Double-vaccinated people who had COVID-19 previously are the group best protected against Delta, data shows.
Regardless of previous infection, two shots of Pfizer or AstraZeneca offer good protection against Delta.
Two doses were always better than one, the researchers said.
The study added to the growing body of evidence that COVID-19 vaccines offer good protection against Delta after two doses, albeit slightly less protection than against Alpha, the formerly dominant coronavirus variant.
The study, led by Oxford University, the UK Office for National Statistics, and the UK health department, found that getting two COVID-19 vaccine doses was the "most effective way" to prevent infections caused by the Delta variant.
Protection against infection was higher when people had previously caught COVID-19, the study authors said in a preprint posted Thursday.
Preliminary data suggested that people who caught COVID-19 while vaccinated had a higher level of antibodies. It would, however, be "completely irresponsible" to deliberately try to get infected after getting your second vaccine, because there is a small chance you'd end up in hospital, the study lead Professor Sarah Walker said.
The research focused on Pfizer and AstraZeneca-Oxford University's two-dose COVID-19 vaccines.
Pfizer offers 85% protection against Delta
The study authors said that two weeks after the second dose, Pfizer's COVID-19 vaccine was 93% effective for people with a prior COVID-19 infection, and 85% effective for people with no prior infection.
Two weeks after the second dose, AstraZeneca's vaccine was 88% effective among those who had COVID-19 before, and 68% effective in those who hadn't, the authors said.
To get the results, the scientists analyzed COVID-19 lab test results from 358,983 people in the UK between May 17 and August 1 - the period when Delta became dominant. Participants were randomly tested, irrespective of symptoms, vaccination status, or whether they'd had COVID-19.
The methods haven't yet been scrutinized by experts in a peer-review.
The researchers said that from four months, the two vaccines offered very similar levels of protection against Delta. Protection from Pfizer's gradually decreased after that point, while AstraZeneca's stayed the same, they said.
Figures for vaccine effectiveness can vary according to the outcome tested. In this case, the Oxford scientists examined protection against COVID-19 infections with and without symptoms in the community.
Previous real-world UK data showed Pfizer's vaccine was 88% effective against COVID-19 with symptoms after two doses, and AstraZeneca's shot was 60% effective by the same parameters.
The Oxford researchers didn't look at the vaccines' ability to stop severe disease and hospitalization.
Protection against Delta was even better at younger ages
The study authors said that protection against Delta was generally better in younger age groups.
Two weeks after the second dose, Pfizer's shot was 90% effective against infection in those aged 18 to 35, and 77% effective in those aged 35 to 64-years-old, they said.
AstraZeneca's vaccine was 73% effective against infection in the younger age group, after two weeks, and 54% effective in those aged 35 to 64.
The researchers cautioned that fully-vaccinated people, regardless of age and previous infection, can still spread Delta. "Whilst vaccinations reduce the chance of getting COVID-19, they do not eliminate it," they said.
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