COVID-19 and the Hunt for Herd Immunity

Now that the Johnson & Johnson COVID-19 vaccine has been authorized in the U.S., along with the Pfizer-BioNTech and Moderna vaccines, and manufacturing and distribution ramps up, the debate over herd immunity is heating up. Currently, vaccination efforts are resulting in fewer new cases, hospitalizations and deaths.

Herd immunity is the point where enough people are protected from a disease, whether by having had it or by vaccination, that it can’t spread through the population. It tends to be more commonly discussed in terms of vaccinated populations.

What percentage of a population needs to be immune in order to achieve herd immunity is a matter of debate among scientists and public health officials. And unfortunately, even before the first vaccine was authorized in late-2020, the topic of herd immunity was politicized.

A scientist with the U.S. Centers for Disease Control and Prevention (CDC), Adam MacNeil, on Friday stated the U.S. was “nowhere close” to achieving herd immunity from COVID-19. MacNeil also argued that the more transmissible variants would push the bar for achieving herd immunity higher, taking longer to achieve it.

“Currently we know that the majority of the U.S. population is not immune to SARS-CoV-2 and variants may cause this portion of the population that is not immune to increase.” MacNeil was speaking at the U.S. Food and Drug Administration (FDA) meeting that reviewed the J&J vaccine for emergency authorization.

Some of his remarks were in response to a Wall Street Journal opinion piece that noted that COVID-19 cases have dropped 77% in six weeks and that herd immunity could be achieved by mid-April. The author of the WSJ op-ed was Marty Makary, professor at the Johns Hopkins School of Medicine and Bloomberg School of Public Health, chief medical adviser to Sesame Care, and author of “The Price We Pay.”

Makary’s calculations argue that testing has only been capturing 10% to 25% of infections, so a much larger proportion of the population already has immunity. So by applying a time-weighted case capture average of 1 in 6.5 to the total 28 million confirmed cases, that means about 55% of Americans have natural immunity. And as of this week, 15% of people in the U.S. have been vaccinated, with numbers going up rapidly. Former FDA Commissioner Scott Gottlieb estimates 150 million people—slightly less than half the U.S. population—will have received about 250 million doses by the end of March. Makary believes that at this rate, COVID-19 will mostly be gone by April.

In addition, he points out that several groups of scientists, some at Sweden’s Karolinska Institute and others in the U.K., have found that people with mild or asymptomatic COVID-19 infection appear to have significant T-cell responses to infection that are stronger than detectable antibody responses, which suggests a much broader immunity.

A number of experts argue that there should be less talk of herd immunity because they don’t want the public to drop their guard or decide not to get vaccinated because there are fewer infections occurring. They believe a sense of urgency is needed to get the public to cooperate with vaccinations, as well as to continue to monitor for new variants and work on developing vaccines against them.

A CDC report estimated that by the end of 2020, 83 million people in the U.S. had COVID-19, which put the country about a third of the way to herd immunity. The pace of vaccinations, by that calculation, would have the country approaching herd immunity around June via a combination of vaccinations and natural immunity.

CNN discussed it with five experts, and their projections for herd immunity ranged from about 65% to 95%, with most agreeing that somewhere in the middle was probably most accurate. All expressed concern about the emergence of variants.

Justin Lessler, associate professor of epidemiology at Johns Hopkins, said, “There are some variants that appear to be escaping that immunity and changing the equation. In a completely susceptible population, the average person with the original strain would spread the virus to three people, so you need two people to be immune to start decreasing the spread. But with some variants, the average person may spread the disease to around five people, so you need four people to be immune to cut the spread.”

That appears to be something of a worst-case-scenario argument, but Aneesh Metha of the Emory Vaccine Center, said, “We think vaccines will provide good immunity to most variants out there, but there are some—particularly the variant first identified in South Africa—that appear to have the ability to get around that and may alter our level of protection. That’s why it’s so important to vaccinate as quickly as we can. The quicker we get to higher levels of protection in our community, the chances for new variants to spread and develop diminishes.”

Although none of the experts were willing to give a solid prediction for the end of the pandemic, all thought that by mid-year and fall things would likely get back to normal and almost for certain by the end of the year.

Jessica Malaty Rivera, science communications lead at the COVID Tracking Project, said, “Vaccinating 70% of the population is going to be an absolute game changer for us in terms of getting back to what we could do similar to pre-COVID days. I’m optimistic that this fall is going to look very different, and hope that means things like travel and mask-free socializing with loved ones. We’re on the road to getting there if we keep the trends down.”