Independent report

REACT-1 study of coronavirus transmission: September 2021 final results

Published 14 October 2021

Methodology

A representative cross-section of volunteers in England, aged 5 and over, tested themselves with swabs from 9 to 27 September 2021 (inclusive). Swabs were analysed using polymerase chain reaction (PCR) for the presence of SARS-CoV-2.

For round 14, there has been a slight change in the method of data collection. Up to round 13, test kits used dry swabs which were then transported via cold-chain distribution to the laboratory. In round 14, we switched to using wet swabs preserved in saline.

Also, at random, 50% of swabs were transported by courier and 50% were sent through the priority COVID-19 postal service. Therefore, we would expect sensitivity to be slightly lower than for previous round, but this would have little to no impact on study findings.

Results

Over the 19 days of testing which form these findings from round 14 of REACT-1, 764 samples tested positive from a total of 100,527 swab results, giving a weighted prevalence of 0.83% (0.76%, 0.89%) or 83 people per 10,000 infected. This compares to a weighted prevalence in the previous round, round 13 (24 June to 12 July) of 0.63% (0.57%, 0.69%).

This round showed limited evidence of exponential growth with an estimated doubling time of 34 days, and an R number of 1.03 (0.94, 1.14). This is significantly lower than for round 13, which had a doubling time of 15 days, and a corresponding R number of 1.28 (1.24, 1.31).

The rate of prevalence was driven primarily by younger age groups. The epidemic was growing in those aged 17 years and below with an R rate of 1.18 (1.03, 1.34) and 0.99 probability that R>1. However, for those aged 18 to 54, the epidemic was decreasing through the testing period with an R of 0.81 (0.68, 0.97).

The highest weighted prevalence in round 14 was found in children aged 5 to 12 years at, 2.32% (1.96%, 2.73%) and 13 to 17 years at 2.55% (2.11%, 3.08%).

At regional level, weighted prevalence ranged from 0.57% (0.45%, 0.72%) in South East to 1.25% (1.00%, 1.57%) in Yorkshire and The Humber. Within round 14 there was evidence of growth in both East Midlands and London with R of 1.36 (1.05, 1.73) and 1.59 (1.23, 1.99) respectively.

Ethnicity, household size and vaccination status were all markers of differing rates of infection.

The highest prevalence was observed in:

• people of Black ethnicity at 1.41% (0.91%, 2.19%) compared with white participants at 0.78% (0.72%, 0.85%)
• those in the largest households of 6 or more people at 1.75% (1.24%, 2.46%) for households with 6 or more persons, compared to 0.33% (0.25%, 0.44%) for single person households

Weighted prevalence was 0.56% (0.50%, 0.62%) in those reporting 2 doses of vaccine compared with 1.73% (1.42%, 2.12%) in unvaccinated people.

Weighted prevalence in round 14 among people who were in contact with a confirmed COVID-19 case was 7.35% (6.50%, 8.31%) compared with 0.43% (0.38%, 0.49%) among those without such contact.

Furthermore, multiple regression analyses in round 14 showed that key workers other than healthcare workers and care home workers had increased risk of testing positive, with R of 1.35 (1.10, 1.66) compared to other workers.

Of the samples which were sequenced for variants, of SARS-CoV-2, 100% were from the Delta lineage (or sub-lineage).

Across rounds 13 and 14 of the REACT-1 study, analysis of weighted prevalence by time since receiving second dose of vaccine indicated higher prevalence at 0.55% (0.50%, 0.61%) for those who received their second dose 3 to 6 months before their swab compared to 0.35% (0.31%, 0.40%) for those whose second dose was within 3 months. Prevalence rates were uncertain for those whose second dose was more than 6 months previously.

The study also examined vaccine effectiveness against infection, comparing those who have received 2 doses of a vaccine against those who are unvaccinated. For all participants and all vaccines combined, vaccine effectiveness against infection was estimated to be 62.8% when adjusted for round, age, sex, index of multiple deprivation, region and ethnicity. Among the subset of participants reporting symptoms, vaccine effectiveness was 66.4% overall.

This is in line with estimates by the UK Health Security Agency (UKHSA) that after 2 doses, vaccine effectiveness against symptomatic disease with the Delta variant is approximately 65 to 70% with AstraZeneca and 80 to 95% with Pfizer-BioNTech.

Conclusion

During the period 9 to 27 September, the number of SARS-CoV-2 infections broadly levelled off compared to previous rounds, with an R of 1.03, and 83 out of 10,000 people estimated to have the virus. The highest rates were observed regionally in London and the East Midlands 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.