COVID-19 Data Dives: The High Cost of Herd Immunity

William P. Hanage, PhD


May 13, 2020

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William P. Hanage, PhD

The excellent Natalie Dean and Carl Bergstrom wrote a recent opinion piece in the New York Times that emphasized a key point: Proponents of "herd immunity" strategies need to be upfront about what that means: a lot of illness and death.

Way back in the early 2000s there was concern following 9/11 about the potential for bioterrorism—in particular, the possibility of a deliberate release of smallpox. Smallpox is caused by an extraordinarily virulent virus that is also extinct in the wild.

Why is it extinct? Because of vaccines. People forget, but the World Health Organization ran a sustained campaign to eradicate smallpox through the 1970s, hunting the virus and ring-vaccinating around it until eventually declaring in 1980 that "smallpox is dead."

As a result, many people have never received a smallpox vaccine. There was just no need to vaccinate once the virus was no longer a risk. But smallpox is not wholly gone from Earth; samples remain in labs with very tight security. Hence the anxiety post 9/11.

A deliberate release of smallpox would be catastrophic. So, widespread vaccination was contemplated, as explained in an article in 2002 featuring, among others, Anthony Fauci, MD, who has been at the center of the national conversation around infectious disease for a long time.

Bottom line: Due to very rare adverse side effects, noted AIDS public health advocate Donald P. Francis, MD, estimated that "vaccine use in the US population would kill 250 to 1,000 people. If someone has HIV and doesn't know it, God knows what would happen."

Just for context, that estimate of 250-1000 people dying throughout the United States was considered too great a cost for herd immunity. According to data from The COVID Tracking Project, at the time I am writing this, there have been more than 1000 COVID-19 deaths in California, Connecticut, Florida, Georgia, Illinois, Indiana, Louisiana, Maryland, Massachusetts, Michigan, New Jersey, New York, Ohio, and Pennsylvania. There have been more than 250 in Alabama, Arizona, Colorado, Minnesota, Missouri, North Carolina, Rhode Island, South Carolina, Texas, Virginia, Washington, and Wisconsin. There have been 18,610 deaths in the state of New York alone. This is the cost.

And do you know the worst thing? Dead people can't even contribute to herd immunity. Herd immunity works because the virus cannot transmit to a contact who is already immune. Dead people don't make contacts.

For now, we don't even know for sure how much immunity is generated by SARS-CoV-2 infection or how it varies by age or the severity of disease or a lot of other stuff.

So, yeah—herd immunity is a thing. How you get it matters.

Bill Hanage is an associate professor at the Center for Communicable Disease Dynamics in the Department of Epidemiology at the Harvard T. H. Chan School of Public Health. He specializes in pathogen evolution. Follow him on Twitter.

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