It is time India assessed where it stands with respect to herd immunity, a natural process by which a large percentage of population (70-80 per cent) are exposed to the Covid-19 virus and acquires immunity. It also needs to understand its real prevalence, percentage population that has attained immunity, and the nature of immunity, or its longevity.

It should also take up antibody testing to determine the presence of antibodies in the covered population in an area broadly categorised into two categories, says Satish Ranjan, Molecular Immunologist, and formerly Senior Scientist with the Institute of Clinical Chemistry and Pathobiochemistry, Medical Faculty, Otto-von-Guericke-University, Magdeburg, Germany, in an Email interaction.

Diagnosed and undiagnosed cases

These categories pertain to those diagnosed with the virus (by RT-PCR Test), having recovered and with or without antibodies; and secondly, those infected unknowingly and not diagnosed, have recovered and have antibodies. “This might help reveal the exact virus prevalence, especially since the second group will be missing in the original data of only diagnosed cases.”

This way, the percentage of population already immune can be estimated, as also the trend of attainment of immunity across various age groups. We will also know at what mortality rate this has been achieved. With this, one might be able to approximately estimate how long it will take to attain herd immunity and with what economic and social restrictions.

This will help, with formulation of policy measures, to open up economic activity by carefully regulating the extent of exposure. “We will get time to build capabilities and prevent the healthcare system from being overwhelmed. Herd immunity can be attained over a period of time with minimum human and economic loss through suitable scientific interventions,” Ranjan says.

Longevity of antibodies

Antibody testing will determine the its longevity in identified persons (both diagnosed and undiagnosed cases). Testing should be done in all identified persons at regular intervals to know how long the antibodies last. “This will lead us to know the right time to collect plasma from them for use in therapy of patients. Absence of scientific data may lead to extraction of plasma from recovered patients who may not have antibodies due to short longevity.”

"We may get to assess the probability of re-emergence of the virus in an already immune person when acquired immunity doesn’t last long, as reported in South Korea and China. We might be able also to predict how long the immunity will last when vaccine becomes available on a later date and estimate the scale of any future vaccination programme. This will help us to estimate the cost of immunisation and formulate policies for future programmes,” says Ranjan.

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