Experts say that the Omicron variant could dash any lingering hopes of populations reaching herd immunity against COVID-19, after a preprint study from South Africa suggests the highly-mutated strain is more capable of reinfecting people than previous strains.

South African researchers have found that Omicron is 2.4 times more likely to reinfect someone who has already had COVID-19 when compared to other studied strains.

The study found a recent spike in reinfections, which is likely attributable to Omicron, since researchers found no increased risk of reinfection during the waves of cases brought about by the Beta and Delta variants.

The study, which was released on preprint server MedRxiv by a cohort of South African labs led by Stellenbosch University, and is yet to be peer reviewed, indicates that Omicron may be adept at evading natural immunity built up by previous exposure to other variants.

"Analysis of routine surveillance data from South Africa suggests that, in contrast to Beta and Delta, the Omicron variant of SARS-CoV-2 demonstrates substantial population-level evidence for evasion of immunity from prior infection," said authors of the study.

Simon Clarke, who is associate professor in cellular microbiology at the University of Reading in the United Kingdom, said that Omicron has "blown a big hole in the controversial argument" that the infection should simply be allowed to spread in an attempt to create immunity.

"Herd immunity now seems like nothing more than a pipe dream," Clarke said.

While more data is needed, experts said that if Omicron is capable of evading natural immunity, there is a chance that current vaccines will prove less effective against the new variant.

"If it can escape natural immunity it is also likely to have substantial escape potential for vaccine-induced immunity," said Paul Hunter, a professor in medicine at the University of East Anglia.

There is currently no indication as to how Omicron is able to better escape natural immunity than previous variants, though Clarke says this is likely to do with the number of mutations on the spike protein.

Omicron has 32 mutations on the spike protein, which is the area of the virus targeted by both the body's immune system and by vaccines. By comparison, the Delta variant has 10 spike protein mutations. Put simply, a highly-mutated spike protein may be harder for the body — or a treatment — to recognize.

Michael Head, who is senior research fellow in global health at the University of Southampton, said that there is a chance that Omicron is better at evading natural immunity than it is at eluding vaccine protection.

"The immune response from vaccination is much stronger when compared with infection-acquired immunity," Head said. "Whilst there is likely to be some impact, it is likely vaccines will still provide some level of protection."

The study did not include data on the severity of illness among the reinfected, which is needed in order to forecast how reinfections might impact hospitalization rates and deaths.