A new study by Imperial College London suggested on Wednesday that about two-thirds of those infected with omicron have previously had COVID-19.

Imperial College London’s study, known as REACT-1, was comprised of 100,000 COVID tests being sent to people across England, alongside a questionnaire.

The questionnaire included asking the individuals if they had coronavirus before, and if they had, if this was confirmed with a test or if they just suspected that they had had it.

Almost two-thirds, 64.5%, of those who tested positive, over 3,500 people, between Jan. 5 to Jan. 20 said they previously tested positive. A further 7.5% said they suspected that they had had it.

Prof. Paul Elliott, who is the director of the REACT program and chairman in Epidemiology and Public Health Medicine at Imperial College London, said these cases were not technically reinfections, because they did not know exactly when they tested positive, and so there was the possibility some people tested positive twice for the same infection.

A reinfection is classified as such when a person tests positive at least 90 days apart.

“They may have had a positive test for omicron, for all I know, in December, and then got tested by us. We don’t know when they had it, it could have been in May 2020, or last week,” Elliott said.

The study also found that omicron has almost entirely replaced the delta variant.

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