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What Is ‘Herd Immunity’ and Can It Stop COVID-19?

A concept bandied about early in the coronavirus pandemic, something called "herd immunity," has surfaced again. President Trump referenced the idea during an ABC News town hall last month, insisting the virus would eventually go away as people develop "a herd mentality." (Okay, he misspoke, but we think we know what he meant.)

Then, during a hearing on the nation's pandemic response, Senator Rand Paul (R-Kentucky) tussled with Anthony Fauci, MD, the nation's leading infectious disease expert, over whether "community immunity" is responsible for New York City's currently low infection rate. "They’re no longer having the pandemic because they have enough immunity in New York City to actually stop it," asserted the state's junior senator. Dr. Fauci, a member of the White House Coronavirus Task Force, forcefully pushed back, arguing that New York's low positivity rate reflects its adherence to task force recommendations, including the use of masks and social distancing, not herd immunity.

Now the head of the World Health Organization (WHO) is weighing in with a strong rebuke. On Monday, Director-General Tedros Adhanom Ghebreyesus said reaching herd immunity by letting the virus spread throughout the population would be "simply unethical," resulting in unnecessary suffering and death. "Never in the history of public health has herd immunity been used as a strategy for responding to an outbreak, let alone a pandemic. It is scientifically and ethically problematic," he said.

So what is herd immunity, anyway, and how does it relate to COVID-19?

Herd immunity (also known as community immunity) is defined by the Centers for Disease Control and Prevention (CDC) as “a situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely.” In other words, where herd immunity exists—when lots of people in an area are vaccinated or have already been infected with a disease—fewer people get sick and fewer germs are able to spread from person to person.

The CDC adds that even people who are not vaccinated, like newborn babies and individuals with chronic illnesses, are afforded some level of protection because the disease can’t spread within the community. 

The theory is that when someone gets vaccinated, it’s not only that person who is protected from infection but others too, because that individual cannot transmit the disease in the community. In that way, herd immunity protects people who cannot be vaccinated, people whose immune systems aren’t strong enough and are therefore the most vulnerable to serious illness. 

A late 1980s measles outbreak among preschool-age children in the US serves as an example of herd immunity via vaccination. Researchers who examined the association between incidence of measles and immunization among preschool-age children concluded that immunizing about 80% of the population may be enough to stop sustained measles outbreaks in an urban community.

Of course, there’s no vaccine for COVID-19 yet. In the context of COVID-19, developing herd immunity would mean protecting the most vulnerable citizens while letting most everyone else catch, and hopefully recover from, the virus.

Allowing people to contract COVID-19—Is that a good idea?

Earlier in the year, UK Prime Minister Boris Johnson had reportedly been considering herd immunity as a strategy. However, on March 17, BBC reported that the UK had shifted gears based on new modeling on the severity of the situation and the number of people who might die. Instead, Johnson ordered a lockdown to control the spread of the virus. Per the Washington Post, the prime minister was absent from public view during most of that period as he battled his own case of coronavirus.

Sweden famously eschewed a lockdown approach in favor of allowing people to take personal responsibility for their health. But as the authors of a recent commentary published by the Journal of the Royal Society of Medicine point out, "herd immunity is nowhere in sight." Rates of infection, hospitalization, and death per million people are much higher than in neighboring Scandinavian countries, they report.

WHO's Ghebreyesus makes it clear that herd immunity is something that's achieved when a certain threshold of vaccination is reached against a virus in a population. "In other words, herd immunity is achieved by protecting people from a virus, not by exposing them to it," he said.

For now, US public health officials continue to emphasize measures shown to curb the spread of infection, and that includes social distancing and wearing face masks.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.

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  • Posted on October 15, 2020