Story at a glance

  • Herd immunity has been discussed as a strategy for responding to the coronavirus in some countries in Europe.
  • Herd immunity protects a population that has a high proportion of people who have immunity against a disease.
  • It’s why vaccines are effective, and is not a response strategy.

In recent weeks, herd immunity has been mentioned in a few countries in Europe in the response to the COVID-19 pandemic. The experts in the U.K. and Sweden have since walked back those comments or claims that they are considering herd immunity as a strategy to fight the pandemic. What is herd immunity, and what does it mean in the context of the coronavirus?

Herd immunity means that a group contains enough individuals who are immune to a certain disease that, if an infected person entered the group, the disease would not spread easily. In this case, the herd as a whole has “immunity” against the disease even if not all individuals are immune.

How many people need to be immune for herd immunity to take effect depends on the contagiousness of a disease. For example, measles vaccination rates need to be 93 to 95 percent of the population to avoid new cases when the disease is introduced by an infected person. In places where vaccination rates drop below that range, measles can spread and turn into an outbreak.

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But herd immunity is almost only talked about in the context of vaccines. “You don’t rely on the very deadly infectious agent to create an immune population,” says virologist Akiko Iwasaki at the Yale School of Medicine to the Atlantic. That’s because a large proportion of the population would need to get infected, and the way the pandemic is going many would die.

We also don’t know whether COVID-19 survivors are immune to the virus, SARS-CoV-2, after they’ve recovered, and, if they are, for how long. Vaccines are tested for safety and also for efficacy, so experts can be more sure of a population being adequately protected. In summary, herd immunity is not a strategy for responding to an ongoing outbreak. Herd immunity is the reason why vaccines are effective in preventing outbreaks.


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In a discussion on TED Connects, Bill Gates says, “There is no middle course on this thing,” meaning that governments cannot go halfway in taking action against the coronavirus. Over half the population would need to get infected, and that would overload the health care system and increase the fatality rate.

We did not act fast enough to avoid a shutdown, says Gates. If the U.S. government had acted quickly at the first reports of cases, it may have been possible to contain the virus. Now that there is widespread transmission, it’s the testing that will tell you if you need to shutdown more or for longer. Or as you open up after a shutdown, testing and contact tracing will let you know how much you can let up.

Gates is positive that we will get through this and that the world as a whole will try to avoid it happening again. Let’s hope that’s true.

For up-to-date information about COVID-19, check the websites of the Centers for Disease Control and Prevention and the World Health Organization. For updated global case counts, check this page maintained by Johns Hopkins University.

You can follow Chia-Yi Hou on Twitter.


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Published on Mar 31, 2020