A large study of blood samples taken from 24,000 people in the United States at the beginning of last year suggests
that the coronavirus arrived in the country in December 2019,
weeks before authorities discovered and certified the first infections.
The analysis is not definitive and some experts remain skeptical, but federal health officials are increasingly inclined to accept that there may be some infections in the United States even before China raised the alert about the mysterious epidemic.
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"The studies are pretty consistent," said Natalie Thornburg, principal investigator with the respiratory virus immunology team at the Centers for Disease Control and Prevention.
"There were probably very rare and sporadic cases here earlier than we knew. But it didn't spread and it didn't become widespread until the end of February," he added.
The coronavirus pandemic emerged in Wuhan at the end of 2019. Officially, the first infection in the United States corresponded to a traveler who returned from that Chinese city on January 15 to the state of Washington, and sought help at a clinic on January 19.
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CDC officials had initially said that the spark that started the outbreak in the United States occurred over a three-week period from mid-January to early February.
But research since then, including some by the CDC, has suggested that a small number of infections occurred earlier.
A CDC-led study published in December 2020 that analyzed 7,000 samples from Red Cross blood donations suggests
that the virus infected some people in the United States in mid-December 2019.
The latest study, published Tuesday in the journal Clinical Infectious Diseases, is from a team that includes researchers from the National Institutes of Health (NIH).
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 one million people in the United States over years to study health.
Like the CDC study, these researchers looked for antibodies in blood taken to look for evidence that there is a coronavirus infection and that they can be detected as early as two weeks after a person is infected.
The researchers say that nine study participants - five from Illinois and four from Massachusetts, Mississippi, Pennsylvania and Wisconsin each - were infected before COVID-19 cases were reported in those states.
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One of the
Illinois
cases
became infected around Christmas
, said Keri Althoff, an associate professor at the Johns Hopkins Bloomberg School of Public Health and lead author of the study.
It can be difficult to distinguish the antibodies that neutralize SARS-CoV-2, the virus that causes COVID-19, from the antibodies that fight other coronaviruses, including some that cause the common cold.
Researchers in the NIH and CDC studies used multiple types of tests to minimize false-positive results, but
some experts say it is still possible that the positive results in 2019 were infections from other coronaviruses
and not from the pandemic strain. .
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"While it is entirely plausible that the virus would have reached the United States much earlier than is believed, it does not mean that this is necessarily strong enough evidence to change our thinking about this," said William Hanage, a researcher at the Harvard University expert in disease dynamics.
NIH researchers have not yet followed up with study participants to see if any had traveled outside of the United States prior to their infection.
But they found noteworthy that
the nine did not live in or near New York City or Seattle, where the first wave of cases in the United States was concentrated.
"The question is how and where did the virus spread," Althoff said. The new study indicates that "it has probably been sown at the same time in various places in our country," he added.
