The latest study, published Tuesday online by the journal Clinical Infectious Diseases, is by a team including researchers at the National Institutes of Health. They analyzed blood samples from more than 24,000 people across the country, collected in the first three months of 2020 as part of a long-term study called “All Of Us” that seeks to track 1 million Americans over years to study health.
Like the CDC study, these researchers looked for antibodies in the blood that are taken as evidence of coronavirus infection, and can be detected as early as two weeks after a person is first infected.
The researchers say seven study participants — three from Illinois, and one each from Massachusetts, Mississippi, Pennsylvania, and Wisconsin — were infected earlier than any COVID-19 case was originally reported in those states.
One of the Illinois cases was infected as early as Christmas Eve, said Keri Althoff, an associate professor at the Johns Hopkins Bloomberg School of Public Health and the study’s lead author.
It can be difficult to distinguish antibodies that neutralize SARS-CoV-2, the virus that causes COVID-19, from antibodies that fight other coronaviruses, including some that cause the common cold. Researchers in both the NIH and CDC studies used multiple types of tests to minimize false positive results, but some experts say it still is possible their 2019 positives were infections by other coronaviruses and not the pandemic strain.
“While it is entirely plausible that the virus was introduced into the United States much earlier than is usually appreciated, it does not mean that this is necessarily strong enough evidence to change how we’re thinking about this,” said William Hanage, a Harvard University expert on disease dynamics.
The NIH researchers have not followed up with study participants yet to see if any had traveled out of the U.S. prior to their infection. But they found it noteworthy that the seven did not live in or near New York City or Seattle, where the first wave of U.S. cases were concentrated.