Clinical trial after clinical trial has established that several vaccines do an outstanding job of preventing severe cases of COVID-19. One urgent question has been whether the shots would also slow transmission of the virus altogether along with heading off its worst effects. New clinical-trial results show that, in the case of AstraZeneca’s candidate, the answer appears to be a resounding yes.
The AstraZeneca vaccine — which is in use in Europe but not yet approved in the United States — results in a greatly reduced positive rate from the virus, which would translate to far less spread among populations, according to a preprint paper released by the company that has not yet been peer-reviewed.
After one dose of the vaccine, nasal-swab results taken from participants showed a 67 reduction in positive PCR tests among participants in the wide-ranging trials, which took place in the U.K., Brazil, and South Africa. Curiously, after two doses, the positivity rate was cut only in half — still very positive news. The virus cannot spread from one person to another if no virus is detected.
AstraZeneca’s methods were not necessarily airtight; CNN notes that it did not measure transmission directly, through contact-tracing methods, relying only on the positive-test measure. And some scientists urged caution at overinterpreting the results. “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,” said Doug Brown, chief executive of the British Society for Immunology.
The AstraZeneca result is also not the first to indicate that vaccines might slow transmission; preliminary results from Pfizer and Moderna have also shown marked decreases. But AstraZeneca’s doses are easier to store than those of some competitors and may be a prime candidate for distribution in poorer countries. That a single dose might halt the spread of the virus so drastically is cause for (cautious) celebration.
The new results also showed that a single dose of the AstraZeneca vaccine by itself provided 76 percent efficacy at preventing COVID cases over a three-month period. That figure rose to 82 percent when a second dose was administered more than 12 weeks after the first one; giving the second dose sooner reduced efficacy. The long lag time had been a point of contention, with British authorities endorsing the approach, which allows authorities to give out more initial doses sooner.