The number of positive PCR tests fell by about half after two doses, according to preliminary results from researchers at the University of Oxford who are not yet to be peer-reviewed.
In the study, transmission was not measured directly – for example, by tracing contacts infected by study volunteers. But the researchers regularly collected nasal swabs from some participants and found that the number of positive PCR tests dropped by half after two doses of the vaccine. After just one dose, the number of positive tests decreased by 67%.
“ Although transmission studies per se were not included in the analysis, in the UK study, swabs were obtained from volunteers every week, regardless of symptoms, to assess the overall impact of the vaccine on the risk of infection and thus a surrogate for possible further transmission, ”the authors write.
If the vaccine simply made infections milder, the PCR positivity would not change, the authors reasoned in the pre-pressure analysis. “A measure of overall PCR positivity is useful for assessing whether there is a reduction in infection pressure.”
Vaccine trials with the coronavirus have mainly looked at the prevention of symptomatic cases of Covid-19. Previously, there was little other public data indicating that vaccines can prevent people from passing the infection on to others.
Helen Fletcher, professor of immunology at the London School of Hygiene and Tropical Medicine, told the UK Science Media Center (SMC) that the data in the study “ indicates that the vaccine may affect transmission, but continued up would be needed to fix this. “
Dr. Doug Brown, chief executive of the British Society for Immunology, told the SMC the study “indicates that the Oxford / AstraZeneca vaccine may be effective in preventing people from transmitting the virus.”
He added, “While this would be extremely welcome news, we need more data before it can be confirmed, and so it is important that we all continue to follow social distance guidelines after we are vaccinated.”
Dose distance
The vaccine showed 66.7% efficacy against symptomatic diseases from two weeks after the second injection, according to researchers at the University of Oxford.
The new analysis adds new trial sites and a month of new data to the mix, building on previous results announced by AstraZeneca that the vaccine had shown an estimated efficacy of 70.4%.
However, the latest research also suggests that the vaccine can provide significant protection after a single injection.
The study estimates an efficacy of 76% up to three months after a single dose. This is based on a subgroup of 88 symptomatic infections, unevenly distributed between the vaccine and placebo groups between 22 and 90 days after vaccination. The study also found relatively stable levels of antibodies during this time period, “with minimal decline at day 90.”
In addition, the authors suggest there could be higher efficacy with more staggered doses. In adults aged 18 to 55 years, the efficacy of the vaccine seemed to be increased when the time between injections was delayed from less than six weeks to more than 12. However, more information is needed to understand the statistical significance of that finding.
Taken together, the findings may bolster the UK’s decision to recommend doses spaced apart for up to 12 weeks, according to a statement by the Oxford Vaccine Trial lead investigator and co-author of the paper, Andrew Pollard.
“This is positive news as it shows that just one dose of this vaccine generates good immunity and that this protection does not appear to be diminishing in the short term,” said Brown of the British Society for Immunology.
“In terms of immunology, this finding is not unexpected, as we know that some other vaccines confer better immunity when doses are more dispersed. While more information is needed to confirm these findings for older age groups, this new research on the The UK’s decision to offer the two doses of this vaccine 12 weeks apart. “
Comparable data is not yet available on delaying the second dose of the Pfizer / BioNTech vaccine until 12 weeks after the first dose, said Dr. Gillies O’Bryan-Tear, former chair of policy and communications at the Faculty of Pharmaceutical Medicine in a commentary on the SMC. But, he added, “most commentators agree that it will likely be the same with that vaccine, and indeed, other two-dose vaccines.”
The authors emphasize that the studies were not initially designed to evaluate efficacy through dose intervals, but the data “emerged from the logistics of conducting large-scale clinical trials in a pandemic setting.”
The primary analysis is based on 332 symptomatic infections that occurred in more than 17,000 trial volunteers in the UK, Brazil and South Africa more than two weeks after their second dose.
AstraZeneca announced last month that it had completed enrollment for its Phase 3 trial in the United States, which will serve as “the primary basis” for the company’s eventual filing with the US Food and Drug Administration.
The vaccine has already been approved in a number of countries, such as the UK and India, but according to Moncef Slaoui of Operation Warp Speed, the license will arrive in the US at the earliest by the end of March.