Now, researchers found that immunity can last up to five months, courtesy of a study led by Ania Wajnberg, MD, the director of clinical antibody testing at Mount Sinai Health System in New York City. The late October study was published in the journal Science.
“Our data shows a slow, expected decline and overall positivity in antibodies over time (vs. what has been reported elsewhere),” Wajnberg tells Verywell, attributing the discrepancy to “various things, including assay used.”
How Is Immunity Calculated?
Immunity is measured by the number, or titer, of antibodies present in a blood sample. Antibodies are proteins produced by the immune system in response to a viral infection. They develop structural adaptations in order to bind to particular types of foreign invaders and mark them for destruction. In the case of COVID-19, antibodies are designed to detect and disable the “spikes” of the SARS-CoV-2 virus that causes the disease.
The authors of the study surveyed 72,401 patients of Mount Sinai Health System for COVID-19 antibodies, 30,082 of whom tested positive for the disease.
The authors then categorized the clinically observed antibody responses by degree of intensity: low, moderate, or high. The overwhelming majority—more than 90%—of the people who had tested positive had moderate or high titers.
“The rate of individuals who do not seroconvert [produce antibodies] after SARS-CoV-2 infection is low, although such individuals may exist, and the majority of responders mount titers of 1:320 or higher,” the authors wrote.
But How Long Would Immunity Last?
To answer this question, the authors solicited plasma donations from 121 of the people who tested positive at two different intervals: an average of 82 days after the onset of symptoms—or around three months—and an average of 148 days—or around five months—after. They found that the majority of donors still had high titers at the time of donation, putting the average duration of immunity at five months at a minimum.
One item of note, however, is that the antibodies measured by the initial titer and the antibodies measured by the second and third titers were likely of different types. The antibodies measured by the first titer were likely produced by plasmablasts, which are cells that act as first responders to a virus and whose strength soon wanes. Since plasmablast-derived antibodies typically die within two or three weeks, Wajnberg speculates that the antibodies measured by the second and third titers were instead produced by long-lived plasma cells that dwell in the bone marrow.
“So far, these findings indicate to us that this virus is behaving like other respiratory viruses from an immune perspective,” she says.
What Do These Results Mean For Public Health Policy?
While the five-month estimate is less conservative than most that came before, Wajnberg tells Verywell it is far from an excuse to slide into complacency with masking and social distancing protocols at the local, state, and federal levels.
“We [will] continue to study these findings and hope over time to see whether and what level may be protective,” she says. “Until that point, these studies will help us understand natural responses and possibly eventually vaccine responses, but people should remain vigilant and follow all guidelines including PPE, social distancing, hand-washing, masking, etc.”
Chunhuei Chi, ScD, MPH, director of the Center for Global Health at Oregon State University’s College of Public Health and Human Sciences, tells Verywell that it is important not to overstate the significance or trustworthiness of any one study, especially because the ones conducted so far have yielded such conflicting results.
In contrast to Wajnberg, Chi attributes the discrepancies in estimates of immunity duration to differences in either virus strain or scientific rigor. “[Studying] different strains of the SARS-CoV-2…may result in differences of the lengths of immunity post-infection, as we know now that there are at least five different strains in the world,” he says.
That said, this study still has the potential to influence U.S. public opinion of the virus and public health policy in several ways, according to Chi. It may be used to determine who should be first in line to receive a vaccine once one becomes available and will help researchers and policymakers predict the physiological effects of vaccination. The findings may color perceptions and assessments of infection risk by locality and region.
“Given that there are still very limited studies and evidence on acquired immunity through infection, most health policymakers will be very cautious not to rely on this early information too heavily, especially when there are inconsistent findings, and wait till more evidence becomes available to make any major policy change,” he says.
“Besides,” he adds, “there [is] already evidence of re-infections in Asia and the United States.”
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