Cases of COVID-19 in England underwent a steep rise in the first half of December when the Omicron variant of 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) reported.
Latest results in Round 16 of the Real-time Assessment of Community Transmission-1 (REACT-1) study, published as a preprint, suggests that an average of 1.41% of people were infected, or around 1 in 70, during the study period.
Latest infection survey figures from the Office for National Statistics today estimated that 1 in 45 people in England had COVID in the week ending December 16.
Omicron Rising Rapidly and Replacing Delta
Out of 275 samples analysed by the ICL team between 1 and 11 December, most were the Delta variant, and 11 were Omicron.
Prof Paul Elliott, director of the REACT programme, who led the investigation, said: that from December 1 “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 to 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 December 1.
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 December 14.
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 to those who had received only two doses.
For those aged 50 and above, having a booster dose reduced the infection risk by around half compared with two doses.
Among people aged 65 to 74, prevalence of infection fell by over 40%, and for those aged 75, 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 that: “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 at 4.74%, similar to the previous study round conducted between October 19 and November 5.
In contrast, prevalence in children aged 12-17 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 vaccine as of December 14.
Vaccine effectiveness against infection was estimated at 57.9% in 12- to 17-year-olds for those who had received one or two vaccine doses.
