DH Deciphers | What is herd immunity? When will India achieve it?

One of the most talked-about weapons in humanity's fight against Covid-19 is herd immunity. And this weapon has become all the more crucial in India where multiple rounds of lockdown have failed to flatten the curve and we do not know when the pandemic will run its course. But what exactly is herd immunity, how does it work and when will India get there? Let's find out...

What is herd immunity?

Herd immunity is the indirect protection from a contagious infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection. (Here, herd means a large group of people). This means that even people who are not vaccinated, or in whom the vaccine does not trigger immunity, are also protected because people around them are immune and can act as buffers between them and an infected person. Once herd immunity is established, the ability of a disease to spread is hindered. For example, if 80% of a population is immune to a virus, four out of every five people who encounter someone having the disease won't get sick (and won't spread the disease any further).

What are the challenges in creating herd immunity?

The more infectious a disease is the greater is the population immunity needed to ensure herd immunity. For example, measles is highly contagious and one person with measles can infect up to 18 other people. This means that around 95% of the people need to be immune in order for the wider group to have herd immunity.

The novel coronavirus has a lower infection rate than measles, with each infected person passing it on to two or three new people, on average. This means that herd immunity should be achieved when around 60% of the population becomes immune to Covid-19.

Should we allow the virus to spread unchecked if we want to achieve herd immunity?

No. Natural herd immunity — achieved through infection rather than vaccination — can be challenging to induce through unchecked infection as there would be a high rate of serious illness and death, with health systems overwhelmed well beyond their surge capacity, even in high-income countries. This is why herd immunity is generally pursued through vaccination programmes.

Even when vaccines are available, it is not always possible to achieve herd immunity for a very long time. Some viruses, such as seasonal flu, mutate frequently (=develop new physical characteristics because of a permanent change in their genes), evading the human body's immune response. So immunity doesn't always last forever, which is why the flu shot is necessary every single year.

Is India anywhere close to achieving herd immunity?

Nationally no, but a few experts such as epidemiologist Jayaprakash Muliyil, a former principal of Christian Medical College, Vellore, believe that signs of herd immunity are being noticed in places like Delhi that reported 23% positive rate in an antibody-based serosurvey. (Serosurvey is a screening test of the serum of persons at risk to determine susceptibility to a particular disease). This means 23% of Delhi's population was exposed to the virus and got cured in the natural process.

Since the last date of the survey was July 10 and the antibody takes about two weeks to develop, the infection probably happened towards the end of June. Once the exposure level reaches 50%, the disease will stop showing its epidemic character and it will become a sporadic one. Doctors can then look after patients without any anxiety and there won't be any need for quarantine. Similar signs were noticed in Dharavi, Mumbai. Signs of herd immunity have also been noticed in the densely populated walled city area of Ahmedabad, according to Dileep Mavalankar, the director of Indian Institute of Public Health, Gandhinagar.

"This epidemic is going to end on its own when we reach herd immunity," Muliyal said.