In what could further underline the importance of vaccination, a new study shows that an infection with the Omicron variant of the Covid virus may not generate broad immunity in unvaccinated individuals that can protect against other variants.

However, the pre-print study from the US shows that in vaccinated individuals, an Omicron infection can boost existing immunity, thereby allowing the individual to fight off another infection better.

The study has been conducted by a team comprising Nobel Laureate Jennifer Doudna, researchers from the University of California, San Francisco; University of California, Berkeley; California Department of Public Health; and Covid-19 testing start-up Curative Inc.

According to the study, mice were infected with the virus-type first identified in the US in 2020, the Delta variant and the Omicron variant, and tested whether their sera (a blood component) could effectively neutralise or fight against the original virus, and the Alpha (first found in UK), Beta (first found in South Africa), Delta (first found in India), Omicron (first found in South Africa) variants of the virus.

The researchers found that mice infected with Delta developed the best protection against the other variants, except the Beta variant which is known to be highly immune evasive. In contrast, infection with Omicron was only effective in fighting off the Omicron variant but did not neutralise enough – meaning did not effectively protect against — other variants, according to the study.

On the other hand, sera from mice infected with the original virus could effectively protect against another infection from the same virus, along with the Alpha and Delta variants. It could not protect effectively against Beta or Omicron infections.

Almost all the vaccines currently in use utilise modified spike proteins from the wild type of the virus to elicit immune response. This could be one of the reasons Omicron is causing a high proportion of breakthrough infections in completely vaccinated individuals.

The vaccines, however, remain effective against severe disease and death. Several countries, including India, have started administering a third booster jab to offer better protection to the most vulnerable people.

After the mice study, the researchers used sera from those who had a breakthrough infection during the Delta wave and Omicron wave to see how much protection they had against the wild type as well as all the other variants.

The sera from those who had a Delta breakthrough infection (infection after complete vaccination) could effectively neutralise all variants, although the neutralisation was low for Omicron.

But the researchers also realised that sera from confirmed Omicron breakthrough infection conferred good protection against all variants.

“These findings suggest that Omicron infection can effectively boost existing immunity conferred by the vaccination against other variants, eliciting “hybrid immunity” that is effective against not only itself but also other variants,” the researchers noted.

The researchers said that since Omicron infection on its own could not offer broad protection against all variants but could boost existing immunity and Delta could create broad immunity, multivalent vaccines using both Omicron and Delta could be developed in future.

Dr Anurag Agarwal, Director of Institute of Genomics and Integrative Biology, said, “Omicron is not nature’s vaccine. Omicron infection does not give protection against infection by other variants, in unvaccinated people. Previously reported effects against Delta were due to enhancement of vaccine induced immune response that protects well against prior variants.”

He has been saying that people must not mistake the virus that is known to cause milder upper respiratory tract symptoms in most patients as a natural vaccine because it may still cause severe disease and death in those with compromised immune systems.

K Srinath Reddy, president of the Public Health Foundation of India and a member of the National Covid-19 Task Force, said, “This would mean that while Omicron itself has an immune evasion capacity due to 36 spike protein mutations, the limited immunity it evokes adds to the prior immunity against spike proteins and may result in sufficiently protective effect against severe disease.”

Dr Samiran Panda, who heads the Department of Epidemiology and Communicable Diseases at the Indian Council of Medical Research and is also part of the Task Force, said, “The findings are interesting, but we cannot extrapolate the results of a mice model to a population as large as India. We have to wait and see what will happen. Of course, when there is an antigenic exposure, there will be immunological changes.”