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Tuesday, January 25, 2022

New research: Omicron and immune response after vaccination

What could explain the rapid spread of Omicron? A study suggests the new Covid-19 variant significantly reduces the neutralising ability of antibodies — those induced by vaccines as well as by hybrid immunity.

Written by Anuradha Mascarenhas | Pune |
Updated: January 11, 2022 7:48:36 am
‘‘Precautionary’ dose given at Maulana Azad Medical College, Delhi. (Express photo by Abhinav Saha)

With its multiple mutations, the Omicron variant of SARS-CoV-2 significantly reduces the neutralising ability of antibodies — those induced by vaccines as well as by hybrid immunity, a study has found. This might explain its rapid spread even amid widespread vaccine coverage, suggests the study, conducted by the Translational Health Science and Technology Institute (THSTI). The researchers have stressed, however, that reduced neutralisation may not translate into drastic reduction in vaccine effectiveness.

The study, ‘Sub Optimal neutralisation of Omicron (B.1.1.529) variant by antibodies induced by vaccine alone or SARS CoV2 infection plus vaccine (hybrid immunity) post six months’, is currently published as a preprint (bit.ly/31Jht9Z).

The comparison

Researchers tested the ability of antibodies to neutralise Omicron among people with vaccination alone, and among vaccinated people who also had had a prior natural SARS-CoV-2 infection. “We included a total of 80 participants — 20 each with ChAdOx1 nCoV-19 (Covishield) and BBV152 (Covaxin) vaccination alone, and 20 each with ChAdOx1 nCoV-19 vaccine plus natural infection and BBV152 vaccine plus natural infection,” said Dr Pramod Garg, Executive Director of THSTI.

The study compared the extent of neutralisation using a measure called geometric mean titre (GMT). For antibodies against the original virus strain, the GMT was 384 in those vaccinated with Covaxin alone, and 383 in those vaccinated with Covishield alone. For the hybrid groups, the values were 795 and 1424 respectively.

Against Omicron, only 5 out of 20 in both vaccine-only groups, 5 out of 19 in the Covaxin-plus-infection group, and 9 out of 20 in the Covishield-plus-infection group exhibited neutralisation titres above the lower limit of quantification. This suggested better neutralisation in those with prior infection.

The proportion of neutralisers was significantly reduced against Omicron compared to the original strain and Delta. Among those without prior infection, GMT was significantly lower against Omicron than against the original strain and Delta. Among those with previous infection, the titres followed the same pattern — but the neutralising ability was better in them than in those without previous infection.

Vaccination does help

“However, reduction in neutralisation may not translate into drastic reduction in vaccine effectiveness. We have to give a balanced message to people, that in general, the body’s own defence mechanism even before vaccines were good enough to prevent symptomatic infections in a large majority of people. Among those clinically infected a large majority had mild disease, due to natural immune defence mechanism in humans,” Dr Garg said. Even if Omicron is spreading, being vaccination prevents severe disease in 80%-90% of the people, Dr Garg said. “Boosting may be required in a sub group of people which is allowed from today onwards.”

There was no difference in the neutralisation levels between the two vaccines.

Antibodies, T cells & boosters

A recent study by THSTI published in The Lancet Infectious Diseases has shown that besides neutralising antibodies, T-cell immune responses are important for controlling SARS-CoV-2 infection. The researchers said this seems to be true for Omicron too, as demonstrated by South African researchers.

“From a policy perspective, a significant correlation between serum IgG titres and neutralisation GMT against the omicron in the face of immune escape by this variant would lend support for an additional dose of vaccine to augment antibody response. This is being implemented in a few countries including India and has the potential to offer better protection in vulnerable people…,” the study said.

Lead author Guruprasad Medigeshi and other researchers have said in the study that the drop in neutralisation might be alarming, but the real-world impact of these reduced neutralisation titres on hospitalisation rates and mortality rates have to be interpreted along with other factors such as pathogenicity of the variant, immunisation uptakes and seroprevalence from natural infection in different geographical regions and the expected role of cellular immune responses to the variant.

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