Dr. Krammer and his colleagues analyzed plasma samples from nearly 5,500 patients who went to Mount Sinai for routine medical appointments, were seen in his emergency department, or were hospitalized weekly ending February 9 to the week ending April 19.
The C.D.C. examined blood samples from people who went for routine medical exams, but only the week ending April 1 for New York. The New York state study recruited people from supermarkets from April 19 to 28.
“When we have three sources that all give you consistent results, it reinforces all the conclusions,” said Eli Rosenberg, epidemiologist at New York State University in Albany and lead author of the state study.
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The figures from the three studies also agree on one crucial point: the vast majority of infections in New York and elsewhere in the country have not been diagnosed. Even in places where epidemics are large, the number of people exposed to the virus is still far from what is needed for collective immunity.
Mount Sinai researchers gathered their samples in different ways and analyzed them using a very precise laboratory antibody test specific to the new coronavirus.
Among people admitted to the emergency room or hospital during the study period, the prevalence of antibodies rose to almost 60% compared to 3.2%, according to the researchers. These numbers are high because they include people who are seriously ill with the coronavirus.
But among people who donated blood for routine appointments or who were admitted to hospital for reasons unrelated to the coronavirus – a group that represents the general population – less than 2% of people had antibodies until the week ending March 29. The rate increased exponentially after that, ending at 19.3% in patients seen in the week ending April 19.