What is herd immunity?

what is herd immunity
Photo via Muenocchio/Flickr (Public Domain)

As scientists race to develop a vaccine for COVID-19, leaders worldwide believe herd immunity could solve the globe’s coronavirus woes. 

Also called community immunity, herd immunity occurs when so many people in a community become immune to a disease that the disease stops spreading. 

According to Healthline, there are two main ways herd immunity can occur: 

  1. A percentage of people contract the virus and eventually build up immunity to it. 
  2.  Many people receive a vaccination that gives them immunity to the virus. 

Administering a vaccine is the ideal option, as far fewer people get sick and run the risk of dying from COVID-19. 

For most diseases, 80-90% of the population must be immune to the illness for it to stop spreading. For example, the Vaccine Knowledge Project reports that 19 out of every 20 people must have the measles vaccines for there to be herd immunity.

Some experts estimate that about 60-70% of the population needs to develop an immunity to COVID-19. That means, absent a vaccine, there’s a good chance that at least 60% of citizens in a country would have to contract, survive, and build up an immunity to this coronavirus for the country to develop a natural herd immunity.  

But the New York Times reported on Aug. 17 that some scientists believe that herd immunity could occur with 50% or less of the population immune to the virus. “I’m quite prepared to believe that there are pockets in New York City and London which have substantial immunity,” Bill Hanage, an epidemiologist at Harvard, told the newspaper.

Collectively, 1.7 million of the 331 million U.S. residents have contracted COVID-19 as of June 1, according to the Centers for Disease Control and Prevention. That’s less than 1% of the population—and more than 100,000 people have already died in the meantime. Even in states where the outbreak has been particularly bad, like New York and New Jersey, the immunity rate remains low.

(As of Aug. 18, 5.44 million Americans had been diagnosed, about 1.6% of the population, with more than 170,000 dead.)

For natural herd immunity to occur, almost 200 million people in the U.S. would have to build a resistance to COVID-19 by contracting the virus. If the death rate remained consistent, 12.5 million people in the U.S. could die before the country developed herd immunity.  

The History of Vaccines says that, so far, the smallest percentage of immune people needed to create herd immunity has been 40%. So in the U.S., the absolute best-case scenario is that around 134 million people need to contract and survive COVID-19. 

Unlike most countries that issued lockdowns to combat COVID-19, Sweden kept its economy open in hopes of developing herd immunity. Sweden had hoped to reach the 60% immunity threshold in May, but NPR said a study released by Sweden’s Public Health Agency showed that 7.3% of people in Stockholm had developed antibodies against COVID-19 by late April. Now, Sweden has a higher mortality rate than even the U.S., and its economy still took a hit from the pandemic. 

And there’s a catch: Natural herd immunity only works if the body can develop antibodies after it catches it the first time, according to Healthline. If the body can create antibodies that combat the disease, the body becomes immune to the disease. But scientists can’t confirm that humans develop an immunity to COVID-19 after recovering from it the first time.  

Natural herd immunity appears to be an implausible way to combat COVID-19. People will probably have to hold out for a vaccine before any community can form herd immunity. In the meantime, social distancing and frequent handwashing are the only known ways to protect yourself and your loved ones from contracting COVID-19. 

Sources: Healthline, Vaccine Knowledge Project, NPR, CDC, History of Vaccines, Fortune

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