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.