HARTFORD, Conn. — While vaccination remains the most important tool in ending Connecticut’s COVID-19 outbreak once and for all, experts say another factor could play a role as well: immunity from previous coronavirus infection, known as natural immunity.
Natural immunity is not absolute and is not indefinite. As with immunity from vaccination, it fades over time. But research suggests those who have had COVID-19 are unlikely to catch it again immediately and therefore unlikely to contribute to a broader outbreak in the near future.
Although estimates vary on what share of Connecticut residents have been infected with COVID-19 since the start of the pandemic, one model suggests the number may be approaching 50%. If that’s the case, experts say, natural immunity among people who have been previously infected could make some difference in ending the pandemic in the state.
How much of a difference, however, remains up for debate.
“We should acknowledge that many people do have some level of immunity (after prior infection),” said Dr. David Banach, an epidemiologist at UConn Health. “But there are still some questions that remain regarding the durability of that immunity, and there’s a lot of variability in terms of natural immunity.”
Given limits on testing, particularly early in the pandemic, and the possibility of undetected asymptomatic cases, no one can say for sure how many Connecticut residents have had COVID-19. Researchers can, however, estimate the true spread of the disease based on seroprevalence studies, reported cases, and death counts.
According to one model run by researchers from the Yale School of Public Health, Harvard T.H. Chan School of Public Health and Stanford Medicine, about 47% of Connecticut residents have been infected with COVID-19 at least once since the start of the pandemic. This projection accounts for not only the 400,000 recorded cases in the state but also an unknown number of unrecorded ones. In some cases, residents may have been infected without experiencing any symptoms.
Other models have the true number of infections significantly lower. Pedro Mendes, a computational biologist at UConn Health, says his model estimates that fewer than 20% of Connecticut residents have been infected and suggests that if his model is on the low end, the one from Yale, Harvard and Stanford is likely on the high end. Dr. Ulysses Wu, an epidemiologist at Hartford HealthCare, says the true total could be anywhere from 10% to 50%.
“When you get (natural immunity and vaccination) together, you will be super immune.”
Regardless of the precise number, what’s clear is that hundreds of thousands of Connecticut residents have had COVID-19 previously, leaving many with some level of protection.
Still, experts say it’s important to note that previous infection does not mean complete immunity against future infection. Because protection wanes over time, people who had the disease during Connecticut’s initial surge in spring 2020 or even during the state’s second wave last winter likely lack immunity today.
“The people who were infected earliest are most likely to have waning immunity over time,” Banach said, “and would be at risk for reinfection.”
In combination with vaccination, experts say, natural immunity may help Connecticut reach a point where there aren’t enough vulnerable people left for an outbreak to occur.
“I think it plays a very big role,” Wu said. “We have to factor that in.”
Wu says he thinks Connecticut will be overwhelmingly protected against COVID-19 when about 90% of residents are immune. So far, about 70% are fully vaccinated, a number that will increase when 5-11 year-olds become eligible as soon as next week. If a large chunk of the remaining unvaccinated residents have some natural immunity, Connecticut may not be too far from Wu’s target.
Although Banach cautions against counting on natural infection to save Connecticut from future outbreaks, he agrees it can at least make a difference at the margins.
“We can acknowledge that people who have had natural infection and the resulting immunity do play a role in terms of their contribution to reducing the spread of infection,” Banach said. “Our level of population immunity incorporates both vaccine-related immunity and natural immunity.”
Still, experts emphasize that widespread natural immunity alone is not enough to protect a community. Dr. Ted Cohn, a Yale School of Public Health researcher who has worked on the model estimating 47% of Connecticut residents have been infected with COVID-19, says people sometimes use that figure to argue, incorrectly, that the state might get to population-level immunity based on prior infection alone.
That, he said, is a “dangerous interpretation.”
“We’ve seen very little evidence that even very high levels of natural infection, or the overlap of natural infection and vaccination, can protect populations from outbreaks of COVID,” Cohn said. “The idea that we’ve reached a sufficient level of previous infection or immunity to protect individuals who haven’t been vaccinated or infected in quite fanciful.”
Despite some research suggesting that immunity from recent COVID-19 infection is at least as effective in protecting against the disease as immunity from vaccination, vaccine mandates implemented by local hospitals and Connecticut’s state government do not allow exemptions for people with a recent positive test.
Asked about this in late September, public health commissioner Dr. Manisha Juthani noted that data around natural immunity remains limited and that protection can vary from person to person.
“Some people may have very little antibodies that wane very quickly, and somebody else may have a lot of antibodies,” Juthani, an infectious disease specialist, said. “Vaccination is the way to make sure that we get to a certain level.”
Other epidemiologists agree, pointing to enduring questions about how much protection natural immunity truly confers.
“We still don’t have an understanding as to the resulting immunity after natural infection because there’s going to be such high variability from person to person and infection to infection,” Banach said. “So it’s really difficult to predict after an infection whether or not there’s going to be a significant protective immunity and how long that might last for.”
Vaccines, on the other hand, provide a high degree of protection to everyone who receives them, Banach said, as demonstrated in extensive clinical trials.
With that in mind, experts urge people who have been infected recently to layer whatever protection they have from that experience with additional immunity from vaccination.
“If you have had natural immunity, it’s certainly not 100%. There are re-infections, and that immunity will wane over time,” Wu said. “But when you get (natural immunity and vaccination) together, you will be super immune.”
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