LONDON — The first studies of Britain’s mass inoculation program showed strong evidence on 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 findings reinforced and went beyond studies out of Israel, which has also reported that the vaccine developed by Pfizer and BioNTech offered significant protection from the virus in real-world settings, and not only in the clinical trials held last year. No other large nation is inoculating people as quickly as Britain, and it was the first country in the world to authorize and begin using both the Pfizer shot and the one developed by AstraZeneca and the University of Oxford.
The studies released on Monday — two on the Pfizer shot and one on it and the AstraZeneca injection — 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 on 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.
“We now need to understand how long lasting this protection is for one dose of the vaccine,” said Arne Akbar, a professor at University College London and the president of the British Society for Immunology.
One of the new studies looked at about 19,000 health workers in England who had received the Pfizer vaccine. Scientists were able to keep an uncommonly close watch on whether or not the subjects had been infected: They were tested regularly for the virus, whether or not they showed symptoms, allowing the scientists to detect asymptomatic cases.
Many of the clinical trials, by contrast, measured only symptomatic infections.
That study showed that a single dose of the Pfizer vaccine reduced the risk of becoming infected by approximately 70 percent. After two doses of the vaccine, protection rose to 85 percent, scientists said, though they cautioned that the low numbers of cases made it difficult to reach precise estimates.
The Pfizer vaccine also appeared to be effective in older people, who were not as well represented in clinical trials and do not always mount strong responses to vaccines. In people over 80 in England, a separate study showed that a single dose was 57 percent effective in preventing symptomatic Covid-19 cases. Protection rose to 88 percent after a second dose.
Older people who had been vaccinated and still became ill had substantially lower odds than unvaccinated people of being hospitalized or dying, suggesting that the Pfizer vaccine blunted the impact of infections even when it did not entirely stop them.
Still, some vaccinated people were hospitalized or killed by the virus, a reminder that “protection is not complete,” said Dr. Mary Ramsay, the head of immunization at Public Health England.
A study conducted in Scotland encompassed both the Pfizer and AstraZeneca injections. The results on AstraZeneca’s vaccine were more limited because it was authorized later in Britain, only coming into use in early January.
Researchers there examined about 8,000 coronavirus-related hospital admissions, and studied how the risk of hospitalization differed among people who had and had not received a shot.
The numbers of vaccinated people who sought care in hospitals were so small, the researchers said, that they could only produce very rough estimates of the vaccines’ effectiveness, and could not compare the shots to each other.
But from 28 to 34 days after the first shot, when it appeared to be at or near peak effectiveness, the AstraZeneca vaccine reduced the risk of Covid-19 hospital admissions by roughly 94 percent. In that same time period, the Pfizer vaccine reduced the risk of hospitalizations by roughly 85 percent, though in both cases, the numbers were too small to be confident of the exact effect.
The findings were a reassuring sign about the effectiveness of the AstraZeneca vaccine, which is the backbone of many nations’ inoculation plans. It is far cheaper to produce and unlike the Pfizer vaccine — and one from Moderna, which is not yet in use in Britain — it can be shipped and stored in normal refrigerators.
But the British studies could not address how long high protection levels from a single dose of either the Pfizer or AstraZeneca vaccine would last.
In the Scottish study, the drop in the risk of people being hospitalized started a week after they received their first shot, and reached a low point four to five weeks after they were vaccinated. But then it appeared to rise again.
“The peak protection is at four weeks, and then it starts to drop away,” said Simon Clarke, a professor in cellular microbiology at the University of Reading, who was not involved in the study.
In England, there was no evidence that protection levels fell after a month. Scientists said that more evidence was needed to establish definitively whether the protection offered by a single dose was likely to wane, and how quickly.
The AstraZeneca vaccine has faced skepticism in parts of Europe; many countries chose not to give it to older people, citing a lack of clinical trial data in that group.
The Scottish study could not offer precise figures on the vaccine’s effectiveness in older people. But the inoculation program there substantially reduced hospital admissions in people over 80, and many older people were given the AstraZeneca vaccine.