London (CNN)The Oxford-AstraZeneca Covid-19 vaccine appears to substantially reduce transmission of the virus, rather than simply preventing symptomatic infections, UK researchers have suggested.The rate of positive PCR tests declined by about half after two doses, according to preliminary results by researchers at the University of Oxford that have yet to be peer reviewed.

Their analysis, released as a preprint Tuesday,also supports spacing out doses and estimates good efficacy after just one shot of the Oxford-AstraZeneca vaccine.Amazon Founder Jeff Bezos will Step Down As CEO / Andy Jassy will Take OverThe study did not measure transmission directly — for example, by tracing contacts who were infected by study volunteers. But the researchers did collect regular nasal swabs from some participants and found that the rate of positive PCR tests fell by half after two doses of the vaccine. After one dose only, the rate of positive tests fell by 67%.“While transmission studies per se were not included in the analysis, swabs were obtained from volunteers every week in the UK study, regardless of symptoms, to allow assessment of the overall impact of the vaccine on risk of infection and thus a surrogate for potential onward transmission,” the authors write.

If the vaccine were simply making infections milder, PCR positivity would not change, the authors argued in the preprint analysis. “A measure of overall PCR positivity is appropriate to assess whether there is a reduction in the burden of infection.”Coronavirus vaccine trials have primarily looked at prevention of symptomatic cases of Covid-19.

Previously, there has been little other public data suggesting that vaccines could prevent people from passing the infection to others.Speaking to the UK’s Science Media Centre (SMC), Helen Fletcher, professor of immunology at the London School of Hygiene and Tropical Medicine, said the data in the study “suggest a possibility that the vaccine could have an impact on transmission but further follow-up would be needed to confirm this.”Dr. Doug Brown, chief executive of the British Society for Immunology, told the SMC the study “hints that the Oxford/AstraZeneca vaccine may be effective in stopping people being able to transmit the virus.”He added: “While this would be extremely welcome news, we do need more data before this can be confirmed and so it’s important that we all still continue to follow social distancing guidance after we have been vaccinated.”

Source: Oxford/AstraZeneca vaccine may cut coronavirus transmission, researchers say