Though herd immunity has been widely touted as the indicator that the pandemic has come to an end, experts at a Wednesday panel said it is more likely that there will be a plateau in infection, with a decrease in prominence over time but continued circulation.
This view has wide support among the scientific community. Almost 90% of scientists surveyed by Nature in February said that COVID-19 will join influenza, malaria and other common diseases by becoming endemic, meaning that it will continue to exist in corners of the population but without propagating on a wide scale.
The event, which featured four keynote speakers, was organized by Knight Hennessy scholar Mallory Harris, a second-year biology Ph.D. student. Her frustration about the confusion and misconceptions around the idea of herd immunity led her to organize a panel of leading experts on the subject to overview key concepts and answer questions from the community.
Harvard epidemiology professor Marc Lipsitch said a view of herd immunity that is limited to the U.S. is insufficient — and once you look globally, it becomes more unclear how the world’s population will all be vaccinated.
“This virus is going to continue to circulate,” he said. “Global coverage is clearly going to be low and slow. The projections for distribution of doses, in the coming months, is in the hundreds of millions of dose range for a planet of billions of people. It’s just not going to be adequate.”
But, according to Lipsitch, achieving herd immunity through vaccination may not be entirely necessary to return to something resembling pre-pandemic normality.
Biologist Lauren Ancel Meyers Ph.D. ’00 agreed: “Maybe the endgame is really not about herd immunity, but it’s the mild endemic state where the vaccines and prior infection have transformed COVID-19,” she said.
By a mild endemic state, Meyers is referring to the point in a population at which the rate of infection of a disease no longer grows or declines exponentially but still causes people to get sick on occasion.
This is in contrast to herd immunity, a phenomenon that occurs when enough of a population has gained immunity against a disease to confer protection to everyone in the population, as the disease has nowhere to spread.
Experts said that vaccines will still play a critical role in reaching that endemic stage, bolstered by encouraging results from vaccine trials.
The Johnson & Johnson vaccine data submitted to the Food and Drug Administration provides further evidence of a reduction in the viral load of those who get vaccinated, according to Lipsitch. The trial found asymptomatic infections to be 74% lower in those vaccinated. Likewise, he said that “the difference between unvaccinated and fully vaccinated people … comes to about a 97% reduction in the amount of viral RNA.”
As demonstrated in the study, Lipsitch explained that vaccinating those at the highest risk is likely to reduce severe outcomes, and the toll on the healthcare system since vaccine effectiveness is high against severe outcomes and in at-risk populations.
In situations where achieving herd immunity is possible, Lipsitch said it is important to keep in mind that that state is fluid: It’s a “continuous factor rather than a binary factor.” The quantity is not fixed; in other words, the goalpost can move.
What it takes for any given community to reach herd immunity will also vary. Meyers showed a map of east Austin ZIP code areas, which are majority Black and Latino, and most west Austin zip codes, which are majority white numbers. “East Austin is expected to have a higher herd immunity threshold just because of the intrinsic risks and suffer a far higher ratio of infections to vaccinations than west Austin,” Meyers explained.
Achieving herd immunity is not guaranteed, especially given complications like emerging variants, pockets of low vaccination coverage, hesitancy and lack of access. “But careful deconstruction of herd immunity can provide insights that get us to a safer, healthier and more equitable and open society in the months ahead,” she said.
Beyond those months, it is unclear how long the protection will last. Emory biology professor Rustom Antia said there will not be sustained immunity in adults and that it is important to empirically conclude whether infection and vaccination will provide protection in the long term.
Instead, the world will reach a point where COVID-19 is “an endemic disease of low pathogenicity in young children, which I think is probably the direction we’re going,” said Julie Parsonnet, a professor in medicine, epidemiology and population health at Stanford.
Rather than “worrying about that specific number,” she suggested focusing on the distribution of vaccines and following the recommendations of public health departments.