The yet-to-be peer reviewed study, posted on the pre-print repository medRxiv on Tuesday, also found that considerable immunity is retained in people who were both vaccinated and previously infected, as reported by news agency PTI. 

Emergence of the Omicron COVID-19 variant is concerning as “based on the large number of mutations in the spike protein and elsewhere on the virus, this variant will have considerable escape from vaccine elicited immunity.” It uses the spike protein to enter and infect the human cells.

“The clinical implications of these important laboratory data need to be determined. It is likely that lesser vaccine-induced protection against infection and disease would be the result,” said Professor Willem Hanekom, executive director at Africa Health Research Institute, South Africa.

“Importantly, most vaccinologists agree that the current vaccines will still protect against severe disease and death in the face of Omicron infection. It is therefore critical that everyone should be vaccinated,” Hanekom said in a statement.

The researchers also looks into the fact whether the new variant escapes antibody neutralisation elicited by the Pfizer mRNA vaccine and if the virus still requires binding to the ACE2 receptor on human cells to infect them.

They used a human lung cell line clone engineered to express the ACE2 receptor to both isolate the virus and test neutralisation. The research finding indicated that ACE2 is required for Omicron entry.

The researcher also tested the ability of plasma from Pfizer vaccinated study participants to neutralise Omicron versus ancestral D614G virus.

They tested 14 plasma samples from 12 participants, with six having no previous record of SARS-CoV-2 infection nor detectable antibodies indicative of previous infection.

The remaining six participants had a record of previous infection in the first SARS-CoV-2 infection wave in South Africa where infection was with ancestral D614G variant.

These samples had very strong neutralisation of D614G virus, consistent with sampling soon after vaccination, according to the study.

However, neutralisation from the vaccine for Omicron saw a 41-fold decline, the researchers said.

Five of the participants who were previously infected showed relatively high neutralisation titers with Omicron, they said.

“Previous infection, followed by vaccination or booster is likely to increase the neutralisation level and likely confer protection from severe disease in Omicron infection,” the authors of the study said.