However, large reductions in infection are evident in secondary school children and older adults because of the impact of vaccination
Cases of COVID-19 in England underwent a steep rise in the first half of December 2021 when the Omicron variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) began to spread rapidly, according to a new preliminary analysis.
However, large reductions in infection were seen in secondary school children and older adults between 23 November and 14 December, largely because of the impact of vaccination, scientists at Imperial College London (ICL) have reported.
The latest results in Round 16 of the Real-time Assessment of Community Transmission-1 (REACT-1) study, published as a preprint, suggest that an average of 1.41% of people in England were infected, equating to around 1 in 70 people, during the study period.
Infection survey figures from the Office for National Statistics on 24 December estimate that 1 in 45 people in England had COVID-19 in the week ending 16 December.
Omicron rising rapidly and replacing Delta
Of 275 samples analysed by the ICL team between 1 and 11 December, most were the Delta variant, and 11 were Omicron.
Professor Paul Elliott, Director of the REACT programme and who led the investigation, said that, from 1 December, ‘the proportion of samples that we sequenced that are Omicron has been rapidly increasing, reflecting both the replacement of Delta by Omicron, but also the rapid rise in Omicron.’
He told a briefing hosted by the Science Media Centre that ‘we’re estimating a daily growth of 66% in the odds, or proportion, of Omicron versus Delta infection, and we estimate there’s around 8–9 days for Omicron to go from a proportion of 10% to 90%, which is around three to four times faster than when we observed Delta replacing Alpha earlier in the year.’
The R number in England was estimated to be 1.13 for the entire study period, rising to 1.27 from 1 December.
London had the highest prevalence of any region, with an R number of 1.62 in December and an estimated daily prevalence of 6.07% of the city’s population on 14 December.
Impact of booster doses
The study found that the risk of infection was much lower in people who had received a third or booster dose compared with those who had received only two doses.
For those aged 50 years and above, having a booster dose reduced the infection risk by around half compared with two doses.
Among people aged 65–74 years, prevalence of infection fell by over 40%, and for those aged 75 years, by two-thirds.
The researchers conclude that ‘the rapidly increasing prevalence of SARS-CoV-2 infections in England during December 2021, coincident with the rapid rise of Omicron infections, may lead to renewed pressure on health services.’
They add: ‘Additional measures beyond vaccination may be needed to control the current wave of infections and prevent health services—in England and other countries—from being overwhelmed.’
Commenting on the study, Kevin McConway, Emeritus Professor of Applied Statistics at The Open University, said: ‘We mustn’t forget that Omicron hadn’t become dominant during this REACT-1 round (though it was present later in the round), and the effectiveness of boosters is thought to be less for Omicron than for Delta.
‘But other investigations have found that there still is a pretty strong protective effect of booster vaccination against Omicron.’
Infection by age groups
By age, the highest prevalence was observed in unvaccinated primary school children aged 5–11 years at 4.74%, similar to the previous study round conducted between 19 October and 5 November.
In contrast, prevalence in children aged 12–17 years fell from 5.35% to 2.31% in the latest study round. The authors note that 76.6% of children in this age category had received at least one dose of a COVID-19 vaccine as of 14 December.
Vaccine effectiveness against infection was estimated at 57.9% in 12–17 year olds for those who had received one or two vaccine doses.
This article originally appeared on Medscape, part of the Medscape Professional Network.
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