LONDON – The first studies of Britain’s mass inoculation program showed strong evidence Monday that coronavirus vaccines were working as intended, offering among the clearest signs yet that vaccines reduce the rate of hospital admissions Covid- 19 and may be reducing the transmission of the virus.
A single dose of the AstraZeneca vaccine or the one made by Pfizer could prevent most coronavirus-related hospitalizations, the British studies found, although the researchers said it was too early to give precise estimates of the effect.
The findings about the AstraZeneca injection, the first to emerge outside of clinical trials, represented the strongest signal yet of the effectiveness of a vaccine that much of the world relies on to end the pandemic.
And separate studies of the Pfizer vaccine offered tantalizing new evidence that a single injection can reduce the spread of the virus, showing that it prevents not only symptomatic cases of Covid-19, but asymptomatic infections as well.
The findings reinforced and went beyond studies conducted in Israel, which also reported that the vaccine developed by Pfizer and BioNTech offered significant protection against the virus in real-world settings, and not just in clinical trials conducted last year. No other large nation is vaccinating people as quickly as Britain, and it was the first country in the world to license and begin using both the Pfizer injection as developed by AstraZeneca and the University of Oxford.
Studies published Monday – two on the Pfizer injection and one on her and the AstraZeneca injection – showed both vaccines to be effective against the most infectious variant of coronavirus that has taken hold in Britain and spread across the world. .
“Both are working spectacularly well,” said Aziz Sheikh, a professor at the University of Edinburgh who helped conduct a study on Scottish vaccines.
Still, the findings contained some warning signs. And even when British lawmakers cited the strength of vaccines when announcing a gradual relaxation of lockdown restrictions, government scientists warned that many more people needed to be injected to prevent cases from spreading to vaccinated vulnerable groups and occasionally causing serious illness and death.
Britain has decided to delay giving people the second doses of the Pfizer and AstraZeneca vaccines until three months after the first, opting to offer more people the partial protection of a single injection.
The trade-offs involved in that strategy were not entirely clear from the evidence published Monday, but government scientists said the greatly reduced hospitalization rates justified the strategy.
But the findings also suggested that people protected themselves better from the coronavirus after a second dose. And they offered mixed answers to the question of how long the high levels of protection from a single dose would last.
“Now we need to understand how long this protection is for one dose of the vaccine,” said Arne Akbar, professor at University College London and president of the British Society for Immunology.
One of the new studies looked at some 19,000 healthcare workers in England who had received the Pfizer vaccine. Scientists were able to uncommonly monitor whether or not subjects had been infected: they were regularly tested 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 about 70 percent. After two doses of the vaccine, protection increased to 85 percent, the scientists said, although they cautioned that the low number of cases made it difficult to obtain accurate estimates.
The Pfizer vaccine also appeared to be effective in older people, who were not as well represented in clinical trials and did not always generate robust responses to vaccines. In people older than 80 in England, a separate study showed that a single dose was 57 percent effective in preventing symptomatic cases of Covid-19. Protection rose to 88 percent after a second dose.
Older people who had been vaccinated and were still getting sick had substantially lower odds than unvaccinated people of being hospitalized or dying, suggesting that the Pfizer vaccine mitigated the impact of infections even when it did not completely stop them.
Still, some vaccinated people were hospitalized or killed by the virus, a reminder that “protection is not complete,” said Dr Mary Ramsay, head of immunization at Public Health England.
A study conducted in Scotland covered both Pfizer and AstraZeneca injections. The results of the AstraZeneca vaccine were more limited because it was licensed later in Britain, and only came into use in early January.
There, the researchers examined around 8,000 coronavirus-related hospital admissions and studied how the risk of hospitalization differed between people who had received a vaccine and those who had not.
The number of vaccinated people who sought care in hospitals was so small, the researchers said, that they were only able to produce very rough estimates of the effectiveness of the vaccines and were unable to compare the injections with each other.
But from 28 to 34 days after the first injection, when it appeared to be at or near its maximum efficacy, the AstraZeneca vaccine reduced the risk of hospital admissions for Covid-19 by about 94 percent. In that same time frame, the Pfizer vaccine reduced the risk of hospitalizations by about 85 percent, although in both cases, the numbers were too small to trust the exact effect.
The findings were a reassuring signal about the effectiveness of the AstraZeneca vaccine, which is the backbone of many nations’ inoculation plans. It is much cheaper to produce and, unlike the Pfizer vaccine, and one from Moderna, which is not yet used in Britain, can be shipped and stored in normal refrigerators.
But the British studies could not address how long the high levels of protection from a single dose of the Pfizer or AstraZeneca vaccine would last.
In the Scottish study, the decrease in the risk of people being hospitalized began one week after receiving their first injection and peaked between four and five weeks after being vaccinated. But then it seemed to rise again.
“Maximum protection is at four weeks and then it starts to wane,” said Simon Clarke, a professor of cell 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. The scientists said more evidence was needed to definitively establish whether and how quickly the protection offered by a single dose was likely to decline.
The AstraZeneca vaccine has faced skepticism in parts of Europe; many countries chose not to give it to older people, citing the lack of clinical trial data in that group.
The Scottish study was unable to provide precise figures on the effectiveness of the vaccine in older people. But the inoculation program substantially reduced hospital admissions for people over 80 years old, and many older people received the AstraZeneca vaccine.