Trending Clinical Topic: Herd Immunity

Ryan Syrek


October 16, 2020

Each week, we identify one top search term, speculate about what caused its popularity, and provide an infographic on a related condition. If you have thoughts about what's trending and why, share them with us on Twitter or Facebook. Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

Although the term is more than a century old, herd immunity is this week's top trending clinical topic. Also referred to as community or population immunity, the phrase describes the point at which enough people are sufficiently resistant to a disease that spread becomes unlikely. At that point, the entire community — including those who are not immune — is considered protected.

In late September, Anthony Fauci, MD, had a heated exchange with Senator Rand Paul, MD, about whether restrictive social isolation measures in the United States were actually detrimental in ending the COVID-19 pandemic. Paul pointed to Sweden, which did not institute a lockdown or other significant measures to control the spread of the coronavirus in the hopes of achieving herd immunity. Fauci responded by saying that Sweden's death rate was far worse than that of other comparable countries.

In fact, the death rate in Sweden was 57.64 per 100,000 as of September 23, which was much higher than the rate of 11.06 per 100,000 in Denmark, 5.02 in Norway, and 6.18 in Finland, according to mortality analyses performed by Johns Hopkins. Findings suggest that Sweden has still not yet achieved herd immunity and may not be particularly close to doing so. Neither has New York, according to Fauci, who told Paul that immunity in that city has been estimated to be about 22%, adding, "If you believe that 22% is herd immunity, I believe you are alone in that."

The exact percentage of the population necessary to achieve herd immunity against COVID-19 is thought to be less than the 94% necessary for highly contagious diseases like measles (see Infographic below). The most common means of achieving community protection is vaccination, not widespread illness. Experts agree that pursuing herd immunity without a vaccine would require a high amount of deaths, if such a thing is even possible at all. Reports of reinfection combined with studies that show a lack of protective antibodies after COVID-19 infection suggest that letting the disease "run its course" would not necessarily provide the desired result.

Specific to the United States, Caitlin M. Rivers, PhD, an epidemiologist at Johns Hopkins, explained on a recent episode of the Medicine and the Machine podcast that "we are very far from herd immunity." Even for places around the world that have had a high degree of virus circulating, Rivers said, "It's really not time to let up off our control measures. It's just as important now as it was in March."

Frustrations with COVID-19 and its related restrictions have left most hoping for a quick end to the pandemic. This may explain the recent surge in interest regarding herd immunity, even if achieving such a thing may not be possible in the near future.

Read more about the epidemiology of COVID-19.


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