Background
The prevalence of SARS-CoV-2 infection continues to drive rates of illness and
hospitalisations despite high levels of vaccination, with the proportion of cases caused by the
Delta lineage increasing in many populations. As vaccination programs roll out globally and
social distancing is relaxed, future SARS-CoV-2 trends are uncertain.
Methods
We analysed prevalence trends and their drivers using reverse transcription-polymerase
chain reaction (RT-PCR) swab-positivity data from round 12 (between 20 May and 7 June
2021) and round 13 (between 24 June and 12 July 2021) of the REal-time Assessment of
Community Transmission-1 (REACT-1) study, with swabs sent to non-overlapping random
samples of the population ages 5 years and over in England.
Results
We observed sustained exponential growth with an average doubling time in round 13 of 25
days (lower Credible Interval of 15 days) and an increase in average prevalence from 0.15%
(0.12%, 0.18%) in round 12 to 0.63% (0.57%, 0.18%) in round 13. The rapid growth across
and within rounds appears to have been driven by complete replacement of Alpha variant by
Delta, and by the high prevalence in younger less-vaccinated age groups, with a nine-fold
increase between rounds 12 and 13 among those aged 13 to 17 years. Prevalence among
those who reported being unvaccinated was three-fold higher than those who reported being
fully vaccinated. However, in round 13, 44% of infections occurred in fully vaccinated
individuals, reflecting imperfect vaccine effectiveness against infection despite high overall
levels of vaccination. Using self-reported vaccination status, we estimated adjusted vaccine
effectiveness against infection in round 13 of 49% (22%, 67%) among participants aged 18
to 64 years, which rose to 58% (33%, 73%) when considering only strong positives (Cycle
threshold [Ct] values < 27); also, we estimated adjusted vaccine effectiveness against
symptomatic infection of 59% (23%, 78%), with any one of three common COVID-19
symptoms reported in the month prior to swabbing. Sex (round 13 only), ethnicity, household
size and local levels of deprivation jointly contributed to the risk of higher prevalence of
swab-positivity.
Discussion
From end May to beginning July 2021 in England, where there has been a highly successful
vaccination campaign with high vaccine uptake, infections were increasing exponentially
driven by the Delta variant and high infection prevalence among younger, unvaccinated
individuals despite double vaccination continuing to effectively reduce transmission.
Although slower growth or declining prevalence may be observed during the summer in the
northern hemisphere, increased mixing during the autumn in the presence of the Delta
variant may lead to renewed growth, even at high levels of vaccination.