REACT-1 is one of the largest population surveillance studies in the world. It measured the prevalence of SARS-CoV-2, the virus that causes COVID-19, in the general population in England during the COVID pandemic. Each month, more than 100,000 people were invited to take part. Individuals were randomly selected from across all 315 local areas in England to make sure the sample represented the wider population.
Participants were sent nose and throat swabs which were then analysed in a laboratory to detect genetic material of the virus that causes COVID-19, known as a PCR test. The data were used to estimate how many people were infected across the whole population, and to calculate the R number, a measure of how quickly the virus is spreading. The study also identified which people are more likely to be infected and was able to estimate the effectiveness of vaccination against infection.