Nearly two-thirds of people in England who tested positive for Covid-19 in January previously had the virus or suspected they had it, a large long-running infection survey revealed Wednesday.
The finding, in the latest report on coronavirus transmission by Imperial College London, is seen as further evidence of the Omicron variant’s ability to dodge the immune systems of those previously infected with the virus.
As part of its regular assessment, Imperial received around 100,000 valid swap tests self-administered by a random sample of people across England between Jan 5 and 20.
Approximately 4,000 showed a positive result, and nearly 3,600 of those individuals specified whether or not they had had Covid before.
Around 65% of those respondents had previously tested positive, while a further 7.5% said they suspected they had caught the virus previously but had not received a confirmatory test.
“Past infection was associated with high risk of reinfection with Omicron,” the study’s authors noted in its abstract.
However Paul Elliott, who directs the Imperial study, noted not all these cases could be confirmed as reinfections because some could be residual illness, meaning a person had tested positive twice for the same infection.
But the numbers reporting reinfections in the study are markedly higher than those published by the UK Health Security Agency, a public health protection body.
Its latest figures show 11% of all English cases were reinfections.
The UKHSA said Wednesday it would begin including reinfections data on the government’s rolling coronavirus data dashboard from the end of the month.
A reinfection is defined as a person testing positive twice at least 90 days apart.
The Imperial survey also confirmed Omicron, which has swept the UK since late November, is now responsible for nearly all new infections in England.
Positive cases among its randomly selected sample were three times higher in the January sampling period than the previous month, it found.
“We observed unprecedented levels of infection with SARS-CoV-2 in England in January 2022 and almost complete replacement of Delta by Omicron,” the report said.
However, the study detected that the prevalence of the virus among adults was starting to decline.
England on Thursday will lift the last of various virus curbs reimposed last month as Omicron surged nationwide, including the requirement to wear masks in most settings.
A so-called Covid pass system for nightclubs will also be scrapped, as the country returns to its lowest level of restrictions of any stage of the pandemic.