One shot of Pfizer and BioNTech's vaccine reduced the risk of coronavirus infection by 70% in a UK study.
The COVID-19 vaccine's protection increased to 85% in those who got two shots.
The data showed Pfizer's vaccine worked against a mutated variant found in the UK and US, B.1.1.7.
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Pfizer and BioNTech's two-dose COVID-19 vaccine provides strong protection against infection with just one shot, and probably stops people spreading the virus, according to the results of a large-scale, real-world study published Monday.
"One dose reduces the risk of catching infection by more than 70%, rising to 85% after the second dose," Public Health England (PHE) said in a press release Monday. "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 authors said that "significant protection from infection" kicked in from 10 days after the shot, and plateaued after 21 days.
The findings show that Pfizer's vaccine works against the coronavirus variant in the UK, called B.1.1.7, which was highly prevalent in the UK during the study period, the authors said.
B.1.1.7 is estimated to be 30-50% more contagious, and has spread worldwide, including to 44 states in the US, according to the Centers for Disease Control and Prevention (CDC).
More than 32.8 million doses of Pfizer's shot has been rolled out in the US so far, according to the CDC .
PHE's ongoing SIREN study has followed more than 23,000 UK health care workers for two months, testing them each week for COVID-19 irrespective of whether they had symptoms. They had fortnightly lab polymerase chain reaction (PCR) tests, the gold standard, and twice weekly rapid antigen tests.
The study then looked at the length of time to COVID-19 infection, confirmed by PCR, and compared those who were immunized with those who were not. The frequent testing meant the researchers could pick up people infected with the virus without symptoms, which they used as "a proxy for transmission" - one in three people who have coronavirus do not have symptoms but could pass it on without realizing it.
The study is a pre-print awaiting scrutiny of its methods and conclusions by other experts prior to publication in the Lancet medical journal.
Another study of health workers in Israel published in the Lancet on February 18 found Pfizer's vaccine was effective at protecting against symptomatic infection 15-28 days following the first dose, but it did not evaluate asymptomatic transmission.
The SIREN study participants were working age, mostly white females, and three-quarters did not have coexisting medical conditions, so the findings might not be generalizable to the general population or older people.
PHE said in the press release that early data from routine testing showed that one dose of Pfizer's shot was 57% effective at protecting against symptomatic COVID-19 disease in those aged over 80, increasing to more than 85% with a second dose.
This effect occurred from about three to four weeks after the first dose. The lower efficacy number, compared with the SIREN study findings, is likely to be because the immune system responds less well to a vaccination as we get older. Hospitalization and death from COVID-19 was reduced by over 75% in those who have received a dose of the Pfizer-BioNTech vaccine, PHE added.
A study in Scotland reported similar findings Monday. The study looked at the rate of hospitalization following immunization, and found Pfizer's shot reduced hospital admissions by up to 85% four weeks after the first dose.
Dr. Mary Ramsay, head of immunisation at PHE. said that there was "strong evidence" that the Pfizer-BioNTech vaccine was stopping people from getting infected, while also protecting against hospitalization and death.