With Omicron spreading faster around the world than any previous variant, cases of re-infection among people who contracted Covid-19 earlier in the pandemic are increasing.
Neil Ferguson, an epidemiologist at Imperial College London, estimated last week that between 10 and 15 percent of Omicron cases were among people already infected with another variant. Other scientists have called it a credible estimate.
Almost all re-infections so far will have been from people who originally contracted another strain of the Sars-Cov-2 virus. No evidence has yet been found that anyone has been infected twice by Omicron itself, including from South Africa where it latest variant of concern the longest circulating – for at least two months.
But health officials are concerned that Omicron’s increased portability and ability to evade immune protection will lead to cases of re-infection with the same variant. They are also concerned about co-infection – concomitant infection with Omicron and another variant – in this phase of the pandemic.
Kingston Mills, professor of experimental immunology at Trinity College Dublin, said it was “too early” for people infected with Omicron to clear the virus and then get it again.
“The test will come in six months when we know if people who had Omicron will be infected with Omicron again,” he said. “My guess is that if they are vaccinated they will not do it, but if they are not vaccinated, there is a chance that they will do it.”
All current evidence for re-infection with Sars-Cov-2 comes from people infected with Delta and other variants – but here the data is incomplete.
Some of the world’s best evidence comes from the UK Health Security Agency, which said on Thursday that its latest available data had been identified 268 517 “possible re-infections” in England – defined as a positive Covid-19 test at least 90 days after a previous positive – compared to 11.1 million primary infections since the onset of the pandemic.
In the week to January 2, when England had about 1 million Covid cases, UKHSA identified 59,783 possible reinfections. But only a complete genomic analysis can prove that a case is a true reinfection, in which someone has completely cleared the virus and is re-infected.
Covid reinfection statistics
Estimated ratio of Omicron cases among people already infected with Covid-19
‘Possible re-infections’ in England so far, compared to 11.1 million primary infections, according to UKHSA data
Possible re-infection cases among the approximately 1 million Covid-19 cases in England until 2 January
Population-based studies probably underestimate the incidence of reinfection because many secondary infections are mild and transient, without producing symptoms. Several studies have shown that a first infection with Sars-Cov-2 provides good immune protection against disease if you are re-infected with another strain of the virus such as Omicron. Indeed, proof shows that other coronaviruses, which cause common colds, provide some protection.
“Natural infection can provide better cross-reactive immunity than current Covident substances against other variants, because it produces T cells that recognize internal proteins in the virus, as well as the peak proteins, ”said Charles Bangham, professor of immunology at Imperial College London.
Although reinfections are usually less severe than first infections, they can still be fatal, especially to people who have not been vaccinated. The First reported Omicron death in US was from a man in his fifties who had previously been infected with Covid-19 but had not been vaccinated.
Bangham warned against relying on natural immunity alone to defend against Covid-19. “You need to take the vaccines that are offered to you because it will significantly improve your immune protection,” he said.
Re-infection seems to be more common among children as adults. “Elementary school children have much lower or no immunity to vaccination in most states,” Mills said. “So the most vulnerable to infection are the 4- to 11-year-olds, then the adolescent groups that did not get a boost are the next most vulnerable.”
Much less common than reinfection is co-infection – infection with two strains at the same time. Christopher Coleman, assistant professor of infection immunology at the University of Nottingham, said: “Viruses tend to prevent competition because when a virus infects a cell, it does not want another virus there. “It does not take away resources. It makes changes to the cell that make it less attractive for another virus to attach to our cells.”
The human inflammatory response to infection can also block access by other viruses. “On a biological level, even if you have very mild symptoms, your immune system is going to overdo it and take a lot of protective measures, which makes it very difficult for another virus to get in and infect you while you have any infection,” said Coleman.
However, the prospect of co-infection worries some virologists, as the two strains can swap genes through a genetic recombination process, increasing the risk of developing dangerous new variants. Scientists therefore hope that the more transferable Omicron will have enough selective advantage over Delta and other variants to replace rather than coexist with them.
A recent unpublished study in South Africa suggested that this is likely to happen. It found that blood plasma taken from Omicron patients not only neutralized Omicron itself, which reduced the chance of re-infection with the same variant, but also showed significantly increased neutralization of Delta, making re-infection with that variant less likely. .
“These results are consistent with Omicron moving the Delta variant,” the researchers said.
In the longer term, most virologists and immunologists expect Sars-Cov-2 to become an endemic virus that periodically re-infects humans for the indefinite future, as it mutates and slowly weakens immune protection over time – like other respiratory pathogens, including the four other human coronaviruses causing mild colds and sniffing. Re-infection will only cause serious illness in a very few people.
“There is good reason to believe that the virus will reinfect us every few years or so, but our immune system, especially T cells, will still provide good protection against serious diseases,” Bangham said.