Why it matters that the latest Covid data includes possible reinfections

The government's daily Covid figures now include possible reinfections for the first time, meaning someone who tests positive for the virus more than 90 days after a previous positive result will be recorded as a new case again.

Until now, the daily figures for cases included only people who had reported themselves as testing positive for the virus for the first time.

What happens to the data on the dashboard now?

The number of cases reported in the UK now includes possible reinfections in England and Northern Ireland, with data for Scotland and Wales to be added in the near future.

The change has been applied to previous data on the dashboard showing the number of cases by specimen date – when a person tested positive – but not historic data for cases reported each day.

But the cumulative number of UK cases since the pandemic began has been adjusted to incorporate all possible reinfections.

It means it jumped from 16.5 million as of January 30 to 17.3 million as of January 31.

Coronavirus: What you need to know

Why the change?

The UK Health Security Agency said: "As the pandemic continues and more variants emerge, it is more likely that people will be reinfected with Covid-19.

"UK public health agencies are now updating surveillance data to count infection episodes, including reinfection episodes.

"It is right that our daily reporting processes reflect how the virus has changed."

So will all infections of Covid in the UK be covered?

While the number of cases announced each day will now be slightly higher than previously, it will still be under-reporting the actual number of people getting infected with the virus.

That's because the dashboard data is based only on people who have reported themselves as testing positive for the virus – and is therefore affected by how many people are deciding to take a test, how many are choosing to report their results, and how many are taking a test because they know they have symptoms.

The most reliable estimate of the number of cases of coronavirus per day across the country is published as part of the weekly infection survey by the Office for National Statistics (ONS).

Their estimates are based on analysis of nose and throat swabs taken from a representative sample of more than 150,000 people in private households.

The same people are sampled every week, regardless of whether they know they have Covid-19 or have reported a positive result.

Professor of health protection at Anglia University, Paul Hunter, outlines what the change has done to the data - and how it will impact the speed of the epidemic:

"The fact that reinfections have become much more common since the emergence of the Omicron variant meant that not reporting such reinfections was no longer tenable," he said.

Up to Sunday evening there were a total of 14,257,268 first episodes (equivalent to cases under the prior definition in England and 588,114 possible reinfections. This represents an overall increase of 4.1% in headline reported numbers.

But the number of reinfections relative to first infections has increased considerably in the past two months with Omicron.

Prior to 1 December 2021, there were 8,872,932 primary and only 82,080 (0.9% increase) possible reinfections. In December there have been 2,616,148 primary, 200,176 (7.7% increase) reinfections reflecting the much greater risk of reinfections associated with Omicron.

So far in January there have been 2,768,188 primary and 305,858 (11.0% increase) reinfections continuing this upward trend in proportion of infections due to reinfection.

The addition of reinfections have not much changed estimates of the speed of increase or decrease in the epidemic over recent weeks. 

Fortunately, reinfections are usually, though not always, less severe than primary infections and consequently we can expect the severity of Covid to decrease further as more and more of the daily infections are reinfections.