Covaxin Booster Improves Vaccine Effectiveness Against Delta, Omicron Variants: ICMR Study

The evidence from the study shows that Covaxin booster immunisation tends to broaden the protective immune response and reduce disease severity against infection with Delta and Omicron variants.

Administration of the booster dose of Bharat Biotech's Covid-19 vaccine Covaxin will enhance the vaccine effectiveness against infection caused by the Delta variant of SARS-CoV-2 and confer protection against the Omicron variants BA.1.1 and BA.2, according to a new study led by researchers at the Indian Council of Medical Research (ICMR). The protective efficacy of Covaxin against the Delta variant following the second dose was compared with that following the third dose. The researchers also studied the efficacy of Covaxin against Omicron variants in a Syrian hamster model. 

A preprint of the study is available on bioRxiv, a preprint server for biology. 

What Factors Were Considered In The Study?

The immunity acquired after natural infection with SARS-CoV-2 or vaccination against the virus tends to wane with time. Also, vaccine effectiveness varies with the variant of infection. ICMR experts studied the antibody response, clinical observations, viral load reduction, and lung disease severity following the Delta variant challenge. This is a trial which involves deliberately infecting healthy variants with the Covid-19 virus to assess the efficacy and effectiveness of a vaccine.

The animals used in the study were divided into three groups: two-dose Covaxin immunised group, three-dose Covaxin immunised group and placebo group.

Protective response in terms of the reduction in lung viral load and lung lesions was observed in the two-dose Covaxin immunised group. A reduction in lung viral load and lung lesions was also observed in the three-dose Covaxin immunised group.

Lung Disease Severity Was Considerably Reduced In The Animals Who Received Coavaxin Booster

The neutralising antibody response against the vaccine strain in the two-dose immunised group was comparable to that in the three-dose immunised group. However, considerable reduction in the lung disease severity was observed in the three-dose immunised group following the Delta variant challenge. 

This indicates the involvement of cell-mediated immune response in protection, the study said. Cell-mediated immunity is an immune response that does not involve antibodies but rather involves the activation of phagocytes, antigen-specific cytotoxic T lymphocytes, and the release of various cytokines in response to an antigen. Phagocytes are cells that protect the body by ingesting harmful foreign substances such as bacteria or dead cells. Cytotoxic T lymphocytes are immune cells that directly kill other cells, including foreign cells, and cells infected with a virus. Cytokines are small proteins that are secreted by certain cells of the immune system and have an effect on other cells. Cytokines are produced in response to invading pathogens to stimulate, recruit and proliferate immune cells, and are key modulators of inflammation. 

In the vaccine efficacy study against the Omicron variants BA.1 and BA.2, lesser virus shedding, lung viral load and lung disease severity were observed in the immunised groups compared to the placebo groups, the authors noted. Viral shedding is the expulsion of virus particles into the environment by an infected person while talking, coughing, sneezing, or exhaling. 

What Is Covaxin?

Whole-Virion Inactivated SARS-CoV-2 Vaccine or BBV152 is India's first indigenous, whole-virion, inactivated vaccine developed by Bharat Biotech in collaboration with the ICMR and the National Institute of Virology (NIV) for protection against Covid-19. BBV152 is the research name for Covaxin.

The fact that Covaxin is an inactivated whole virion vaccine means that it contains viruses whose genetic material has been destroyed by heat, chemicals or radiation, so that they cause no disease, but provide a good immune response. Covaxin is licensed in India and 13 other countries. 

After the successful completion of the third phase of human clinical trials, Covaxin received emergency use authorisation from the World Health Organization (WHO). An interim analysis of the Phase III controlled trials showed that two doses of Covaxin provide 77.8 per cent protection against symptomatic Covid-19, two weeks after receiving the second dose.

Why Is It Important To Monitor Protective Efficacy Of Vaccines Against Newly Evolving Variants?

The vaccine efficacy data of Covaxin against Omicron is not available. Approved vaccines should be monitored for their protective efficacy against newly evolving variants because the vaccine response tends to vary with variant divergence and immunological memory, which is the ability of the immune system to respond more rapidly and effectively to pathogens that have been encountered previously. Therefore, ICMR researchers performed an animal model challenge study to assess the protective efficacy of Covaxin against Delta and Omicron variants. In the first study, the scientists compared the protective efficacy in the two-dose and three-dose immunised groups against the Delta variant. In the second study, protective response against the Omicron variants BA.1.1 and BA.2 following three doses was measured. 

Important Findings Of The Delta Infection And Vaccine Efficacy Studies

In the Delta infection study, the researchers found that administration of the booster dose enhances the effectiveness of Covaxin against the Delta variant. Lung disease severity was found to be reduced to a greater extent after the third dose, compared to the reduction in lung disease severity following the second dose of the vaccine. 

In both the two-dose and three-dose immunised groups, the virus shedding and viral load were considerably reduced. This indicates vaccine efficacy against the Delta variant.

According to the study, the proportion of antibody-secreting memory B cells in the three-dose group was found to be more compared to that in the two-dose recipients.

The booster dose of Covaxin was found to improve the neutralising antibody response against the Delta and Omicron variants. The findings also demonstrate the protective response of Covaxin against the Omicron variant.  In the vaccine efficacy study, reduced lung disease severity was observed in the vaccinated animals. 

The authors concluded that the evidence from the study shows that Covaxin booster immunisation tends to broaden the protective immune response and reduce disease severity against infection with Delta and Omicron variants.