Explained | Herd Immunity and Who Gets the COVID-19 Vaccine First

FIT speaks with Dr Jayaprakash Muliyil, Dr K Srinath Reddy and Dr Shahid Jameel about efficacy of COVID vaccines.

Herd immunity helps break the cycle of transmission in contagious infections.

(Photo: iStock)

"We never said we'll vaccinate the entire country," said the union health secretary Rajesh Bhushan on Tuesday while replying to a question on how long it will take immunise all Indians with COVID-19 vaccine.

Indian Council of Medical Research’s (ICMR) Director General Balram Bhargava added that what was needed was to identify a ‘critical mass’ of people who will be vaccinated to ‘break the cycle of transmission.’

This raises important questions: Will the entire country not get the vaccine? Who will be a part of the 'critical mass'? And does the science hold up when it comes to such statements?

FIT reached out to top epidemiologists and virologists to get the answers.

'Herd Immunity' and Vaccines

When a large subset of a population is immune to a disease, it can provide protection to the 'herd' that is not immune. That threshold, when herd immunity kicks in, is different for each infectious disease and can vary from 50 to nearly 100 percent.

What determines this threshold is what’s called the R Nought or R0 (rate of transmission). The more the R0, the greater the immunity threshold.

If you take polio and smallpox, the R0 was 5-7. And the herd immunity threshold was 80-85 percent of the population. Being a very severe disease, it has taken us 100 years to achieve that, not through acquired immunity via infections, but through vaccines.

In terms of Ebola and Influenza, the R0 was 1-2. There the herd immunity threshold is 30-50 percent. According to various experts, for COVID-19 R0 was 3 when the outbreak first happened. So, the herd immunity threshold is 60-70 percent, though some believe it could be as low as 50 percent.

So, could the statements by the Health Ministry be referring to this 'critical mass' that will be inoculated.

Dr Shahid Jameel, a leading virologist and Chairperson, Trivedi School of Biosciences, Ashoka University, says he agrees with Dr Bhargava and the entire country doesn't need the vaccine.

“The concept of herd immunity is that if a significant number of people in a population are immune, they will break transmission of the pathogen. And as a result even those not exposed earlier would be protected without getting the vaccine.”

Dr Shahid Jameel

But there has been little clarity on who those people should be who get the vaccine first. This, despite repeated calls from public health experts for the government to share their distribution plan and put it up for scrutiny.

If Not Everyone, Then Who Gets the Vaccine?

There are several questions that the state and central governments must examine in greater detail.

Speaking with FIT, Dr K Srinath Reddy, Chairperson, Public Health Foundation of India, said,

"If the intention is to break the cycle of transmission, we need to first establish who are the people we are going to vaccinate? What proportion, where are they located and in which cities? If you cover only the healthcare workers, the elderly and the vulnerable, that's 20 percent of the population, and it is not enough to break the cycle of transmission. If you immunise people in cities and not in smaller towns and villages, you don't extend the protection of herd immunity to them."

Dr Jayaprakash Muliyil, chairperson of the Scientific Advisory Committee of the National Institute of Epidemiology, says that while healthcare workers, 60 plus and those with severe comorbidities have to be a priority, vaccine rollout has to be a carefully calibrated plan.

He warns we should not rush into vaccine distribution without proper protections and safety protocols in place. In the US, vaccine distribution plans include preparing beds in case of adverse events.

Should Highly Infected Cities Get Vaccines First or Low Infected Areas

This, Dr Muliyil says, is an important area of study. While talking to FIT, he says, areas where the infection is waning, for eg in Tamil Nadu and Karnataka, vaccine distribution should be more calibrated. The reason being, these areas may have already reached a herd immunity threshold by the time the vaccine plan rolls out.

Areas where infection rates are rising, and where infections are low, could possibly be prioritised.

Should Those Already Infected be Excluded?

Dr Jayaprakash Muliyil believes we should exclude all those who have already been infected with COVID-19.

“Several studies from various universities have established that this disease is fairly immunogenic and provides long lasting immunity. A bulk of those who have been infected would not need a vaccine.”

Dr Jayaprakash Muliyil 

At the press conference on Tuesday, Heath Secretary Rajesh Bhushan said, “Should the vaccination be preceded by antibody tests? It's a question before the National Expert Group on Vaccination but there is no final decision.”

Experts raise some red flags on limitations of antibody testing.

“If it is decided that persons who have already been infected do not need to be immunised, it raises the questions on whether they have developed and retained adequate levels of immunity. Will this group of already infected include asymptomatic persons who were detected in antibody surveys some months ago? Or, will we follow the WHO recommendation and immunise them too?”

Dr K Srinath Reddy

Dr Jameel points to the various sero-surveys that suggest we may already have about 150 million COVID infected people in India.

"What the government should decide is whether it would vaccinate everyone in priority groups or choose those still unexposed. If it chooses the former, it wastes precious vaccine doses on those who would have no more than a booster effect. If it chooses the latter, it would have to test for antibodies, which adds another layer of logistics and adds to cost. WHO recommends vaccinations without prior testing," he added.

While lists are being drawn of healthcare workers, elderly and vulnerable, a scientific approach is essential to protect the entire country from a possible next phase of this pandemic. As one expert said, "be sensible with big decisions."

(The article was first published in FIT and has been republished with permission)