Independent report

REACT-1 study of coronavirus transmission: October 2021 interim results

Updated 12 November 2021

Methodology

A representative cross-section of volunteers in England, aged 5 and over, tested themselves with swabs from 19 to 29 October 2021 (inclusive). Swabs were analysed using polymerase chain reaction (PCR) for the presence of SARS-CoV-2.

For round 15, there has been a slight change in the method of data collection. Up to round 13, test kits were returned by courier to the lab. For round 14, 50% of samples were transported by courier and 50% were sent through the priority coronavirus (COVID-19) postal service. For round 15, all samples have been returned by post, but we expect this will have little impact on study findings.

Results

Over the 11 days of testing which form these round 15 interim findings (round 15a) of REACT-1, 1,021 samples tested positive from a total of 67,208 valid swab results, giving a weighted prevalence of 1.72% (1.61%, 1.84%) or 172 people per 10,000 infected. This compares to a weighted prevalence in the previous round, round 14 (9 to 27 September 2021), of 0.83% (0.76%, 0.89%).

Using an exponential model for data between round 14 and round 15a the R number was estimated at 1.12.

As was the case in round 14, in round 15a the rate of prevalence was driven primarily by younger age groups.

Highest weighted prevalence by age was observed in those aged 5 to 12 years at 5.85% (5.10%, 6.70%) and those aged 13 to 17 years at 5.75% (5.02%, 6.57%).

High rates were also found in those aged 45 to 54 years at 1.53% (1.29%, 1.80%) and 35 to 44 years at 1.48% (1.21%, 1.80%).

Weighted prevalence in in those aged 65 to 74 years was 0.82% (0.68%, 0.99%) and at 75 years and over it was 0.67% (0.50%, 0.89%), both representing increases of approximately twofold from round 14.

At regional level, weighted prevalence in round 15a ranged from 2.18% (1.84%, 2.58%) in the South West to 1.31% (1.06%, 1.63%) in London. Across round 14 and round 15a, the epidemic was clearly found to be growing with an R number greater than 1 in all regions except Yorkshire and the Humber where it was estimated to be 1.03 (0.98, 1.08).

Household size, symptom status and recent contact with a confirmed case, were all showing contrasted prevalence of infection. The highest prevalence was observed:

• in larger households including 5 people at 3.68% (3.05%, 4.43%) and 6 or more people at 3.03% (2.24%, 4.09%) compared to 0.78% (0.62%, 0.98%) in single-person households
• in households with one or more children at 3.09% (2.80%, 3.41%) compared to 0.75% (0.67%, 0.84%) in households without children
• in those having been in contact with a confirmed COVID-19 case at 10.1% (9.19%, 11.2%) compared to 0.83% (0.74%, 0.92%) for those without such contact
• in those reporting classic COVID-19 symptoms in the month prior to testing at 8.69% (7.90%, 9.55%) compared to 0.71% (0.62%, 0.81%) in those without symptoms

Additional analyses in round 15a show that:

• females are less likely to be infected than males in this period with an observed odds ratio of 0.87 (0.77, 0.99) compared to round 14 where there were similar chances of being infected if you were female or male with an odds ratio close to 1
• that essential/key workers were more likely to test positive compared to other workers with an odds ratio of 1.25 (1.03, 1.51); and
• persons in households with one or more children were twice as likely to test positive with an odds ratio of 2.09 (1.63, 2.68) compared to households without children.

Lineages were determined for 126 positive samples collected up to 23 October 2021. All were found to be Delta or a sub-lineage of Delta; 13 (10.3%) within Delta were found to be AY.4.2.

Conclusion

During the 11 days covered by the period of this interim report, the number of SARS-CoV-2 infections rose to its highest ever level in the REACT study, with 171 per 10,000 people infected. The highest rates were observed regionally in the South West and in those aged 5 to 17 years nationally. All positive samples from this round that could be sequenced were found to be the Delta variant.