New York,  Without vaccination, an infection with Omicron fails to confer robust immunity against other Covid-19 variants, finds a study.

In experiments using mice and blood samples from donors who were infected with Omicron, researchers at Gladstone Institutes and UC San Francisco (UCSF) in the US, found that the Omicron variant induces only a weak immune response.AIn vaccinated individuals, this response - while weak - helped strengthen overall protection against a variety of Covid strains. In those without prior vaccination, however, the immune response failed to confer broad, robust protection against other strains, revealed the study published in the journal Nature."

In the unvaccinated population, an infection with Omicron might be roughly equivalent to getting one shot of a vaccine," said Melanie Ott, director of the Gladstone Institute of Virology.

"It confers a little bit of protection against Covid-19, but it's not very broad," Ott added.

The team of researchers found that despite the milder symptoms, the immune system in mice infected with Omicron still generated the T cells and antibodies typically seen in response to other viruses.

Further, to gauge how the immune response against Omicron fared over time, the researchers collected blood samples from mice infected with the ancestral, Delta, or Omicron variants of SARS-CoV-2 and measured the ability of their immune cells and antibodies to recognise five different viral variants - ancestral (WA1), Alpha, Beta, Delta, and Omicron.

Blood from uninfected animals was unable to neutralise any of the viruses - in other words, block the ability of any of the viruses to copy themselves. Samples from WA1-infected animals could neutralise Alpha and, to a lesser degree, the Beta and Delta virus - but not Omicron. Samples from Delta-infected mice could neutralise Delta, Alpha and, to a lesser degree, the Omicron and Beta virus.

However, blood from Omicron-infected mice could only neutralise the Omicron variant. The team confirmed these results using blood from ten unvaccinated people who had been infected with Omicron - their blood was not able to neutralise other variants. When they tested blood from 11 unvaccinated people who had been infected with Delta, the samples could neutralise Delta and, as had been seen in mice, the other variants to a lesser extent.

When they repeated the experiments with blood from vaccinated people, the results were different: vaccinated individuals with confirmed Omicron or Delta breakthrough infections all showed the ability to neutralise all the tested variants, conferring higher protection.

"When it comes to other variants that might evolve in the future, we can't predict exactly what would happen, but based on these results, I'd suspect that unvaccinated people who were infected with Omicron will have very little protection," Ott said.

"But on the contrary, vaccinated individuals are likely to be more broadly protected against future variants, especially if they had a breakthrough infection."