A new study that pushes back against some accepted notions of how the coronavirus is transmitted argues that herd immunity might already have been reached in some parts of the nation.
The study, “Persistent heterogeneity not short-term overdispersion determines herd immunity to COVID-19,” was published Aug. 10 in the Proceedings of the National Academy of Sciences.
It pointed out that current infection models assume there are so-called “super-spreaders,” which are a small number of individuals who, because of their social activity, pass the virus along to large numbers of other people.
In this model, everyone everywhere has an equal chance of infection.
Writing at Reason, Ronald Bailey offered an interpretation of what the study thinks is happening instead.
Bailey wrote that the researchers define a concept they call heterogeneity “as the biological and social susceptibility of individual members of the population to COVID-19 viral infection.”
The study relies on what is called “biological heterogeneity,” which includes “the strength of immune responses, genetics, age, and comorbidities,” and social heterogeneity, which measures close contacts among individuals, according to Reason.
“Taking the effects of biological and social heterogeneity on COVID-19 transmissibility, the researchers calculate that the herd immunity threshold is likely somewhere between 20 and 30 percent of the population,” Bailey wrote.
The researchers then conducted a modeling exercise of what might happen this fall, which has been cited as a time when a feared second wave of coronavirus infections might take place.
In what the study’s authors called a “worst-case scenario” in which virtually everything has returned to pre-pandemic ways of life, the study predicted “virtually no second wave of COVID-19 cases in New York City which indicates that herd immunity has likely been achieved there,” Bailey wrote.
“By applying our theory to the COVID-19 epidemic we found evidence that the hardest hit areas such as New York City, have likely passed the heterogeneity-modified herd immunity threshold,” the study said.
Bailey noted that the researchers “calculate that Chicago has not passed the herd immunity threshold,” but found that “the effects of biological and social heterogeneity would still result in a substantial reduction of the magnitude of the second wave there, even under the worst-case scenario.”
The study suggested that moderately restrictive measures — such as social distancing and wearing masks — might be enough to avoid a second wave in areas that are close to herd immunity but have not yet achieved it.
“While the coronavirus may not just fade away, these calculations imply that the U.S. has a good chance to avoid a potentially disastrous second wave this fall if the public maintains reasonable social distancing and mask-wearing efforts,” Bailey wrote.
There is a potential catch: Herd immunity to the virus may not last forever.
“This means we need to be careful about declaring the epidemic over in a given location simply because cases have started to decline,” Justin Lessler, an associate professor of epidemiology at the Johns Hopkins University School of Public Health, wrote in an Op-Ed in The Washington Post on Friday.
“What may be adequate immunity to stem the virus in summer may not be enough to stop epidemic spread in the winter — a phenomenon seen in each of the four modern influenza pandemics, where an initial summer wave quickly receded only to be followed by a large epidemic in the fall or winter.”
Lessler said community immunity will emerge and reduce the severity of each wave of the disease.
“The long fight against COVID-19 is far from over, and we will likely be living with the virus in some form for years to come. But each round we fight with this virus will get easier due to the accumulating effects of community immunity, even if we never reach a promised land where herd protection makes epidemics a thing of the past.
“Hopefully, we will soon have a vaccine that accelerates this process, but even when one arrives, it will take time to distribute to large portions of the population. As we wait, every day and every person who becomes immune makes the fight easier. This growing protection will allow us to slowly roll back control measures and return to a more normal life,” he wrote.
“But we should remember that community immunity is a journey, not a destination, and we have to proceed carefully on our voyage back to the way things were before the virus spread around the globe.”
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