As the omicron variant of the coronavirus continues to spread, many people who have already contracted the virus once are picking it up again. But a key question remains unresolved: for how long after a covid-19 infection are people likely to be protected from reinfection?
A lot hinges on the answer, as new omicron subvariants called BA.4 and BA.5 are on the rise in South Africa. It is still unclear if this will translate into a large wave in that country or elsewhere.
With some infections, such as measles, if someone has had the disease once, they rarely catch it a second time. Coronaviruses don鈥檛 seem to provoke such 鈥渟terilising immunity鈥�, although a first infection does reduce people鈥檚 risk of a second one.
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In unvaccinated people, for instance, a first infection , according to a large US study. This is similar to the level of protection provided by two doses of the mRNA covid-19 vaccines and was stable over the nine months of the study. 鈥淚 was surprised that we found such a high level of protection,鈥� says at the University of Chicago.
Ridgway鈥檚 study was done while the delta variant was the dominant virus strain circulating globally, though. Omicron causes more reinfections than past variants, because of mutations in its spike protein that help it to evade immunity. During the UK鈥檚 recent omicron wave, reinfections were , according to the Office for National Statistics (ONS).
This still doesn鈥檛 tell us how long most people can expect to wait between covid-19 infections, however. Indeed, it isn鈥檛 yet possible to calculate this figure with any accuracy, although we can get a hint from existing data. For instance, the ONS has , covering the whole of the pandemic up to mid-December 2021, when omicron had been dominant for several weeks. The study found the interval between reinfections ranged from 90 to 650 days, with the average being 343 days, or nearly one year.
But these figures could have two sources of error. First, they could be too low an estimate because many people haven鈥檛 yet had two infections. When they have, and their data can be added to the calculations, the average interval between infections should increase.
Second, some reinfections will be going unrecognised, as the ONS classes someone as being reinfected only if more than 90 days has passed since a previous infection confirmed by a PCR test. That is because these tests can lead to false-positive results within this period. This approach 鈥渋dentifies the majority of reinfections while excluding those people who may still have virus detected for some time after their initial infection鈥�, says at the UK Health Security Agency.
But reinfections within 90 days can happen, as shown by a , which experienced a surge of omicron beginning in November 2021, caused by peaks of omicron subvariants BA.1 and BA.2 merging into each other. at the Statens Serum Institute in Copenhagen, Denmark, and his colleagues managed to find 47 people who were infected with both variants, with intervals of 20 to 60 days. 鈥淚t does seem to be rare,鈥� says Rasmussen.
A reason why there is so much interest in the reinfection question is concern that repeated covid-19 infections might have a cumulative toll on people鈥檚 health. Studies suggest that subsequent bouts with the coronavirus are less severe than the first. Research in Qatar, for instance, found that second infections were linked with a .
But that is no guarantee of safety, says at the University of Leeds, UK, who is a member of iSAGE, a group that offers independent scientific advice on covid-19 to the government. It is still unclear how likely people are to develop long-lasting symptoms from an infection, but if someone catches covid-19 several times then, overall, their chance of long covid may rise, says Griffin. 鈥淭he less likely things are more likely to happen with a greater number of times you get reinfected,鈥� he says.
Rasmussen believes it is too soon to know the effect of repeat infections on people鈥檚 risk of long covid or anything else. 鈥淚t feels like [the pandemic] has been going on for hundreds of years, but it鈥檚 really been an incredibly short amount of time,鈥� he says. 鈥淏ut it does not look as if it鈥檚 going to go away.鈥�
