavirus after a second dose. And they offered mixed answers to the question of how long high protection levels from a single dose would last.