Independent report

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

Published 19 November 2021

Methodology

A representative cross-section of volunteers in England, aged 5 and over, tested themselves with swabs from 19 October to 5 November 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, which is expected to have little impact on study findings.

Results

Over the 18 days of testing which form these round 15 findings of REACT-1, 1,399 samples tested positive from a total of 100,112 valid swab results, giving a weighted prevalence of 1.57% (1.48%, 1.66%) or 157 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 of growth for data between round 14 and round 15 the R number was estimated at 1.09. As was the case in round 14, in round 15 the rate of prevalence was driven primarily by younger age groups.

Highest weighted prevalence by age was observed in those aged 13 to 17 years at 5.21% (4.61%, 5.87%) and those aged 5 to 12 years at 4.95% (4.39%, 5.58%).

High rates were also found in those aged 45 to 54 years at 1.32% (1.14%, 1.52%) and 35 to 44 years at 1.29% (1.09%, 1.51%).

Weighted prevalence in in those aged 65 to 74 years was 0.84% (0.72%, 0.99%) and in those aged 75 years and over it was 0.63% (0.48%, 0.82%) both representing increases of approximately twofold from round 14.

At regional level, weighted prevalence in round 15 ranged from 1.97% (1.69%, 2.29%) in the South West, increasing more than threefold from round 14 at 0.59% (0.43%, 0.80%), to 1.23% (1.03%,1.47%) in London.

Across round 14 and round 15, the epidemic was clearly found to be growing with an R number greater than 1 in all regions, except the East Midlands and Yorkshire and the Humber.

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 3.12% (2.66%, 3.66%) and 6 or more people at 2.95% (2.27%, 3.82%) compared to 0.78% (0.65%, 0.94%) in single-person households

• in households with one or more children at 2.62% (2.41%, 2.85%) compared to 0.74% (0.67%, 0.81%) in households without children

• in those having been in contact with a confirmed COVID-19 case at 9.13% (8.35%, 9.96%) compared to 0.83% (0.75%, 0.90%) for those without such contact

• in those reporting classic COVID-19 symptoms in the month prior to testing at 7.84% (7.22%, 8.51%) compared to 0.67% (0.60%, 0.75%) in those without symptoms

Vaccine effectiveness was estimated by comparing vaccinated and unvaccinated groups at ages 18 to 64, as almost all individuals aged over 65 are vaccinated.

The report found that:

• the unweighted prevalence of breakthrough infections in those who had received 2 doses of vaccine more than doubled between round 13 and round 15 from 0.41% (0.35%, 0.48%) to 1.10% (1.01%, 1.20%)
• people who have received a booster shot are on average around two-thirds less likely to be infected compared to those who have had 2 doses of vaccine
• among children aged 12 to 17 years, vaccine effectiveness against infection from a single dose of vaccine was estimated at 56.2% (41.3%, 67.4%)
• the unweighted prevalence of infection in unvaccinated adults decreased from 1.49% (1.09%, 2.00%) in round 13 to 1.16% (0.60%, 2.02%) in round 15 – to note, 4.0% of participants in round 13 were unvaccinated, whereas it was only 1.1% in round 15

Additional analysis of the findings from round 15 showed that:

• females are less likely to be infected than males in this period with an observed odds ratio of 0.88 (0.79, 0.98) compared to round 14 where there were similar chances of being infected

• essential and key workers were more likely to test positive compared to other workers with an odds ratio of 1.23 (1.05, 1.44)

• persons in households with one or more children were twice as likely to test positive with an odds ratio of 1.97 (1.60, 2.42) compared to households without children

Lineages were determined for 841 positive samples collected up to 5 November 2021. All were found to be Delta or a sub-lineage of Delta, 99 (11.8%) of these were found to be the AY.4.2 lineage.

Conclusion

During the 18 days covered by the period of this interim report, the number of SARS-CoV-2 infections rose to the joint highest level observed in the REACT study, with 157 per 10,000 people infected. The highest rates were observed regionally in the South West and in those aged 13 to 17 years nationally. All positive samples from this round that could be sequenced were found to be the Delta variant.