Herd immunity & COVID - 19 in India
What is herd immunity?
When majority of a population develops immunity against a contagious disease, it provides an 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 - 8 out of every 10 people who encounter someone with the disease would neither get sick nor spread the disease any further. Depending how contagious an infection is, usually 50% to 90% of a population (700 million to 1.26 billion people in India) needs to be immune to the disease to achieve herd immunity.
How have we achieved herd immunity for other infectious diseases?
Polio, Measles, Diphtheria, Pertussis and others are some of the infectious diseases that were once very common in India. Universal immunization programmes and vaccines have helped establish 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 emphasising that the immunity is either short lasting or infection with a new strain is eminent.
Vaccination for SARS-CoV-2?
There is currently no evidence for lasting protective immunity against the SARS-CoV-2 after natural infection, and a recent paper in The Lancet Journal warns that this waning immunity, as a result of natural infection, would not end the COVID-19 pandemic.
Currently, we have two vaccines approved in India on an emergency basis, Covishield, with an efficacy of 70% after first dose, going up to 90% after second dose; and Covaxin which is undergoing phase-3 of trials.
To develop herd immunity with a vaccine, at least 70% to 80% of a population, has to be vaccinated. Considering both the vaccines have an efficacy of approximately 80%, it is required to vaccinate approximately 2.5 million people every day (700 million to 1.26 billion people in India) to achieve herd immunity by August, 2021.
What happens if India does not reach high vaccine coverage levels?
First, SARS-CoV-2 will become endemic at a low level, the precise level depending on the degree of vaccine uptake, with peaks during winter and troughs during summer.
Second, policy makers will have to consider whether to mandate vaccination and to generate some certificate to record immunisation at educational institutions and the workplace.
Given the vaccine intake hesitancy among people, the creation of herd immunity via the vaccination is likely to be challenging.