The story so far: Bharat Biotech, the makers of Covaxin, and the Indian Council of Medical Research (ICMR) reported results of a study that showed the vaccine appeared to be working against B.1.1.7, better known as the U.K. variant or a new variant of SARS-CoV-2. Pfizer and Moderna, whose vaccines have been approved after demonstrating efficacy, have also undertaken studies to check if their vaccines are effective against new variants.
Why and how are such studies done?
Covaxin, among the two vaccines being used to inoculate Indian health-care workers — the other being Covishield — was still in clinical trial mode when it was approved for restricted emergency use. Among the reasons advocated by the government for doing so was the possible infection by mutant strains, like the U.K. variant. The B.1.1.7 variant, with many mutations, some in the spike protein, was linked to a sharp spike in coronavirus cases in the United Kingdom and many other countries. In India, the variant has been confirmed in 165 people (January 28), all with a travel history to the U.K. Another variant, B.1.351, first identified in South Africa, has been reported from several countries. With most vaccines targeting the spike protein, a variant with mutations that helps SARS-CoV-2 evade the immune system could defeat their purpose. One way to find out the ability of a specific antibody to neutralise a virus is through a serological test called the Plaque Reduction Neutralisation Test (PRNT).
Blood serum, that contains antibodies, is taken from people who have been infected or have been vaccinated. This antibody soup is diluted and mixed with the virus and evaluated in customised plates to see the extent to which neutralising antibodies were successful in containing the virus spread. To check the antibodies’ capabilities against mutant variants, scientists at the National Institute of Virology, Pune, used antibodies obtained from people who had been vaccinated in the Covaxin trials and tested them against virus extracted from those infected with the U.K. strain and another un-named strain, hCoV27 19/India/2020Q111, reportedly a strain with provenance from Iran.
What did the study find?
The objective of PRNT is to find out the antibodies needed to reduce a given amount of virus by a fixed proportion. What emerged was that irrespective of the SARS-CoV-2 strain tested, roughly the same number of antibodies were needed to kill the virus. This implies that Covaxin generated antibodies that were as effective against the mutant U.K. variant as the virus used to make the vaccine. Tests by Pfizer and Moderna had found similar results, though there has been no such test reported on the AstraZeneca Covishield vaccine.
What does it imply?
The latest study attests to the fact that Covaxin is a promising candidate to be tested in large trials. A laboratory study that quantifies the antibodies generated against a virus does not correlate with a vaccine’s efficacy or effectiveness in the field. That can only be done via a phase-3 trial that compares it in different sets of the population. A limitation of the Covaxin study, which has not been peer reviewed, is that it does not discuss other important aspects. For instance, an important mutation in the South African variant, called E484K, has been found to help the virus evade detection.
How long does antibody protection last?
There is considerable uncertainty if antibody protection can last over six months. It is not clear whether a mutant can evade lower concentrations of antibodies. The promoters of Covaxin and scientists at the ICMR say the strength of a vaccine lies in it being a whole-inactivated virus — that means a larger surface area of the virus is visible to the immune system and thereby a wider range of antibodies are produced. However, the evidence so far is that antibodies from all vaccines are largely targeted at the spike protein, or the operative region where the virus binds to the cell; therefore, more does not necessarily mean better.
Also read | Indian scientists divided over nod for Covaxin
What remains to be tested?
A challenge governing all vaccines is that it must induce a calibrated response from the immune system — just enough so that antibodies guard against a future, actual infection and not so much that too many weakly neutralising antibodies (aiding a condition called antibody-dependent enhancement) are produced.