Reinfections are being added now because they are much more prevalent than in previous waves, when people were only counted once to avoid recording the same infection twice in the data.
Findings published last week by the ONS suggest the risk of reinfection was 16 times higher in the omicron-dominant period in December and January compared to the delta-dominant period.
Now, if someone tests positive 90 days or more after a first infection it will count as a new infection.
Estimates of how many reinfections have been missed out of the data since omicron emerged are currently ranging between 10 and 64 per cent depending on which study you use.
There have been around six million positive cases reported in Britain since omicron got a foothold at the start of December, so we can expect hundreds of thousands of missed cases to be added, although only the English data will be updated on Monday.
There is a chance that the new case additions could be in the millions. Data from Imperial College London’s React-1 survey suggest that two thirds of the people infected with omicron have previously already had Covid.
Poised for ‘dramatic increases’
More than 3,500 people who tested positive between January 5 and January 20 were asked if they had previously tested positive for the virus, and almost two-thirds (64.5 per cent) said they had.
A further 7.5 per cent said they suspected they had caught the virus previously, but had not received a positive test.
In contrast, UK Health Security Agency figures show 11 per cent of all cases were reinfections, however experts said they are poised for “dramatic increases”.
As well as a large rise in the cumulative total, daily figures will also be noticeably higher.
Prevalence of Covid reached an all-time high in January, peaking at more than four per cent of the population, equivalent to around one infected person per 23 people. About one quarter of the population has tested positive at least once.
As well as a fall in death rates, the change will also bring a fall in the ratio of cases to hospital admissions.
At the peak of the 2021 winter wave, the number of cases ending up in hospital rose to 12 per cent, but it has since fallen to 1.95 per cent and will come down again with new cases added.
We can be thankful that these figures are only coming to light now when we already know that this wave was mild and did not overwhelm the NHS.
Had we been given this data before Christmas, the Government would have found it tricky to resist calls for more restrictions. It is the one time in this pandemic when knowing too little may have been a blessing.