Health

Latest round of REACT-1 finds SARS-CoV-2 prevalence has halved since previous round

Researchers in the UK say the latest round of the REal-time Assessment of Community Transmission-1 (REACT-1) study has found that the prevalence of infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) – the agent that causes coronavirus disease 2019 (COVID-19) – has fallen by 50%, compared with the previous round conducted in March of this year (2021).

“We observed marked reductions in prevalence from March to April and early May 2021 in England reflecting the success of the vaccination program and despite easing of restrictions during lockdown,” says Steven Riley from Imperial College London and colleagues.

Between the previous round 10 and the latest round 11, SARS-CoV-2 prevalence fell by two-thirds among those aged 55 to 64 years.

On the other hand, prevalence rates among those aged 25 to 34 years were similar between the two rounds, despite the overall reduction in prevalence. The team says this may reflect the increase in contact rates in this age group, for which vaccine coverage was low while social mixing restrictions were eased.

The researchers warn that further easing of lockdown regulations and the presence of the B.1.617.2 lineage that originally emerge in India could lead to an increase in prevalence, in which case policy makers will need to assess the potential impact on hospitalizations and deaths.

A pre-print version of the research paper is available on the medRxiv* server, while the article undergoes peer review.

Factors that drive dynamics of national infection patterns

National patterns of SARS-CoV-2 infections are driven by the degree of recent indoor mixing, vaccine coverage, the intrinsic properties of circulating lineages, and prior history of infection.

For example, India is currently experiencing a large wave of COVID-19 cases and deaths due to the emergence of the B.1.617 variant, following a period of relatively increased levels of social mixing and low vaccination coverage. Meanwhile, the incidence is low in Israel, despite social mixing restrictions having been largely relaxed, which is likely driven by the effects of vaccination.  

In England, the rate of infections, hospitalizations and deaths fell this year during the first two stages of social mixing regulations being relaxed on March 8th and April 12th as part of the “roadmap” for exiting the third national lockdown. During the same period, a national vaccination program prioritizing individuals according to their risk of death once infected has seen 35.5 million people (67.6% of the adult population) receive at least one dose as of the 10th of May.

Despite the predominant lineage of SARS-CoV-2 during this period (B.1.1.7) being more transmissible than the previous wild-type strain, the incidence of hospitalizations and deaths has dropped considerably – to only 2.6% and 0.7%, respectively from the peak values in January.

The third step of the roadmap in England took place on the 17th of May, with indoor hospitality reopening, the travel ban being lifted, and some mass events being allowed to restart.

Prevalence of national swab-positivity for England estimated using a P-spline for all eleven rounds with central 50% (dark grey) and 95% (light grey) posterior credible intervals. Shown here only for the entire period of the study with a Log10 y-axis. Unweighted observations (black dots) and 95% binomial confidence intervals (vertical lines) are also shown. Note that the period between round 7 and round 8 (December) of the model is not included as there were no data available to capture the late December peak of the epidemic.

More about round 11 of the REACT-1 study

Now Riley and colleagues have reported the findings from round 11 of the REACT-1 study where self-administered swabs are obtained from a representative cross-sectional sample of the population in England and tested for SARS-CoV-2 infection by polymerase chain reaction (PCR).

The results of Round 11, which was carried out between the 15th of April and the 3rd of May 2021, were compared with those from round 10 carried out between the 11th and 30th March 2021.

Weighted prevalence of swab-positivity by region for rounds 10 and 11. Bars show 95% confidence intervals.

Weighted prevalence of swab-positivity by region for rounds 10 and 11. Bars show 95% confidence intervals.

In the eleventh round, 115 of 127,408 swabs tested PCR-positive for SARS-CoV-2 infection, giving a prevalence of 0.1%. This compared with a prevalence of 0.2% in round 10, thereby representing a 50% decrease in round 11 since the previous round.

Among 55 to 64 year-olds, the rate of swab positivity fell by two-thirds from 0.17% in round 10 to 0.06% in round 11, which may reflect the effects of the recent vaccination roll-out among this age group.

By contrast, the rate of swab positivity among 25 to 34 year-olds was similar between round 11 (0.21%)  and round 10 (0.18%), despite a background of reducing prevalence.

“This may reflect increased contact rates, due to the easing of restrictions, in this age group where there are low levels of vaccination coverage,” says Riley and the team.

Prevalence was higher among those of Asian ethnicity

Prevalence in round 11 was higher among individuals of Asian ethnicity than among white individuals, at 0.31% versus 0.09%.

Multivariable logistic regression analysis estimated that the likelihood of infection was 88% higher among individuals of Asian ethnicity, compared with white individuals.  

“This suggests that infections may be continuing to spread more rapidly among the Asian community than in other groups in the population,” writes the team.

Based on the sequence data available on positive samples for which a lineage could be identified, the researchers estimated that 92.3% of infections were of the B.1.1.7 lineage and that 7.7% were of the B.1.617.2 lineage.

“Our viral sequencing data indicate that the B.1.1.7 lineage remains dominant in England, but that B.1.617.2 is also circulating in London, consistent with reports from Public Health England,” they write.

Vaccination may be breaking the link between infection, hospitalizations and deaths

Riley and colleagues also observed a divergence from the previous close alignment between infection incidence measured in REACT-1 and the national rate of hospitalizations and deaths recorded by routine surveillance.

The team says the divergence began in January 2021, coinciding with the mass roll-out of vaccination to the elderly and most vulnerable in society.

The researchers say this suggests that the vaccination program may be contributing to lower rates of severe outcomes in older age groups, effectively breaking the link between infection, hospitalizations and deaths.

“However, there is potential upwards pressure on prevalence from the further easing of lockdown regulations and the presence of the B.1.617.2 lineage,” warns the team.


“If prevalence rises in the coming weeks, policy-makers will need to assess the possible impact on hospitalizations and deaths. In addition, consideration should be given to other health and economic impacts if increased levels of community transmission occur,” concludes Riley and colleagues.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information

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