Imperial College London: REACT study shows rapid rise in Omicron while highlighting vaccine success

Results from swab tests taken at home by over 97,000 people in England between 23 November and 14 December showed that 1.41% of people were infected, or around 1 in 70. This is slightly lower than the study’s previous findings when around 1 in 64 was infected, or 1.57% of people, as of 5 November.

These findings reflect slight a fall of infections in November, with large drops observed in secondary school-aged children and older adults, followed by a steep rise in the overall rate of infections in December, when the Omicron variant began to spread rapidly. The R number was estimated to be almost 1.3 in December.

“Compared to the Delta variant, the proportion of Omicron cases is increasing rapidly.”
Prof Paul Elliott
School of Public Health
Infections decreased by 40% or more in 12-17-year-olds and those aged 65 and above, reflecting the impact of the vaccination programme, yet they have been increasing in adults aged 18-54, and have remained relatively stable in primary school-aged children.

Out of 650 samples analysed between 1 and 11 December, almost all were the Delta variant and 11 were Omicron, all of whom were between the ages of 18 and 54 and double vaccinated (reflecting the large numbers of people who have received two doses of vaccine in this age group) but not boosted. The proportion of Omicron infections compared to Delta rapidly increased during this time, mostly concentrated in the South of England. Based on these data, the scientists estimate that there were on average 31,000 prevalent Omicron infections across this period.

The study, led by Imperial College London with Ipsos MORI, also found variation in infection rates across the country with the highest number of infections in London, which had an estimated R number of 1.62 in December.

Rapid spread of Omicron
Professor Paul Elliott, director of the REACT programme from Imperial’s School of Public Health, said: “The results reported in this round of REACT show that Omicron is spreading rapidly in England, especially in London, which now has the highest prevalence of COVID-19 in the country. Compared to the Delta variant, the proportion of Omicron cases is increasing rapidly.

“The positive news is that both the teenage vaccination and booster programmes have already shown encouraging results in protecting against infection (assumed to be mainly Delta in the REACT data so far), with prevalence amongst 12-17-year-olds and those aged 65 and above dropping significantly since the beginning of November.

“However, we have seen prevalence overall rise sharply again since the beginning of December, which aligns with the rapid increase in the Omicron variant and the growing number of COVID-19 cases being reported nationally. It is therefore vital that as many people as possible get vaccinated including their boosters and take sensible precautions such as mask-wearing to reduce the risk of infection.”

These findings from the ongoing REal-time Assessment of Community Transmission (REACT-1) programme, commissioned by the Department of Health and Social Care, are available here in a pre-print report and have been published in the journal Science. Data are continually reported to the Government to inform decision-making.

High and rising infection rates in London
For this latest round of the REACT study, 97,089 people swabbed themselves at home and their samples were analysed by PCR testing. 1,192 of these were positive, giving an overall weighted prevalence of 1.41%. Weighting is where the researchers make adjustments to their calculations to ensure the sample reflects England’s population.

“The REACT study provides vital insight into the prevalence of COVID-19 and, crucially, is helping us understand more about the Omicron variant.”
Dr Jenny Harries
UK Health Security Agency
For the entire study period, the R number was estimated to be 1.13, meaning 100 infected people would on average pass the virus on to 113 people. For December only, R was estimated to be 1.27.

By region, London had a high and increasing prevalence of infection, with the highest rate in the country at 1.84%. R was estimated to be 1.42 across the study period in London, rising to 1.62 in December. The lowest prevalence was found in the North East.

The first Omicron infection detected in this round was found in London, with subsequent cases mostly in the South of England. Of the 11 detected in total, seven were symptomatic (five with classic COVID-19 symptoms: loss or change in sense of taste or smell, fever, persistent cough) and two were asymptomatic. The symptoms for the remaining two infections were unknown.

Dr Jenny Harries, Chief Executive of the UK Health Security Agency, said: “I’d like to say a really big thank you to the members of the public who continue to take part in the REACT study. It provides vital insight into the prevalence of COVID-19 and, crucially, is helping us understand more about the Omicron variant.

“Omicron is spreading fast and the COVID-19 vaccine remains our best line of defence against it. I urge everyone who is eligible to come forward to receive their latest jab without delay – whether that’s a first, second, third or booster dose.”

The impact of COVID-19 vaccination
Infections decreased by more than half in 12-17-year-olds compared to the previous round, from 1 in 19 (5.35%) to 1 in 44 (2.31%). By comparing vaccinated and unvaccinated participants in this age group, the scientists estimated that a single dose reduced infection risk by more than half (55.7%), and by 57.9% for those having received one or two vaccine doses. In this age group, most of the infections were likely to be the Delta variant. The researchers will analyse vaccine effectiveness against the Omicron variant when more data are available.

“The latest REACT-1 data is yet more evidence that boosters are vital in protecting us from the Omicron variant.”
Sajid Javid
Health and Social Care Secretary
The researchers also compared the impact of three versus two doses in adults aged 18 and above, finding that people who had received a booster were three to four times less likely to be infected than those who had had two doses.

Infections also fell by two-thirds in those aged 75 and above (0.63% to 0.21%), and by 40%in those aged 65-74 (0.84% to 0.48%), which is likely largely attributable to booster doses.

Health and Social Care Secretary Sajid Javid said: “The latest REACT-1 data is yet more evidence that boosters are vital in protecting us from the Omicron variant.

“While infections may be rising rapidly across the country, you can protect yourself, your friends, family and community by getting boosted now – like 28 million others across the UK so far.”

Kelly Beaver, CEO at Ipsos MORI, said: “The latest REACT round finds an R number above 1 and high prevalence of COVID-19 in England, so it remains critical that people get vaccinated and boosted.

“We have found a number of cases of the Omicron variant, demonstrating the speed at which it is becoming the dominant variant and highlighting why we must all exercise caution over the festive period to ensure that prevalence does not continue to rise even further in the new year.”

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