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In the early days of the COVID-19 pandemic in the US, officials focused limited testing capacity on symptomatic people who had recently traveled to or been in close contact with someone who had traveled to places with confirmed outbreaks of SARS-CoV-2. A year later, it’s clear that some proportion of viral transmission—perhaps as high as 50 percent—comes from presymptomatic or asymptomatic individuals, making it difficult to trace transmission. In a study published in Clinical Infectious Diseases June 15, researchers analyzed blood collected between January 2 and March 18, 2020, and found antibodies to SARS-CoV-2 in samples from nine people in five US states, meaning that the virus was likely present in the US in late 2019.
“We suspected that there were probably cases that preceded the ones that were diagnosed and confirmed. This is very suggestive that there were probably multiple exposures prior to those initial ...
