Background: It has been nearly a year since the first vaccinations against SARS-CoV-2
were delivered in England. The third wave of COVID-19 in England began in May 2021 as
the Delta variant began to outcompete and largely replace other strains. The REal-time
Assessment of Community Transmission-1 (REACT-1) series of community surveys for
SARS-CoV-2 infection has provided insights into transmission dynamics since May 2020.
Round 15 of the REACT-1 study was carried out from 19 October to 5 November 2021.
Methods: We estimated prevalence of SARS-CoV2 infection and used multiple logistic
regression to analyse associations between SARS-CoV-2 infection in England and
demographic and other risk factors, based on RT-PCR results from self-administered throat
and nose swabs in over 100,000 participants. We estimated (single-dose) vaccine
effectiveness among children aged 12 to 17 years, and among adults compared
swab-positivity in people who had received a third (booster) dose with those who had
received two vaccine doses. We used splines to analyse time trends in swab-positivity.
Results: During mid-October to early-November 2021, weighted prevalence was 1.57%
(1.48%, 1.66%) compared to 0.83% (0.76%, 0.89%) in September 2021 (round 14).
Weighted prevalence increased between rounds 14 and 15 across most age groups
(including older ages, 65 years and over) and regions, with average reproduction number
across rounds of R=1.09 (1.08, 1.11). During round 15, there was a fall in prevalence from a
maximum around 20-21 October, with an R of 0.76 (0.70, 0.83), reflecting falls in prevalence
at ages 17 years and below and 18 to 54 years. School-aged children had the highest
weighted prevalence of infection: 4.95% (4.39%, 5.58%) in those aged 5 to 12 years and
5.21% (4.61%, 5.87%) in those aged 13 to 17 years. In multiple logistic regression, age, sex,
key worker status and presence of one or more children in the home were associated with
swab positivity. There was evidence of heterogeneity between rounds in swab positivity rates
among vaccinated individuals at ages 18 to 64 years, and differences in key demographic
and other variables between vaccinated and unvaccinated adults at these ages. Vaccine
effectiveness against infection in children was estimated to be 56.2% (41.3%, 67.4%) in
rounds 13, 14 and 15 combined, adjusted for demographic factors, with a similar estimate
obtained for round 15 only. Among adults we found that those who received a third dose of
vaccine were less likely to test positive compared to those who received only two vaccine
doses, with adjusted odds ratio (OR) =0.38 (0.26, 0.55).
Discussion: Swab-positivity was very high at the start of round 15, reaching a maximum
around 20 to 21 October 2021, and then falling through late October with an uncertain trend
in the last few days of data collection. The observational nature of survey data and the
relatively small proportion of unvaccinated adults call into question the comparability of
vaccinated and unvaccinated groups at this relatively late stage in the vaccination
programme. However, third vaccine doses for eligible adults and the vaccination of children
aged 12 years and over are associated with lower infection risk and, thus, remain a high
priority (with possible extension to children aged 5-12 years). These should help reduce
SARS-CoV-2 transmission during the winter period when healthcare demands typically rise.