Full vaccination halves the transmission of the Delta variant of coronavirus and is 60 percent effective at preventing symptoms of Covid-19, according to Imperial College’s latest React-1 prevalence survey.
The findings, released on Wednesday and based on a study of 98,000 volunteers in England, provide the most extensive community evidence so far of the way vaccines protect against Delta, which originated in India and has completely displaced the earlier Alpha variant in the UK since the beginning of April. Recent studies have shown 90 percent vaccine effectiveness in preventing hospitalization with Delta. In React-1, 40 percent of participants testing positive were asymptomatic and many others had very mild symptoms. Effectiveness estimates always decrease as researchers include less serious diseases.
React-1, which has been running since April 2020, provides monthly snapshots of Covid by testing random samples of people across England. During the latest survey, from June 24 to July 12, the prevalence of infection, at 0.63 percent, was four times higher than during the previous round from May 20 to June 7. The spread was starting to slow at the end of the sampling period but the study stopped too soon to catch the recent rapid fall in Covid cases.
This is the first time React-1 has estimated vaccine effectiveness. The analysis included both AstraZeneca and Pfizer vaccines but did not break down the figures for each jab.
“The findings, when coupled with other studies demonstrating the impact of coronavirus vaccines on reducing hospitalization and death from Covid-19, are encouraging,” said Tom Wingfield, senior clinical lecturer.