Nearly two-thirds of people infected with the omicron variant in the United Kingdom in January had previously had Covid-19, according to a recent preprint study—findings that align with other research indicating omicron is adept at evading immunity.

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Study details

For the study, researchers looked at 100,607 PCR tests between Jan. 5 and Jan. 20 that were collected as part of Imperial College London's ongoing Covid-19 monitoring study known as React.

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The researchers found that out of the 4,011 people who had a positive PCR test, 64.6% said they previously had recovered from Covid-19, and 7.5% said they suspected they previously had Covid-19 but it was not confirmed by a positive test. Of the positive test swabs that were sequenced, 99% were omicron and 1% were delta, according to the study.

However, the researchers cautioned the study results "are based on self-reported data and therefore it's uncertain what proportion of these are reinfections or recent infection picked up due to the sensitivity of PCR testing."

The researchers also found that during the study period, 4.41% of people tested positive for the coronavirus, the highest positivity rate observed in the study since it began in May 2020.

"Vaccination (including the booster campaign) remains the mainstay of the defense against [Covid-19] given the high levels of protection against hospitalizations," the researchers wrote. "However, further measures beyond vaccination may be required if the very high rates of omicron infection persist, despite omicron appearing to be intrinsically less likely to cause severe disease."

Omicron's ability to evade immunity

The results align with other research that has found omicron is able to more easily evade immunity than previous variants.

One preprint study from researchers in Qatar found that prior Covid-19 infection was 90% protective against symptomatic reinfection with the alpha variant, 92% protective against the delta variant, and just 56% protective against omicron.

However, reinfection with omicron appears to generally be mild, research has found. The Qatari study found prior infection was 69% protective against hospitalization with alpha, 100% protective against delta, and 88% protective against omicron.

Similarly, a CDC study from December 2021 looked at a family of six who all tested positive for omicron in early December. The researchers found that five members of the family, including the one person who was fully vaccinated, previously had Covid-19. Those five members experienced milder symptoms than the one unvaccinated family member who had never previously had Covid-19. (Lonas, The Hill, 1/26; McDermott, New York Post, 1/26; Carbajal, Becker's Hospital Review, 1/26; Taylor, CNBC, 1/26; Owens, Axios, 1/7; Subbaraman/Hookway, Wall Street Journal, 12/28/21)