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REACT-1 round 7 interim report: fall in prevalence of swab-positivity in England during national lockdown

By Steven Riley, Oliver Eales, Caroline E. Walters, Haowei Wang, Kylie E. C. Ainslie, Christina Atchinson, Claudio Fronterre, Peter J. Diggle, Deborah Ashby, Christl Donnelly, Graham Cooke, Wendy S Barclay, Helen Ward, Ara Darzi, Paul Elliott

Posted 02 Dec 2020
medRxiv DOI: 10.1101/2020.11.30.20239806

Background The second wave of the 2020 COVID-19 pandemic in England has been characterized by high growth and prevalence in the North with lower prevalence in the South. High prevalence was first observed at younger adult ages before spreading out to school-aged children and older adults. Local tiered interventions were in place up to 5th November 2020 at which time a second national lockdown was implemented. Methods REACT-1 is a repeated cross-sectional survey of SARS-CoV-2 swab-positivity in random samples of the population of England. The current period of data collection (round 7) commenced on 13th November 2020 and we report interim results here for swabs collected up to and including 24th November 2020. Because there were two distinct periods of growth during the previous round 6, here we compare results from round 7 (mainly) with the second half of round 6, which obtained swabs between 26th October and 2nd November 2020. We report prevalence both unweighted and reweighted to be representative of the population of England. We describe trends in unweighted prevalence with daily growth rates, doubling times, reproduction numbers (R) and splines. We estimated odds ratios for swab-positivity using mutually-adjusted multivariable logistic regression models. Results We found 821 positives from 105,123 swabs giving an unweighted prevalence of 0.78% (95% CI, 0.73%, 0.84%) and a weighted prevalence of 0.96% (0.87%, 1.05%).The weighted prevalence estimate was ~30% lower than that of 1.32% (1.20%, 1.45%) obtained in the second half of round 6. This decrease corresponds to a halving time of 37 (30, 47) days and an R number of 0.88 (0.86, 0.91). Using only data from the most recent period, we estimate an R number of 0.71 (0.54, 0.90). A spline fit to prevalence showed a rise shortly after the previous period of data collection followed by a fall coinciding with the start of lockdown. The national trends were driven mainly by reductions in higher-prevalence northern regions, with prevalence approximately unchanged in the Midlands and London, and smaller reductions in southern lower-prevalence regions. Sub-regional analyses showed variable changes in prevalence at the local level including marked declines in the North, but also local areas of growth in East and West Midlands. Mutually adjusted models in the most recent period indicated: people of Asian ethnicity, those living in the most deprived neighbourhoods, and those living inthe largest households, had higher odds of swab-positivity. Conclusion Three weeks into the second national lockdown in England there has been a ~30% proportionate reduction in prevalence overall, with greater reductions in the North. As a result, inter-regional heterogeneity has reduced, although average absolute prevalence remains high at ~1%. Continued monitoring of the epidemic in the community remains essential until prevalence is reliably suppressed to much lower levels, for example, through widespread vaccination.

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