Herd immunity and the prospect of seeing an end to the COVID-19 crisis

It is the proverbial million-dollar question: When will the United States achieve herd immunity against COVID-19?

The question is so important because many people feel that achieving herd immunity would allow the nation to relax restrictions and begin the economic recovery and return to normalcy. When and even whether this outcome might occur would seem to merit a Las Vegas betting line, given how many highly qualified medical experts and management consulting firms have peered into their crystal balls and shared their respective (and varying) predictions. However, before looking at those predictions, several things need to be considered.

The definition of 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 — to keep it from spreading any further through the population.

The herd immunity threshold

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. That view completely ignores the first component of herd immunity, which is the natural immunity found in that significant percentage of the population comprising those who have already contracted and recovered from the virus.

The prevalence of natural immunity

There is a debate over natural immunity. 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, a study reported by Columbia University estimated that figure had surpassed 120 million people by the end of January — more than a third of the population.^a And even more recently, Marty Makary, MD, 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.^b Makary’s estimate has been contested by some researchers. The truth is probably somewhere between his estimate and the one reported by Columbia University — 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

Herd immunity may be unachievable if we see a proliferation of COVID-19 variants that are strongly resistant to the vaccines. Fortunately, the three vaccines currently approved for use in the U.S. — from Pfizer, Moderna and Johnson & Johnson— have proven fairly effective against the B.1.1.7 variant from the U.K. and the B.1.351 variant from South Africa.^c But these vaccines are not quite as effective against these variants as they are against the original novel coronavirus.

Waning immunity

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

Vaccine availability

On Feb. 27, the FDA granted Emergency Use Authorization to the Johnson & Johnson vaccine. The company promised that — largely due to its partnership with Merck and its two U.S. facilities devoted to manufacturing the vaccine — 100 million doses will be produced by the end of June. Meanwhile, Pfizer and Moderna ramped up manufacturing of their vaccines.

Given these developments, President Joe Biden announced on March 2 that the U.S. would have enough vaccine supply for every adult in America by the end of May. On April 6, he said that all U.S. adults should be eligible for COVID-19 vaccines by April 19, and the New York Times reported on that date that the deadline had been met.^d Also on April 19, the Centers for Disease Control and Prevention reported that 132.3 million people had received at least one dose while 85.4 million people were fully vaccinated.^e  Even with the FDA’s temporary suspension of production of the Johnson & Johnson vaccine, the Biden administration had stated that the U.S. has “more than enough Pfizer and Moderna vaccine supply to continue or even accelerate the current pace of vaccinations.”^f

Herd immunity projections

Based on a broad range of herd immunity projections, the average prediction for achieving herd immunity for the U.S. is August 2021, within an overall range of April to November 2021.^g However, it is important to note that the updated vaccine supply promise made by President Biden was not factored into these projections. The April 19 New York Times article notes the United States is on track to vaccinate 70% of its population by mid-June, achieving the low end of the most commonly cited range for the herd immunity threshold, although vaccine hesitancy remains a risk.

A reasonable prognosis

In the final analysis, taking into consideration all the forces that could accelerate or delay achievement of herd immunity, the nation could expect to see an end to at least the first round of this bout against COVID-19 during the summer of 2021. Then, the future will depend on continued societal vigilance and development of new tests, therapeutics and vaccines. 

Footnotes

^a Columbia University Mailman School of Public Health, “Active COVID cases may be ten times official count: Study,” Feb. 9 2021.

^b Makary, M., “We’ll have herd immunity by April,” The Wall Street Journal, Feb. 18, 2021.

^c Prior to hfm’s press time for the print version of this column,  vaccinations with the Johnson & Johnson vaccine were temporarily paused due to rare clotting cases. Only a short time aftyer press time, the vaccine was reistated on the recommendation of an expert panel.

^d Anthes, E., et al., “Adults in all U.S. states are now eligible for vaccination, hitting Biden’s target. Half have had at least one dose.,” The New York Times, April 19, 2021.

^e Nadeem, D., “U.S. administers 211.6 mln doses of COVID-19 vaccines – CDC,” Reuters, April 19, 2021.

^f White House COVID-19 Response Team press briefing, April 14, 2021..

^g This prediction is based on projections by Marty Makary, MD; Moncef Slaoui, PhD, former head of Operation Warp Speed; Robert M. Wachter, MD, of the Department of Medicine at UCSF; the Public Health Informatics, Computational and Operations Research team at the City University of New York School of Public Health and Health Policy; Gonzalo Bearman, MD, of Virginia Commonwealth University and the management consulting companies Oliver Wyman and McKinsey. Of these prognosticators, Makary is most optimistic and Wachter is least optimistic.