PUNE: Recent findings by the Indian Council of Medical Research, indicating lower levels of neutralizing antibodies against the Delta variant in a small portion of Covishield recipients, may just tip the balance in favour of a third booster shot, experts have said.
The ICMR study indicated a 4.5-fold and 3.2-fold reduction in neutralising antibody levels against Delta in those who were given one and two doses of Covishield, respectively, compared to an earlier SARS-CoV-2 version that had the D614G mutation (detected early last year).
Dr Samiran Panda, chief of the ICMR’s epidemiology and communicable diseases division, said, "These kinds of observations have the potential of informing immunization programmes, including the possible need for a third booster dose for vaccination against Covid-19."
For the study, serum samples were collected from healthy individuals who had received one or two doses of Covishield. Samples were also collected from Covid-recovered patients who had received one or two doses.
The fifth category of individuals whose samples were drawn were breakthrough cases — people who contracted Covid after vaccination.
Dr Pragya Yadav, from National Institute of Virology and a member of the research team, said the study indicated that serum of Covid-recovered participants (given either one or two doses) and the breakthrough cases had higher neutralizing antibody titres compared to non-Covid participants who had received one or two shots of Covishield.
Dr Panda said, "This indicates that neutralizing antibodies in the serum of individuals who had contracted Covid, in a way, worked like the first dose. A full vaccination schedule after that resulted in higher levels of neutralizing antibodies. Similarly, people who had received two doses of Covishield and contracted Covid after that (breakthrough infections), had higher levels of neutralizing antibodies."
In both cases, Dr Panda said, the immune system was exposed to the viral antigen thrice, generating a greater immune response. "The ICMR study could then help support discussion on whether or not a third Covishield booster dose could be needed in future for some cases.”
Dr Yadav said, “Another important aspect of the study was the finding that breakthrough cases have significantly less severity and mortality. Also, it showed that even one dose is sufficient for long-term protection in case of those who have recently recovered from Covid-19."
She added that a majority of healthy individuals vaccinated with one or two doses of Covishield did show neutralizing antibodies, while only a small portion did not have any detectable antibodies.
For the study, serum samples were collected four weeks post-vaccination for all categories of participants, except breakthrough cases. In case of the latter category (tested via RT-PCR), samples were collected two weeks after completing a two-dose schedule.
