Some areas of the United States may be close to reaching herd immunity, even if the country as a whole is still far off.
Herd immunity is achieved when enough of the population is either vaccinated or has natural immunity from infection.
Matthew Ferrari, a biology professor and director of Center for Infectious Disease Dynamics at Penn State University, said the process of herd immunity began when the first person in the U.S. was infected with the coronavirus and survived. Each person who is infected and survives reduces the risk of other people catching the virus by a tiny amount.
“The question most people are now asking is, ‘Have we achieved a herd immunity threshold?’” Ferrari said. “The threshold is a sufficient level of immunity in a population such that outbreaks are unlikely to start. We’re in an outbreak right now, so it is a little absurd to be talking about it.”
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Most experts say 80% of the population must be immune to achieve herd immunity, which makes the U.S., with only 40% of the population fully vaccinated, still a long way off.
Yet, certain areas may be getting close.
“I would say that there are absolutely communities in the country that have reached that level through either vaccination, natural infection, or both,” Ferrari said.
For example, long-term care facilities may be close to reaching herd immunity as 2.8 million residents and staff have been fully vaccinated. Weekly COVID-19 cases in long-term care facilities have fallen from 33,659 in December to 701 in mid-May, according to the Centers for Medicare and Medicaid Services.
Susan Hassig, an epidemiology professor at Tulane University School of Public Health and Tropical Medicine, pointed to states in the Northeast as “places where we are very, very close [to herd immunity]…because we see very few cases and hospitalizations occurring there.”
Connecticut, for example, reported only 88 new COVID-19 cases on Wednesday and seven hospitalizations. Massachusetts reported 211 new cases and 253 hospitalizations. The states have fully vaccinated 52% and 51% of their entire population, respectively.
The percentage of immunity in those states increases when the number of COVID-19 infections is included: 339,100 in Connecticut and 660,298 in Massachusetts. However, it is not a simple matter of adding the confirmed number of cases. The actual number of cases are likely two to four times higher given the number of people with asymptomatic infections or mild symptoms who were never tested. There is also some overlap among people who have been infected and vaccinated.
Assuming that actual cases are three times higher than confirmed cases and that half of those have also been vaccinated, about 66% of the population in Connecticut may be immune and 65% in Massachusetts.
While some areas may be nearing herd immunity, others will continue to be vulnerable because of lower vaccination rates.
“We are going to see pockets and outbreaks of infections occur as long as there is a vulnerable population and there are small amounts of the virus in communities,” said Dr. Manoj Jain, an infectious disease physician at the Rollins School of Public Health.
Hassig suggested that in her state of Louisiana, New Orleans is likely to get close to herd immunity while the rest of the state remains vulnerable. Orleans Parish, which includes New Orleans, has a vaccination rate of almost 47%. Many other parishes have not yet achieved 30%.
Jain, an adviser to the Memphis-Shelby County COVID-19 Joint Task Force, said communities need to do more surveillance of blood samples to determine if herd immunity is being reached.
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Jain shared with the Washington Examiner data from about 300 blood samples taken by primary-care physicians in Shelby County. In February, barely 40% of the county had coronavirus antibodies either from vaccines or natural immunity. By April, that had increased to 67%.
