Understanding the Concept of ‘Herd Immunity’

Niagara Falls (Dr. Noorali Bharwani)
Niagara Falls (Dr. Noorali Bharwani)

According to WHO website, ‘herd immunity’, also known as ‘population immunity’, is a concept used for vaccination, in which a population can be protected from a certain virus if a threshold of vaccination is reached.

If 90 per cent of the population is vaccinated then the rest of the population is generally protected by the concept of herd immunity. Herd immunity is achieved by breaking the chain of transmission of the virus by immunization.

Important point is herd immunity is achieved by protecting people from a virus, not by exposing them to it.

Unfortunately, herd immunity cannot be achieved with COVID-19.

Currently, we do not have a vaccine against the virus. Vaccines train our immune systems to create proteins (antibodies) that fight disease, just as would happen when we are exposed to a disease. Vaccines work without making us sick.

The percentage of people who need to have antibodies in order to achieve herd immunity against a particular disease varies with each disease. For example, herd immunity against measles requires about 95 per cent of a population to be vaccinated. The remaining five per cent will be protected by the fact that measles will not spread among those who are vaccinated. For polio, the threshold is about 80 per cent, says WHO website.

There are two ways to herd immunity for COVID-19 – vaccines and infection.

Vaccines: Using the concept of herd immunity, vaccines have successfully controlled deadly contagious diseases such as smallpox, polio, diphtheria, rubella and many others. People who oppose vaccines do pose a real challenge to herd immunity.

Natural infection: Herd immunity can also be reached when a sufficient number of people in the population have recovered from a disease and have developed antibodies against future infection.

However, there are some major problems with relying on community infection to create herd immunity.

It isn’t yet clear if infection with the COVID-19 virus makes a person immune to future infection. Even if infection with the COVID-19 virus creates long-lasting immunity, a large number of people would have to become infected to reach the herd immunity threshold.

Experts estimate that in the U.S., 70 per cent of the population – more than 200 million people – would have to recover from COVID-19 to halt the epidemic. If many people become sick with COVID-19 at once, the health care system could quickly become overwhelmed. This amount of infection could also lead to serious complications and millions of deaths, especially among older people and those who have chronic conditions.

In most countries, less than 10 per cent of the population have been infected with COVID-19. It would be scientifically problematic and unethical to expose millions of people to the virus to achieve herd immunity. Letting COVID-19 spread through populations, of any age or health status will lead to unnecessary infections, suffering and death.

We are still learning about immunity to COVID-19. Most people who are infected with COVID-19 develop an immune response within the first few weeks, but we don’t know how strong or lasting that immune response is, or how it differs for different people. There have also been reports of people infected with COVID-19 for a second time.

Until we better understand COVID-19 immunity, it will not be possible to know how much of a population is immune and how long that immunity last for, let alone make future predictions.

It is crucial to slow the spread of COVID-19 virus and protect individuals at increased risk of severe illness. Wear a mask in public, maintain social distance, avoid big gatherings and wash your hands frequently.

Prevention is better than cure. If you have cough, fever, and difficulty breathing, seek medical care early.

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