One dose of the Oxford/AstraZeneca vaccine provides sustained protection against Covid for at least three months and cuts transmission of the virus by two-thirds, according to research that appears to support the UK’s decision to delay booster shots. Analysis of fresh data from three trials found that the first shot conferred on average 76% protection against symptomatic infections from three weeks until 90 days, and reduced transmission of the disease by 67%. The findings are preliminary, and still under review at The Lancet, but if they stand up to scientific scrutiny would reassure public health officials that prioritising more vulnerable people for a first shot of the Oxford/AstraZeneca vaccine is a sound strategy. While people were originally due to be given two shots of coronavirus vaccines three or four weeks apart, in December the Joint Committee on Vaccination and Immunisation (JCVI), which advises ministers, recommended delaying second doses so more vulnerable people could receive a first shot. Less is known about the impact of spacing out two shots of the Pfizer vaccine, which is part of the UK rollout. But the latest data on the Oxford/AstraZeneca vaccine, drawn from clinical trials in the UK, Brazil and South Africa conducted up until 7 December, show that it may be more effective if the second shot is delayed. Researchers running the trial recorded no evidence for the 76% level of protection waning between three weeks and three months after the first dose. Delaying the second shot for at least three months boosted protection to an average of 82%, they concluded, compared with 62% in the interim trial when volunteers had their shots four weeks apart. The paper suggests vaccine efficacy rose from about 55% when the booster was given no more than six weeks after the first shot, to 82.4% when given more than three months later. Prof Andrew Pollard, chief investigator of the Oxford trial and co-author on the paper, said the results were reassuring and an important verification of interim data that UK and other regulators used to grant emergency authorisation for the vaccine. A similar effect is seen in vaccines for the flu and Ebola, which are more potent when the two shots are more spaced out. “This is excellent news and very much what was expected from what we know about other vaccines,” said David Matthews, a virologist at the University of Bristol. As part of the UK arm of the trial, the researchers obtained weekly nose and throat swabs from volunteers which were used to check for infections. These revealed that after a single dose of vaccine, PCR test positive cases of Covid fell between 49% and 78%, raising hopes that the vaccine may help to reduce the spread of the disease as well as preventing deaths. Deborah Dunn-Walters, professor of immunology at Southampton University and chair of the British Society for Immunology’s Covid-19 taskforce, said: “It is very reassuring news that just one shot has 76% efficacy. The efficacy still improves at the second dose so it is still important to get that to consolidate immune defences. The indication that virus carriage is reduced is also very encouraging.” However, Azra Ghani, professor of infectious disease epidemiology at Imperial College London, urged caution over the results. She pointed out that the trial was not designed to assess different dosing gaps or vaccine efficacy at one versus two doses. This means the participants were not randomised and that other factors could explain the findings. For example, those who received the single dose were younger and more likely to be female, and more likely to be healthcare workers, and more likely to be white, than those who received two shots. “This means that it is not sensible to compare the efficacy estimates from a single dose with those from two doses,” she said. A further trial, looking specifically at different dosing schedules, was needed to know the answer for sure, she added. Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, said the extra data on the Oxford vaccine was “very helpful”. “The data definitely provide some evidence to suggest that the eventual protection from two doses of this vaccine are not worsened by having a longer than 28- or 42-day period between doses and tend to confirm what had been shown before, that if anything the eventual efficacy was better. The data also do not provide any evidence that efficacy wanes after the first dose,” he said.