What Exactly is Herd Immunity?

Ken Perez
Vice President of Healthcare Policy and Government Affairs at Omnicell

All About Herd Immunity

You’ve probably heard the term “herd immunity” debated and discussed in the news. But for such a popular term to be bandied about on a daily basis, one common question remains: what actually is “herd immunity?”

Herd immunity, also known as community immunity, is an epidemiological concept that refers to the point when enough people are immune to the virus—either because they have natural immunity from prior illness or because they have been vaccinated—so that the virus can no longer spread through the population.

What percentage of the population needs to be immune to the virus in order for herd immunity to be achieved? This is a much-debated topic, with the low end at 50% of the population (according to the World Health Organization) and 90% at the high end. The most commonly cited range is 70% to 85%.

However, some experts, in an attempt to motivate the public to get vaccinated, have erroneously presented the 70% to 85% range as the target proportion of the population that needs to be vaccinated, ignoring completely the significant percentage of the population that has already recovered from the virus.

Regarding the reality of achieving herd immunity, here are some factors and projects:

The Prevalence of Natural Immunity

There is a debate over the first component of herd immunity, the number of people who have natural immunity because they have contracted and recovered from the virus. The Centers for Disease Control and Prevention (CDC) estimates that at the end of December 2020, 83.1 million people—about 25% of the population—had had an infection.

More recently, researchers at Columbia University estimated that by the end of January, that figure had risen to 118 million people—36% of the population. And even more recently, Marty Makary from Johns Hopkins University, citing 28 million confirmed cases via testing and applying a time-weighted case capture average of 1 in 6.5, estimated that about 55% of Americans—roughly 180 million people—have natural immunity. Makary’s estimate has been contested by some researchers. The truth is probably somewhere between his estimate and the one from Columbia University at the end of January—perhaps 40% to 45% of the population.

Vaccine Hesitancy

As for the second component of herd immunity, the number of people who are vaccinated, the public’s willingness to get vaccinated has been in question. According to a February poll conducted by the Kaiser Family Foundation, at least 55% of Americans have received the vaccine or plan to get vaccinated as soon as possible up from 47% the previous month. An additional one in five Americans say they are willing to get vaccinated but plan to wait, a decrease from 31% in January. These trends suggest that eventually about 80% of Americans will get vaccinated, which bodes well for the achievement of herd immunity.

The Variants

Unfortunately, herd immunity may be unachievable if COVID-19 variants proliferate and the vaccines are significantly less effective against them. Fortunately, the three vaccines currently approved for use in the United States—from Pfizer, Moderna, and Johnson & Johnson—are fairly effective against the B.1.1.7 variant from the United Kingdom and the B.1.351 variant from South Africa, though less than against the original novel coronavirus. Pfizer has expressed optimism that its vaccine is effective against the variants, and Moderna has developed variant-specific vaccine candidates.

A recent study published in the journal Science has found that natural immunity can last up to eight months. As for the duration of vaccine-induced immunity, it is hoped that the Pfizer and Moderna vaccines will confer immunity for 2-3 years, though they (and other approved vaccines) will probably require annual booster shots.

Vaccine Availability

On Feb. 27, the U.S. Food and Drug Administration (FDA) granted Emergency Use Authorization to Johnson & Johnson’s vaccine. The company has promised, in large measure due to its partnership with Merck in which the latter company will devote two U.S. facilities to manufacture the vaccine, that 100 million doses will be produced by the end of June. Moreover, on March 2, President Joe Biden emphasized that the U.S. will have enough vaccine supply for every adult in America by the end of May—two months earlier than previously forecast—which indicates that Pfizer and Moderna are ramping up their production volumes significantly.

Herd Immunity Projections

Based on herd immunity projections issued by management consulting firms Oliver Wyman and McKinsey, Moncef Slaoui (former head of Operation Warp Speed), Bob Wachter of UCSF, Medscape, Makary, PHICOR at the City University of New York School of Public Health & Health Policy, and Gonzalo Bearman of Virginia Commonwealth University, the average prediction for achievement of herd immunity for the U.S. is August 2021, with the most optimistic, April 2021 (Makary), and the most pessimistic, November 2021 (Medscape). However, it is important to note that the projections did not factor in the FDA’s recent approval of the Johnson & Johnson single-shot vaccine nor the updated vaccine supply promise made by President Biden.

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DISCLAIMER

The views and opinions expressed in this blog are those of the authors and do not necessarily reflect the official policy or position of any other agency, organization, employer or company. Assumptions made in the analysis are not reflective of the position of any entity other than the author(s). These views are always subject to change, revision, and rethinking at any time and may not be held in perpetuity.