With reinfections now included in the daily Covid case numbers for most countries in the UK, we take a look at why the shift matters.
How are reinfections counted?
Covid reinfections can involve either the same or a different variant. Since 31 January reinfections have been included in the daily reported case numbers for England and Northern Ireland. Reinfections are expected to be included for Scotland in weeks, and were already included in figures for Wales.
While the definition of a reinfection for England and Northern Ireland is a positive specimen at least 90 days after the last one, for Wales that gap is at present defined as at least 42 days.
The UK Health Security Agency (UKHSA) says a gap of at least 90 days captures the majority of reinfections, while excluding those for whom fragments of the virus are detected for a long period of time. The agency says the interval will be standardised across UK countries in the coming weeks.
How many reinfections have there been?
According to UKHSA data for England, at present, possible reinfections account for about 10% of new cases reported each week. An estimated 532,570 reinfection episodes have been identified in England between the beginning of the pandemic and 23 January this year. This is a small figure compared with the 14.2m cases believed to be first infections.
However, these figures are based on people who have come forward for a Covid test. Not all those with a Covid infection do so – while during the first wave, access to tests was very limited, meaning both cases and reinfections were underestimated.
Are some people more likely to be reinfected than others?
According to data from the Office for National Statistics (ONS) – which uses an episode interval of 120 days, or four consecutive negative tests, in its definition of a reinfection – unvaccinated people are about twice as likely to have a reinfection as those who had their second Covid jab between 14 and 89 days ago. Reinfection also appears to be more likely among people whose previous infection involved a lower quantity of the virus, and among those who had their Covid jabs more than 90 days ago.
But other factors are also at play. “Reinfection is a result of a combination of exposure and risk once exposed,” said Prof Rowland Kao, an epidemiologist at the University of Edinburgh and member of the modelling group Spi-M, adding that the risk once exposed can be affected by elements such as the time since last infection, as well as past health.
“It seems that a lot of those factors do come together to potentially increase observed reinfection rates – so many of the people who are in high-exposure jobs also are more likely to have health issues because the factors allied to deprivation are related to both,” he said.
Has Omicron had an effect on reinfections?
Yes. According to ONS data, the risk of reinfection was 16 times higher when Omicron became dominant in the UKcompared with the period when Delta dominated. That is not surprising: the variant has been found to be able to dodge the body’s immune responses to a significant degree.
However, many of the new reinfections would have been among people who had Delta, or another variant, previously, given that the first Omicron cases were only picked up in the UK in late November. The question remains how well those who have had Omicron are protected against a second Omicron infection, or infections with new variants.
