Independent report

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

Published 23 December 2021

Methodology

A representative cross-section of volunteers in England, aged 5 and over, tested themselves with swabs from 23 November 2021 to 14 December (inclusive). Swabs were analysed using polymerase chain reaction (PCR) for the presence of SARS-CoV-2.

Results

Over the 22 days of testing which form these round 16 findings of REACT-1, 1,192 samples tested positive from a total of 97,089 valid swab results, giving a weighted prevalence of 1.41% (1.33%, 1.51%) or 141 people per 10,000 infected. This compares to a weighted prevalence in the previous round, round 15 (19 October to 5 November), of 1.57% (1.48%, 1.66%).

Lineages were determined for 650 positive samples collected up to 11 December 2021. Eleven Omicron variants were detected; all other lineages were the Delta variant or a sub-lineage of Delta.

In REACT-1 round 16 data, an Omicron infection was first detected in London on 3 December.

Subsequent Omicron infections appeared concentrated mainly in the South of England.

The 11 Omicron cases were all aged 18 to 54 years, double-vaccinated (reflecting the large numbers of people who have received 2 doses of vaccine in this age group), 9 were men, 5 lived in London and 7 were symptomatic (5 with classic COVID-19 symptoms), 2 were asymptomatic, and symptoms were unknown for 2 cases.

The proportion of Omicron (versus Delta or Delta sub-lineages) appears to have rapidly increased the odds of receiving an Omicron result (as opposed to Delta) for those receiving a positive test increased by 66% each day (32.7%, 127.3%).

Based on 11 Omicron variants detected out of 275 positives sequenced, for swabs collected from 1 to 11 December 2021, the estimated average prevalence of Omicron infections in England was 31,000 (17,000, 55,000), assuming 100% sensitivity and a weighted prevalence of 1.38% over that period.

In round 16 the rate of prevalence for SARS-CoV-2 infections was higher in younger age groups.

Highest weighted prevalence by age was observed in those aged 5 to 11 years at 4.74% (4.15%, 5.40%).

High rates were also found in those aged 12 to 17 years at 2.31% (1.91%, 2.80%).

Weighted prevalence in in those aged 65 to 74 years fell from 0.84% (0.72%,0.99% in round 15 to 0.48% (0.39%, 0.59%) in round 16.

Weighted prevalence in those aged 75 years and over fell by approximately two-thirds from 0.63% (0.48%, 0.82%) in round 15 to 0.21% (0.13%, 0.32%) in round 16.

At regional level, weighted prevalence in round 16 ranged from 1.84% (1.59%, 2.12%) in London (compared with 1.23% (1.03%, 1.47%) in round 15), to 1.00% (0.70%,1.42%) in North East.

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 2.73% (2.25%, 3.32%) and 6 or more people at 2.65% (2.00%, 3.50%) compared to 0.88% (0.72%, 1.09%) in single-person households

• households with one or more children at 2.43% (2.23%, 2.65%) compared to 0.85% (0.76%, 0.95%) in households without children

• those having been in contact with a confirmed COVID-19 case at 8.00% (7.25%, 8.82%) compared to 0.81% (0.73%, 0.89%) for those without such contact

• those reporting classic COVID-19 symptoms (loss or change of sense of smell or taste, fever, persistent cough) in the month prior to swabbing at 6.96% (6.32%, 7.67%) compared to 0.62% (0.55%, 0.70%) in those without symptoms

Vaccine effectiveness against infection in children aged 12 to 17 years was estimated at 55.7% (41.3%, 66.6%) for those having received a single vaccine dose, and at 57.9% (44.1%, 68.3%) for those having received 1 or 2 vaccine doses. Adults aged 18 years and over having received a third vaccine dose were at lower risk of testing positive than those having received 2 vaccine doses with an unadjusted risk of 0.23 (0.19, 0.28) and fully adjusted at 0.27 (0.22, 0.34).

Conclusion

During the 22 days covered by the period of this interim report, the prevalence of SARS- CoV-2 infections overall was 140 per 10,000 people infected and was rapidly increasing in December 2021. The highest rates were observed regionally in the London and in those aged 5 to 11 years nationally. Of all the positive samples sequenced in this round for swabs collected to 11 December 2021, 11 of those were Omicron variant – all other lineages were the Delta variant or a sub-lineage of Delta.