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Herd Immunity

  • Herd immunity, or community immunity, is when a large part of the population of an area is immune to a specific disease. If enough people are resistant to the cause of a disease, such as a virus or bacteria, it has nowhere to go.
  • While not every single individual may be immune, the group as a whole has protection. This is because there are fewer high-risk people overall. The infection rates drop, and the disease peters out.
  • Herd immunity protects at-risk populations. These include babies and those whose immune systems are weak and can’t get resistance on their own.

How is herd immunity achieved?

There are two paths to herd immunity for COVID-19 — vaccines and infection.


  • A vaccine for the virus that causes COVID-19 would be an ideal approach to achieving herd immunity. Vaccines create immunity without causing illness or resulting complications.
  • Herd immunity makes it possible to protect the population from a disease, including those who can’t be vaccinated, such as newborns or those who have compromised immune systems.
  • Using the concept of herd immunity, vaccines have successfully controlled deadly contagious diseases such as smallpox, polio, diphtheria, rubella and many others.
  • Reaching herd immunity through vaccination sometimes has drawbacks, though. Protection from some vaccines can wane over time, requiring revaccination. Sometimes people don’t get all of the shots that they need to be completely protected from a disease.
  • In addition, some people may object to vaccines because of religious objections, fears about the possible risks or skepticism about the benefits. People who object to vaccines often live in the same neighborhoods or attend the same religious services or schools. If the proportion of vaccinated people in a community falls below the herd immunity threshold, exposure to a contagious disease could result in the disease quickly spreading. Measles has recently resurged in several parts of the world with relatively low vaccination rates. Opposition to vaccines can 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.
  • For example, those who survived the 1918 flu (influenza) pandemic were later immune to infection with the H1N1 flu, a subtype of influenza A. During the 2009-10 flu season, H1N1 caused the respiratory infection in humans that was commonly referred to as swine flu.
  • However, there are some major problems with relying on community infection to create herd immunity to the virus that causes COVID-19. First, it isn’t yet clear if infection with the COVID-19 virus makes a person immune to future infection.
  • Research suggests that after infection with some coronaviruses, reinfection with the same virus — though usually mild and only happening in a fraction of people — is possible after a period of months or years. Further research is needed to determine the protective effect of antibodies to the virus in those who have been infected.
  • 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.
  • 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.

Herd immunity in COVID-19:

  • Over the months, scientists have seen that COVID-19’s reproduction number has varied from 1.5 to 3.5 in different parts of the world.
  • And since large-scale surveillance programs have not been done, and since the disease has not run its full course anywhere, experts can only approximate mortality rates from symptomatic patients and not from all those who actually have been infected by the virus.
  • So, after many months, we only have a reasonable understanding of how long the person is infectious and the possible modes of transmission. Some debates about sexual and other possible modes of transmissions, and other disease parameters are still ongoing in the scientific community.
  • But beyond all these, there are big questions still plaguing the scientific community — the longevity of the immunity and how quickly the virus transforms itself through mutations.
  • Since it is still early days, it is not only difficult to determine but would also yield an inaccurate estimate of these two factors. A clear understanding of these aspects is necessary to gear up the systems and take appropriate policy decisions.
  • It may so happen that after several mutations, COVID-19 could behave like the influenza virus that warrants the need for a yearly flu shot, if/when a vaccine becomes available. It must be noted that in case of the yearly flu shot, even though some people have less risk, they are urged to take the shot to achieve and maintain herd immunity so that the risk of spread is minimised.
  • In the absence of a vaccine or drug, and without a clear understanding of the disease pathology, seeking to achieve herd immunity through infection is a dangerous strategy.
  • Allow the disease to spread too quickly, it overwhelms the health system and causes many people to die “unnecessarily”; do it too slowly, and it takes that much longer for life to come back to “normal”. Therefore, for almost all countries, at this juncture, it is a cruel choice between saving lives and saving livelihoods.

What Are the Challenges to Developing Herd Immunity to COVID-19?

  • The main obstacle to herd immunity to COVID-19 right now is that the virus that causes the disease is “novel,” or new. That means that it hasn’t infected humans before and everyone is at risk of infection. There’s no existing immunity to build on.
  • Another potential barrier is that we don’t know how strong the immune protection is or how long it will last in people who’ve had COVID-19. Early research on monkeys showed that they made antibodies to the virus that protected them from a second infection a month later. If the coronavirus is like the flu, we can expect a few months of protection.
  • With no vaccine to protect against COVID-19 yet, a large number of people would need to catch the virus, get sick, and recover before we can have herd immunity.

How have we achieved herd immunity for other infectious diseases?

  • Measles, mumps, polio, and chickenpox are examples of infectious diseases that were once very common but are now rare because vaccines helped to establish herd immunity.
  • Sometimes see outbreaks of vaccine-preventable diseases in communities with lower vaccine coverage because they don’t have herd protection
  • For infections without a vaccine, even if many adults have developed immunity because of prior infection, the disease can still circulate among children and can still infect those with weakened immune systems. This was seen for many of the aforementioned diseases before vaccines were developed.
  • Other viruses (like the flu) mutate over time, so antibodies from a previous infection provide protection for only a short period of time.
  • For the flu, this is less than a year. If SARS-CoV-2, the virus that causes COVID-19, is like other coronaviruses that currently infect humans, we can expect that people who get infected will be immune for months to years, but probably not their entire lives.