Will Herd immunity work for COVID - 19 in India?

Dr Srikanta J T, Consultant- Paediatric Pulmonology, Aster CMI Hospital

What is herd immunity?

When majority of a population develops immunity against a contagious disease, this provides indirect protection—or herd immunity—to those who are not immune to the disease.

For example, if 70 to 80% of a population is immune to a virus, approximately 7 to 8 out of every 10 people who encounter someone with the disease would neither get sick nor spread the disease any further. So, depending on how contagious an infection is, usually 50% to 90% of a population (700 million to 1.26 billion people in India) need immunity to achieve herd immunity.

How have we achieved herd immunity for other infectious diseases?

Polio, Measles, Diphtheria, Pertussis, among others, are some of the infectious diseases that were once very common in India. But because of universal immunization programmes, vaccines helped establish the herd immunity.

Viruses (causing flu and common cold) mutate over time, so antibodies from a previous infection provide protection for only a short period of time. In patients infected with SARS-CoV-2, we are seeing a number of cases of re-infections emphasizing that immunity is either short-lasting or infection with a new strain is eminent.

Will herd immunity work with SARS-CoV-2?

There is currently no evidence for lasting protective immunity to SARS-CoV-2 after natural infection, and a current Lancet paper warns that this diminishing immunity as a result of natural infection would not end the COVID-19 pandemic.

With diseases that are either less severe or don’t carry a high mortality rate, like chickenpox, achieving immunity by intentional exposure was a norm. But the situation for SARS-CoV-2 is very different: COVID-19 carries a much higher risk of severe disease and even death.

The death rate for COVID-19 is unknown, but current data suggest it is 10 times higher than that of flu. It’s higher still among vulnerable groups like the elderly and people with comorbid conditions.

So the approach of herd immunity and the concept of getting an infection and “get it over with” is not a good idea. Until we have an effective vaccine which is the only safe way of developing herd immunity, all the preventive measures underlined by all the agencies (ICMR, WHO, CDC) has to be followed with utmost precautions.