New York,   Vaccines developed by US drugmakers Pfizer, Moderna, Johnson and Johnson against Covid-19 have proved to be less effective against the new super mutant Omicron variant that has now spread to 77 countries.

The Omicron variant, harbouring up to 36 mutations in spike protein, is known to evade vaccine efficacy.

The study led by researchers at Massachusetts General Hospital (MGH), Harvard and MIT tested blood from people who received the Moderna, Johnson & Johnson and Pfizer/BioNTech vaccines against a pseudovirus engineered to resemble the Omicron variant.

They included individuals that were both vaccinated recently or had recently taken booster doses, and also had prior SARS-CoV-2 infection.

The findings, posted on preprint Memedrxiv, meaning not peer-reviewed yet, showed that the neutralisation of Omicron was 'undetectable' in most vaccinated individuals.

"The study demonstrates that Omicron drastically escapes vaccine-induced immunity after primary vaccination series with mRNA-1273 (Moderna), BNT162b2 (PfizerBioNTech) or Ad26.COV2.S (Johnson & Johnson/Janssen) and exhibits increased infectivity in vitro, raising the potential for increased transmissibility," said Wilfredo F. Garcia-Beltran, Department of Pathology, MGH at Boston.

However, individuals boosted with mRNA vaccines exhibited potent neutralisation of Omicron only 4-6-fold lower than wild type, suggesting that boosters enhance the cross-reactivity of neutralising antibody responses.

The team also found that Omicron pseudovirus is more infectious than any other variant tested. Overall, this study highlights the importance of boosters to broaden neutralising antibody responses against highly divergent SARS-CoV-2 variants.

Further, the study showed that recipients of mRNA boosters following J&J vaccine exhibited markedly improved neutralising titers, but did not achieve the breadth of response seen with mRNA primary vaccination followed by mRNA booster.

"Our results would suggest that these recipients of J&J vaccines may benefit from additional mRNA vaccine doses with the potential to further raise titers and broaden their neutralizing activity," the researchers said.

Omicron breakthrough infections may result in attenuated disease severity in vaccinated people due to cellular and innate immunity, they noted.

"These findings support the need for rapid and synchronized widespread deployment of mRNA boosters as a public health measure to curtail the emergence and spread of highly mutated SARS-CoV-2 variants," Garcia-Beltran said.

The results are in line with other studies recently published by researchers at the University of Oxford who said that they found the two-dose Pfizer and AstraZeneca Covid-19 vaccine regimens do not induce enough neutralising antibodies against the new variant.