One dose of a Pfizer-BioNTech or AstraZeneca-Oxford jab cut COVID-19 infections by 65% in a study.
Effectiveness rose to 72% against COVID-19 with symptoms in the UK survey of about 375,000 people.
Antibody responses lasted at least 10 weeks for both vaccines, the study found.
A single dose of a COVID-19 vaccine can give people significant protection against the disease for at least 10 weeks, a UK study of about 375,000 people suggested.
There was a 65% reduction in COVID-19 infections overall after one dose of either the AstraZeneca-Oxford University or Pfizer-BioNTech vaccine, study authors from Oxford said in a press briefing on Thursday. Protection from a single dose probably holds up for at least 10 weeks, based on measurements of antibody levels.
The vaccines are intended to be given as two injections, spaced weeks apart. But the UK has pursued a strategy of delaying the second vaccine dose for up to 12 weeks to give more people a first injection. The study provides evidence supporting that strategy, said Sarah Walker, a professor of medical statistics and epidemiology at Oxford, who led the survey.
"But people must get their second dose," she added.
Getting a second dose of the Pfizer vaccine boosted people's protection. Researchers found that in people who'd had both doses, the vaccine reduced the number of overall infections by 90%. There's not enough data yet to draw any conclusions on the AstraZeneca shot. Doses of Pfizer's shot are supposed to be given three weeks apart, while AstraZeneca jabs can be given four to 12 weeks apart.
The studies provide more evidence that COVID-19 vaccines work
The data provide the latest real-world evidence that coronavirus vaccines are highly effective at halting the virus. Another real-world study from Israel published in February found that Pfizer's vaccine was 94% effective at preventing COVID-19 with symptoms when two doses were given 21 days apart.
The UK results come from the National COVID-19 Infection Survey conducted by Oxford University, the Office of National Statistics, and the Department of Health and Social Care. The Oxford researchers who led the study did not work on the coronavirus vaccine with AstraZeneca.
The researchers collected 3.9 million random nasal coronavirus swabs from more than 218,000 UK households from December to April. They then looked at how many people got infected with the coronavirus and checked whether those people were immunized.
"The benefit of this study is learning real-world effectiveness," Walker said. "The scale of the study is helpful here in determining what is going on in a range of different ages and groups."
The results were released as two studies posted as preprints on Friday and have not yet been scrutinized by other experts in a peer review.
To figure out how long protection may last, the authors also looked at the antibodies the body creates in response to being vaccinated. Antibodies are one part of the natural defenses against infection that vaccines are intended to stimulate.
Antibodies from the vaccines lasted at least 10 weeks
The researchers found that after one dose of either vaccine, antibodies persisted for at least 10 weeks, across all age groups. A second dose of Pfizer's shot boosted antibody levels. Results aren't provided for a time period longer than 10 weeks.
Pfizer has released data showing that protection from its vaccine lasts for at least six months. Still, some medical experts have suggested that people may need annual boosters to maintain protection over time.
The UK study didn't look at whether the vaccines prevented hospitalization or death. Another preprint study from Scotland posted on February 22 suggested that one vaccine dose helped protect against hospitalizations, but effectiveness waned after five weeks.
The Oxford scientists cautioned that immunized individuals could still get COVID-19 and transmit the virus, even if at a lower rate than those who have not had a COVID-19 shot.
They did find that a single dose of either vaccine provided more protection against infections associated with a high level of virus particles, known as a viral load. People with a higher level of the virus may be more likely to transmit the virus to others.
The vaccines also worked well at preventing symptomatic cases of the coronavirus. There was a 72% reduction in symptomatic infections, but just a 57% reduction in infections that didn't cause symptoms, after one dose.
"The people with symptoms are those who could potentially end up in hospital," Walker said.
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