As COVID-19 cases continue to increase across the country, many people are asking whether so-called herd immunity might be the best strategy to defeat the disease.
Public health experts say talk of herd immunity is premature until we have a vaccine for the deadly virus. Here’s what you need to know:
What is herd immunity?
Herd immunity, or community immunity, happens when a sufficient proportion of a population (the percentage varies by disease) is immune to an infectious disease through vaccination or prior illness to make its spread from person to person unlikely, according to the Centers for Disease Control and Prevention.
Even individuals not vaccinated (such as newborns and those with contraindications for the vaccines) are offered some protection because the disease has little opportunity to spread within the community.
This assumes that COVID-19 immunity through infection is long-lasting, said Christelle Ilboudo, MD, infectious disease expert at MU Health Care. Some studies have shown a progressive decline in antibodies after infection with other types of coronaviruses, but scientists are still not certain whether infection provides long-term protection from the virus.
What is an example of herd immunity?
Measles provides a good example. It is a highly contagious infectious disease for which we have very effective vaccines. Public health experts say if 93-95% of the population is immune to measles, a feasible target with the available vaccine, then that will protect the entire population.
This herd or community immunity acts as a barrier against the disease, with the immune people breaking the potential chain of transmission so those vulnerable populations are unlikely to get it.
How close are we to herd immunity for COVID-19?
The U.S. population is 328 million. There have been roughly 11 million cases of COVID-19 to date, and that equals roughly 3% of the population. Experts estimate that herd immunity would require at least 60-70% of the population to have COVID-19 immunity, and that translates to roughly 200 million people.
In the absence of a vaccine to provide immunity, that number would have to come from infections, and with the current death rate of 2%, that prevalence of infection would mean roughly 4 million preventable deaths.
Health experts say the cost in human life is too high to pursue this strategy, and Ilboudo noted there are no examples of community immunity to any pandemic viral infectious diseases without a vaccine.
“We have not achieved any herd immunity through a natural disease process to most major infectious diseases that affect the population to this scale,” she said. “All of the major infections I know of have required vaccination.”
Will we ever achieve herd immunity?
Once there is a vaccine for COVID-19, the immunity threshold should be within reach.
Until a vaccine is available, the best course of action is to continue following public health measures — physical distancing, masking in public and good handwashing hygiene — as those also break the virus transmission chain.