REACT-1 round 9 final report: Continued but slowing decline of prevalence of
SARS-CoV-2 during national lockdown in England in February 2021
Steven Riley
1,2,
*, Haowei Wang
1,2
, Oliver Eales
1,2
, David Haw
1,2
, Caroline E. Walters
1,2
,Kylie
E. C. Ainslie
1,2,12
, Christina Atchison
1
, Claudio Fronterre
3
, Peter J. Diggle
3
, Deborah Ashby
1
,
Christl A. Donnelly
1,2,4
, Graham Cooke
5,6,7
, Wendy Barclay
5
, Helen Ward
1,6,7
, Ara Darzi
6,7,8
,
Paul Elliott
1,6,7,9,10,11
*
1
School of Public Health, Imperial College London, UK
2
MRC Centre for Global infectious Disease Analysis and Abdul Latif Jameel Institute for
Disease and Emergency Analytics, Imperial College London, UK
3
CHICAS, Lancaster Medical School, Lancaster University, UK and Health Data Research,
UK
4
Department of Statistics, University of Oxford, UK
5
Department of Infectious Disease, Imperial College London, UK
6
Imperial College Healthcare NHS Trust, UK
7
National Institute for Health Research Imperial Biomedical Research Centre, UK
8
Institute of Global Health Innovation at Imperial College London, UK
9
MRC Centre for Environment and Health, School of Public Health, Imperial College
London, UK
10
Health Data Research (HDR) UK London at Imperial College
11
UK Dementia Research Institute at Imperial College
12
Centre for Infectious Disease Control, National Institute for Public Health and the
Environment, Bilthoven, The Netherlands
*Corresponding authors: Steven Riley and Paul Elliott, s.riley@imperial.ac.uk,
p.elliott@imperial.ac.uk, School of Public Health, Imperial College London, Norfolk Place,
London, W2 1PG
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Abstract
Background
England will start to exit its third national lockdown in response to the COVID-19 pandemic
on 8th March 2021, with safe effective vaccines being rolled out rapidly against a
background of emerging transmissible and immunologically novel variants of SARS-CoV-2.
A subsequent increase in community prevalence of infection could delay further relaxation of
lockdown if vaccine uptake and efficacy are not sufficiently high to prevent increased
pressure on healthcare services.
Methods
The PCR self-swab arm of the REal-time Assessment of Community Transmission Study
(REACT-1) estimates community prevalence of SARS-CoV-2 infection in England based on
random cross-sections of the population ages five and over. Here, we present results from
the complete round 9 of REACT-1 comprising round 9a in which swabs were collected from
4th to 12th February 2021 and round 9b from 13th to 23rd February 2021. We also compare
the results of REACT-1 round 9 to round 8, in which swabs were collected mainly from 6th
January to 22nd January 2021.
Results
Out of 165,456 results for round 9 overall, 689 were positive. Overall weighted prevalence of
infection in the community in England was 0.49% (0.44%, 0.55%), representing a fall of over
two thirds from round 8. However the rate of decline of the epidemic has slowed from 15 (13,
17) days, estimated for the period from the end of round 8 to the start of round 9, to 31 days
estimated using data from round 9 alone (lower confidence limit 17 days). When comparing
round 9a to 9b there were apparent falls in four regions, no apparent change in one region
and apparent rises in four regions, including London where there was a suggestion of
sub-regional heterogeneity in growth and decline. Smoothed prevalence maps suggest large
contiguous areas of growth and decline that do not align with administrative regions.
Prevalence fell by 50% or more across all age groups in round 9 compared to round 8, with
prevalence (round 9) ranging from 0.21% in those aged 65 and over to 0.71% in those aged
13 to 17 years. Round 9 prevalence was highest among Pakistani participants at 2.1%
compared to white participants at 0.45% and Black participants at 0.83%. There were higher
adjusted odds of infection for healthcare and care home workers, for those working in public
transport and those working in education, school, nursery or childcare and lower adjusted
odds for those not required to work outside the home.
Conclusions
Community prevalence of swab-positivity has declined markedly between January and
February 2021 during lockdown in England, but remains high; the rate of decline has slowed
in the most recent period, with a suggestion of pockets of growth. Continued adherence to
social distancing and public health measures is required so that infection rates fall to much
lower levels. This will help to ensure that the benefits of the vaccination roll-out programme
in England are fully realised.
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Introduction
Most European countries started 2021 with high numbers of COVID-19 cases [1] and
stringent interventions [2]. During the first eight weeks of the year, cases fell across Europe
before starting to increase again during February [1]. England entered its third national
lockdown on 5th January 2021 [3] with the seven-day rolling average of cases at 95 per
100,000. By the 22nd February 2021, cases had fallen to an average of 18 per 100,000
during the previous seven days [4]. Over the same period, daily hospital admissions for
COVID-19 fell from 3,592 to 949 (seven-day average) [4]. By the 22nd February, 15,113,158
people in England had received at least one dose [4] of either the BNT162b2 mRNA [5] or
ChAdOx1 [6] COVID-19 vaccines.
On 22nd February 2021, the UK government announced a plan (roadmap) for the gradual
easing of the lockdown to start on 8th March 2021 with the opening of schools before
proceeding in four additional steps [7]. The roadmap leaves five-week minimum periods
between steps to allow time for the impact of each set of relaxations on the epidemic to be
assessed against four criteria: successful continuation of the vaccine roll-out programme,
good efficacy of the vaccine against hospitalisations and deaths, no substantial change in
the overall risk assessment of the pandemic because of SARS-CoV-2 variants, and no
evidence that an increase in infections in the community may lead to a surge in
hospitalisations such that healthcare services would be placed under unsustainable
pressure.
Largely in line with cases, prevalence of infections in the community in England dropped
substantially during January and early February 2021 [8,9]. Any large uptick in the
prevalence of infections would signal a potential threat to the smooth continuation of the
roadmap; but with high vaccine uptake [4] and encouraging early estimates of vaccine
efficacy [5], the link between infections and strain on healthcare services will likely be
weakened in the near future.
The PCR self-swab arm of the REal-time Assessment of Community Transmission Study
(REACT-1) is designed to measure community prevalence of SARS-CoV-2 infection in
England [10]. Here, we present results from the complete round 9 of REACT-1 comprising
round 9a in which swabs were collected from 4th to 12th February 2021 and round 9b from
13th to 23rd February 2021. We also compare the results of REACT-1 round 9 to round 8, in
which swabs were collected mainly from 6th January to 22nd January 2021.
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Results
REACT-1 round 9 included 165,456 individuals with a valid swab result of whom 689 tested
positive, giving a weighted prevalence overall of 0.49% (0.44%, 0.55%), down by over two
thirds from 1.57% (1.49%, 1.66%) in round 8 (Table 1). The round 9 data comprised 388
positives from 87,408 swabs in round 9a, with weighted prevalence of 0.51% (0.44%,
0.59%) and 301 positives from 78,047 swabs in round 9b, with a weighted prevalence
slightly lower at 0.47% (0.40%, 0.55%) .
Using a constant growth rate model, we estimated halving times and R numbers for England
using two time periods: from the second half of round 8 (8b) to round 9a and within round 9
(Table 2). We estimated a halving time of 15 (13, 17) days, corresponding to an R of 0.73
(0.69, 0.76) between rounds 8b and 9a, whereas from round 9a to 9b we estimated a
halving time of 31 days (lower confidence limit 17 days) and a corresponding R of 0.86
(0.76, 0.97). We thus observe a slowing in the rate of decline of the epidemic in England on
comparing these estimates (probability of difference in Rs > 0.99, not accounting for overlap
in the time periods), although note that the estimate of R in the most recent period is still
reliably below one. This slowing is reflected in the fitted P-spline which shows a flattening off
in the most recent period (Figure 1).
The decline in prevalence from round 8 to round 9 was seen in all age groups, which was
50% or more over this period (Table 3, Table 4, Figure 2). In the latter half of round 9 (9b),
prevalence varied from 0.21% (0.14%, 0.31%) in those aged 65 and over to 0.71% (0.34%,
1.45%) in those aged 13 to 17 years.
We observed differences in patterns of regional prevalence between rounds 8 and 9 when
compared with patterns between rounds 9a and 9b (Table 3, Table 4, Figure 3). Between
rounds 8 and 9 there were substantial falls in weighted prevalence in seven of the nine
regions with smaller apparent falls in Yorkshire and The Humber and in North East.
However, between rounds 9a and 9b, while there were apparent falls in North East, North
West, East of England and South West, and no apparent change in Yorkshire and The
Humber, there were apparent rises in London, South East, East Midlands and West
Midlands. Using a constant regional growth rate model, we found evidence suggestive of
growth (80% or greater probability) in London and South East and robust evidence of
decline in North West (Table 5). These patterns were also reflected in the regional fitted
P-splines (Figure 4).
Maps for rounds 9a and 9b of unweighted, unsmoothed swab-positivity at the level of
lower-tier local authorities (LTLA) suggest reductions in prevalence in eastern part of North
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West region, but are otherwise difficult to interpret with highly fragmented groups of high and
low prevalence areas (Figure 5). We therefore used nearest neighbours within the study
sample to smooth LTLA prevalence. We aimed to reveal underlying spatial structure and
thus allow visualisation of apparent growth or decline. The resultant maps suggest large
contiguous areas of apparent increasing or decreasing prevalence when comparing rounds
9a and 9b (Figure 6). A long tract of apparent increasing prevalence runs from the south
coast, through south and west London (Figure 7) into the Midlands and then on to the west
side of Yorkshire and the Humber. One contiguous area of decreasing prevalence was seen
in north and east London, the southern part of East of England and the northern part of
South East, with another in the western part of South West. We also observed sharp
declines in the conurbations in the North West reflecting the overall decline in prevalence in
that region.
We observed robust patterns in the prevalence of swab-positivity for ethnicity subgroups in
both rounds 8 and 9 (Table 3b). In round 8, unweighted prevalence among Bangladeshi
participants was very high at 6.1% (4.0%, 9.3%) compared to 1.2% (1.2%, 1.3%) in white
participants. However unweighted prevalence in Bangladeshi fell to 0.85% (0.29%, 2.5%) in
round 9 (we report unweighted prevalence for Bangladeshi participants because of small
numbers of positives in round 9). In round 9, the highest weighted prevalence was amongst
Pakistani participants at 2.1% (1.0%, 4.2%) compared with white participants at 0.41%
(0.37%, 0.46%). Corresponding odds ratios adjusted for: age, sex, region, deprivation (core
variables), showed similar patterns (Table 6).
We also give prevalence and adjusted odds ratios for occupation for both rounds 8 and 9
(Table 3c, Table 4, Table 7). Healthcare workers and care home workers had higher adjusted
odds of infection at 1.48 (1.25, 1.77) in round 8 and 1.37 (1.02, 1.86) in round 9 (Table 4)
compared with other workers. Higher adjusted odds were seen in participants who worked in
public transport at 2.17 (1.58, 2.97) in round 8, and 2.14 (1.20, 3.83) in round 9, compared
with those who did not; higher adjusted odds were also seen in those working in education,
school, nursery or childcare at 1.20 (1.03, 1.39) in round 8 and 1.43 (1.07, 1.91) in round 9
compared with participants not working in those settings (Table 7). Lower adjusted odds of
swab-positivity were seen among those not currently required to work outside their home at
0.67 (0.61, 0.74) in round 8 and 0.64 (0.54, 0.76) in round 9 compared with those currently
required to work outside their home.
Community prevalence of infection, as measured by REACT-1, shows strong apparent
correlation with hospital admissions in England. With a fitted lag of 18 (18, 20) days between
dates of swab and admissions, the national trend in swab-positivity and hospital admissions
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