Cases of the Omicron variant may be in decline across the UK, but the strain still makes up the vast majority of Covid-19 infections.
Omicron has been shown to typically cause less serious illness, but also brings with it an increased risk of reinfection.
A study published by the World Health Organisation (WHO) and Imperial College London in December showed Omicron is five times more likely to reinfect people than the previously dominant Delta variant.
Analysis by the UK Health Security Agency (UKHSA) published in December showed that around one in 10 people with the Omicron variant in England had previously contracted Covid-19.
And a more recent Imperial study showed two-thirds of people who have had Omicron had previously been infected with a different strain.
So we know that it is very possible to catch Covid twice, and that Omicron makes that more likely, but how quickly can you be reinfected? Here is what you need to know.
How quickly can you get reinfected with Covid?
Research has shown that for people who recover from Covid-19, immunity can last for about three months to several years, making it very unlikely someone could catch the virus twice in a month – but not necessarily impossible.
Dr William Schaffner, professor of preventive medicine and infectious diseases at Vanderbilt University Medical Centre: “There are some people who have the notion that after you’ve a natural infection, you will be permanently protected against Covid-19 as if this were measles.
“But the two viruses that cause these infections are very, very different. The coronavirus protection wanes naturally after a period of time.”
An October 2021 study by the Yale School of Public Health reported that unvaccinated people should have immunity against reinfection for between three and 61 months.
However, this data was recorded and published before the emergence of the Omicron variant.
What officially counts as reinfection?
More than one positive test for any variant of Covid-19 within a 90-day time period are considered to be part of the same case episode, and if you test positive a matter of weeks after first getting a positive test it is more likely this is from the residual effects of the initial infection.
Positive tests outside the 90-day period are now counted as a reinfection.
This is a change to the previous UKHSA methodology, by which people who tested positive for Covid-19 were counted only once in case numbers published on the daily dashboard, when they first tested positive.
Repeat positive tests were not included.
Why is reinfection so common?
UK studies into Omicron have suggested that previous Covid infection provides poor protection against the new strain.
Researchers at Imperial College London found that the Omicron variant largely evades immunity from past Covid infection or two vaccine doses.
A study of all the PCR test-confirmed Covid cases in England between 29 November and 11 December concluded that patients who had already contracted coronavirus only had around 19 per cent protection against Omicron.
The figure was roughly in line with two doses of a Covid vaccine, which scientists estimated were as much as 20 per cent effective against the new variant.
Professor Neil Ferguson, from Imperial College London, said: “This study provides further evidence of the very substantial extent to which Omicron can evade prior immunity given by both infection or vaccination.”
Ministers noted that the data showed the importance of the booster jab, which is thought to provide protection against around 55 to 80 per cent of symptomatic cases.
Is it possible to get the Omicron variant twice?
At present there is not enough data to tell whether people can get reinfected with Omicron.
This is because most public health bodies define reinfection as two positive test results for the same individual 90 or more days apart.
Residual signs of Covid infection can show for up to three months on a PCR test, so the only reliable way of talking about reinfection is to include a 90-day window between positive tests.
The first Omicron case was only identified in the UK on 27 November, so there has not been enough time to reliably work out whether it is possible for people to get reinfected.
Omicron was first reported to the WHO) from South Africa on 24 November – just three days before it was first identified in the UK – so there is no reliable data from there either.
However, the UKHSA told i it was “definitely possible” to get Omicron more than once, in the same way that it is possible to get any Covid variant twice – but that UK studies have yet to look into how likely this is.
Dr Amesh Adalja, senior scholar at Johns Hopkins Centre for Health Security in the US, agreed: “It’s unclear, at this point, what level of immunity occurs after an Omicron infection. I suspect over time, yes, you probably can get reinfected. But we don’t have that data yet because Omicron has only been around since October/November.”