Authors: Johns Hopkins Dr. Marty Makary
The news about the U.S. Covid pandemic is even better than you’ve heard. Some 80% to 85% of American adults are immune to the virus: More than 64% have received at least one vaccine dose and, of those who haven’t, roughly half have natural immunity from prior infection. There’s ample scientific evidence that natural immunity is effective and durable, and public-health leaders should pay it heed.
Only around 10% of Americans have had confirmed positive Covid tests, but four to six times as many have likely had the infection. A February study in Nature used antibody screenings in late summer 2020 to estimate there had been seven times as many actual cases as confirmed cases. A similar study, by the University of Albany and New York State Department of Health, revealed that by the end of March 2020—the first month of New York’s pandemic—23% of the city’s population had antibodies. That share necessarily increased as the pandemic spread.
Natural immunity is durable. Researchers from Washington University in St. Louis reported last month that 11 months after a mild infection immune cells were still capable of producing protective antibodies. The authors concluded that prior Covid infection induces a “robust” and “long-lived humoral immune response,” leading some scientists to suggest that natural immunity is probably lifelong. Because infection began months earlier than vaccination, we have more follow-up data on the duration of natural immunity than on vaccinated immunity.
Skeptics of natural immunity point to Manaus, capital of the Brazilian state of Amazonas, where reports in January suggested a wave of re-infections despite herd immunity. But the initial estimate of those infected was incorrect because it was based on antibody testing among those who donated convalescent plasma—an unrepresentative subgroup of the population. A follow-up study debunked the re-infection hypothesis and found only three confirmed re-infections in the entire state, whose population exceeds four million. Other studies have confirmed that re-infections are rare and usually asymptomatic or mild.
Should the previously infected be vaccinated? My clinical advice to healthy patients with natural immunity is that one shot is sufficient, and maybe not even necessary, although it could increase the long-term durability of immunity. A University of Pennsylvania study of people previously infected with Covid found that a single vaccine dose triggered a strong immune response, with no increase in that response after a second dose. A separate study from New York’s Mount Sinai School of Medicine concluded that “the antibody response to the first vaccine dose in individuals with pre-existing immunity is equal to or even exceeds the titers found in naïve”—never-infected—“individuals after the second dose.”
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