At this point in the COVID-19 pandemic, about 60 to 70 percent of unvaccinated adults in the U.S. have been infected. Still, there are significant differences in hospitalization rates among the vaccinated and unvaccinated, indicating natural immunity does not hold up, according to Ashish Jha, MD.
"We see large gaps in hospitalizations between vaccinated and unvaccinated," Dr. Jha, dean of Brown University's School of Public Health in Providence, R.I., wrote in a series of tweets Jan. 30. "At this point, probably two-thirds [of adults] have been previously infected. And yet, we still see 50X differences in hospitalizations between vaccinated and unvaccinated."
One possible explanation, Dr. Jha said, is COVID-19 hospitalizations currently accumulating are among the quickly fading group of unvaccinated people who have not previously been infected.
"But much more likely, it means infection-induced immunity is not holding up," he said, estimating that natural acquired immunity starts to diminish three months after infection, and "certainly" six months after infection, which explains why hospitals are now seeing unvaccinated patients get reinfected.
"If infection-induced immunity was great," or comparable to vaccines, the large gap in hospitalization rates between the vaccinated and unvaccinated would start to close as more unvaccinated people became infected.
"Is infection-induced immunity real? Absolutely. Does it help prevent infections/hospitalizations? Probably … for a while," Dr. Jha said, adding that beyond 90 days, the protection from natural acquired immunity is "probably not that much."
CDC data shows about 64 percent of the U.S. population had been vaccinated as of Jan. 30.