Two-thirds of those who have recently been infected with the Omicron variant report they have previously had Covid.
The findings came from React, a big, ongoing study in which thousands of participants in England were swab-tested.
Most research is needed to determine how many are real reinfections, but the findings do suggest which demographics are more likely to contract Covid again.
Healthcare workers and families with children or several people living under one roof are among them.
In total, over two million people have been tested as part of the project.
The most recent findings, for the first two weeks of 2022 (round 17), are based on around 100,000 polymerase chain reaction (PCR) tests that were sent to participants and then returned.
Around 4,000 people tested positive, the highest number since the pandemic began.
When a sample of them was sequenced to see what type of Covid was to blame, it turned out that nearly all of them were Omicron, the highly infectious form first discovered in South Africa and responsible for a large winter wave of infections in the UK.
It’s unclear how many of the participants who tested positive had received all of their vaccinations.
Two doses provide minimal protection against Omicron infection, although they do provide some protection against severe disease.
However, since Omicron’s arrival, booster dosages have been rapidly distributed to supplement people’s protection. Two out of every three infected volunteers (65%) stated that they had previously tested positive for Covid. It’s possible that many of these were reinfected.
Although, in other cases, the most recent PCR testing may be catching up ancient viral traces.
According to some estimations, one out of every ten Omicron cases could be a recurrence.
Currently, the government’s daily Covid case data, which reflect the number of infections detected by persons who come forward for testing, do not account for reinfections in all of the UK’s countries.
Coronavirus infections have slowed recently, but they are still common, especially among children and early teenagers, according to the React findings.
Prof Paul Elliott of Imperial College London, the director of the React programme, said: “Prevalence among children is rapidly increasing as they mix more after the start of the school year, and prevalence among older people, aged 65 and up, has increased since December… potentially leading to an increase in hospitalizations.
“It is therefore critical that we continue to keep a careful eye on the issue.”