REACT-1 study of coronavirus transmission: May 2021 final results
Updated 7 July 2021
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Methodology
A representative cross-section of volunteers aged 5 years and older in England tested themselves with swabs from 20 May to 7 June inclusive (round 12). Swabs were analysed using polymerase chain reaction (PCR).
Note that the sampling method in which volunteers were selected to participate in changed in round 12. Participants were selected randomly in proportion to population at Lower Tier Local Authority (LTLA) level; previously similar numbers of participants in each LTLA were sought.
This revised sampling methodology gives more weight to higher population density LTLAs in urban areas versus lower population density LTLAs in rural areas. Temporal analysis is then run to adjust findings to be representative of the population of England in each round. This change may affect comparison of unweighted prevalence across the rounds, but should not affect comparisons of weighted estimates.
Results
Over the 19 days of round 12 testing, 135 out of 108,911 swab results were positive, giving a weighted prevalence overall of 0.15% (0.12%, 0.18%), or 15 people per 10,000 infected. This is an increase from 0.10% (0.08%,0.13%) in round 11 (15 April to 3 May).
Using constant growth rate models, during round 12 there is strong evidence for a recent increase in R nationally with a more than 99% probability that R was greater than one.
Within round 12, R was 1.44 (1.20, 1.73) with a doubling time of 11 days (7.1 to 23). There was evidence of an increase over the period of round 11 to round 12 (20 May to 7 June), as the R between rounds was 1.07 (1.03, 1.12).
Regionally, the R number between rounds 11 and 12 was estimated to be greater than 1 in North West, East Midlands and East of England. For other regions in the same period there was no strong evidence R was either less than or greater than one and insufficient data to estimate regional R within round 12 itself.
Substantial variation in prevalence between the regions was observed. Weighted prevalence was higher in the North West at 0.26% (0.16%, 0.41%) than the South West, which was calculated at 0.05% (0.02%, 0.12%). Notably, the locations of positive samples in the North West suggest a cluster in Greater Manchester and the east Lancashire area.
A smoothing parameter known as P-splines were fitted to the daily prevalence data for the 5 northernmost regions combined, and the 4 southernmost regions combined which revealed a sustained upward trend in the North and recent growth in the South.
With regard to age, weighted prevalence in round 12 was five-fold higher in 5 to12 and 18 to 24 year olds at 0.35% (0.23%, 0.54%) and 0.36% (0.20%, 0.64%) respectively compared with people aged 65 and above. Looking at the 5 to 49 year olds as a single group revealed that prevalence was over two-fold higher at 0.20% (0.16%, 0.26%) compared with those aged 50 years and above at 0.08% (0.06%, 0.11%). There were recent upturns in prevalence shown through the use of P-spline estimates for those aged 5 to 49 years and those aged 50 years and above, but at a higher level in the younger age group.
Viral sequencing was carried out for 46 of the 135 positive samples. This revealed that 36 (78.3%) were Delta (B.1.617.2) variant and 10 (21.7%) were Alpha (B.1.1.7) variant.
Conclusion
During the period 20 May to 7 June (round 12), SARS-CoV-2 virus was circulating with a higher prevalence at 15 in 10,000 infected than round 11 (15 April to 3 May). Delta has become the dominant variant circulating in the population of England.