One shot of Pfizer and BioNTech's vaccine reduced the risk of coronavirus infection by 70% in a UK study.
The vaccine's effectiveness increased to 85% in those who got two shots, the study found.
The data suggested the vaccine worked against the B.1.1.7. variant first found in the UK.
Pfizer and BioNTech's two-dose COVID-19 vaccine provides strong protection against infection with just one shot and probably helps to stop people from spreading the virus, early data from a large real-world study in the UK published on Monday suggested.
"Data shows one dose reduces the risk of catching infection by more than 70%, rising to 85% after the second dose," Public Health England said in a press release summarizing the findings. "This suggests the vaccine may also help to interrupt virus transmission, as you cannot spread the virus if you do not have infection."
The study's authors said that "significant protection from infection" kicked in 10 days after the shot and plateaued after 21 days.
The study found that the vaccine worked against the B.1.1.7. coronavirus variant, which was highly prevalent in the UK during the study period, the authors said. That variant, estimated to be up to 50% more contagious, has spread to more than 70 countries and 44 US states, according to the Centers for Disease Control and Prevention.
More than 32.8 million doses of Pfizer's shot have been administered in the US so far, according to the CDC.
PHE's ongoing Siren study has followed more than 23,000 UK healthcare workers for two months, testing them each week for COVID-19 regardless of whether they had symptoms. They had regular lab polymerase-chain-reaction tests - the gold standard - and rapid antigen tests.
The study then looked at the length of time to COVID-19 infection and compared those who were immunized with those who were not. The frequent testing meant the researchers could pick up infected people who weren't showing symptoms, which they used as a "proxy for reduction in transmission" - about one in three people who get COVID-19 do not have symptoms and could pass it on without realizing it.
The study is a preprint, meaning it is awaiting experts' scrutiny of its methods and conclusions before publication in the Lancet medical journal.
A study of health workers in Israel published in The Lancet on Thursday found that Pfizer's vaccine was effective at protecting against symptomatic infection 15 to 28 days after the first dose, but it did not evaluate its effect on asymptomatic transmission.
Participants in the Siren study were working-age, mostly white women, and three-quarters of them did not have coexisting medical conditions, so the findings might not be generalizable to the population or to older people.
PHE said that early data from routine testing suggested that one dose of Pfizer's shot was 57% effective at protecting against symptomatic COVID-19 in people over 80 about three to four weeks after the shot, increasing to more than 85% with a second dose.
The lower efficacy number compared with the Siren study's findings could be because the immune system responds less well to a vaccination as we get older.
PHE estimated that "hospitalisation and death from COVID-19 will be reduced by over 75% in those who have received a dose of the Pfizer-BioNTech vaccine."
A study in Scotland published on Monday looked at hospitalization rates following immunization and found that Pfizer's shot reduced the risk of hospitalization by up to 85% four weeks after the first dose.
Dr. Mary Ramsay, the head of immunization at PHE, there was "strong evidence that the Pfizer-BioNTech vaccine is stopping people from getting infected, while also protecting cases against hospitalisation and death."
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