A study has found that two-thirds of people recently infected with the Omicron variant have had coronavirus before.
The findings come from a large, continuing study, React, swab-testing thousands of volunteers in England.
Two out of every three (65%) of the infected volunteers said they had already previously tested positive for Covid.
It has found groups that appear to be more likely to catch the virus for a second time - including healthcare workers, households with children, or homes that have lots of people living under one roof.
More than two million people were tested in the study.
The latest findings, for the first two weeks of 2022 - round 17 - are based on about 100,000 PCR tests posted to volunteers and then returned.
About 4,000 were positive, by far the highest rate seen since the pandemic began.
And when a selection of them were sequenced to check what type of Covid was to blame, virtually all were Omicron - the highly infectious variant, first identified in South Africa, causing a big winter wave of infections in the UK.
It is not yet clear how many of the volunteers who tested positive had been fully vaccinated, but the study did find that two shots offer little protection against catching Omicron, although protection against severe disease wanes less.
But booster doses have been rolled out at speed since Omicron hit, to top up people's protection.
A GP has explained why some people are catching Covid-19 more than once.
Dr Alan Stout, chair of the British Medical Association's GP committee in Northern Ireland, said people "can absolutely get Covid twice" and it explained why case numbers here remain high. Dr Stout told Belfast Live : "One of the difficulties with the variants is that having one of the previous ones doesn't necessarily protect you from the next.
"That has become quite clear as the previous Delta infection we know now gives you virtually no protection or natural immunity from Omicron. That's the simplest reason why people are contracting Covid twice - it's down to the different variants.
"That's why the booster has been so important for Omicron because we've known that the level of immunity given by the first two vaccine doses has started to wane. Hence the booster was giving that extra level of protection but it was actually giving protection above and beyond, which is then preventing people getting Omicron or certainly becoming seriously unwell with it."
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