A new study of blood samples from 24,000 Americans taken early last year is the latest and largest to suggest that the new coronavirus appeared in the United States in December 2019 — weeks before cases were first identified by health officials.

The analysis is not conclusive, and some experts remain skeptical, but federal health officials are increasingly accepting a timeline in which a small number of COVID-19 infections occurred in the United States before the rest of the world was aware of a dangerous new virus erupting in China.

According to Natalie Thornburg of the Centers for Disease Control and Prevention, “the studies are pretty consistent.”

“There were probably very rare and sporadic cases here before we were aware of them. However, it was not widespread and did not become widespread until late February,” said Thornburg, the principal investigator of the CDC’s respiratory virus immunology team.

In late 2019, a pandemic coronavirus emerged in Wuhan, China. Officially, the first U.S. infection was identified in a traveler — a man from Washington state who returned from Wuhan on January 15 and sought treatment at a clinic on January 19.

However, subsequent research, including that conducted by the CDC, has suggested that a small number of infections occurred earlier.

A CDC-led study that analyzed 7,000 samples from American Red Cross blood donations and was published in December 2020 suggested that the virus infected some Americans as early as the middle of December 2019.

The latest study, published online by the journal Clinical Infectious Diseases on Tuesday, was conducted by a team of researchers from the National Institutes of Health. They analyzed blood samples from over 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,” which aims to track 1 million Americans over time to study health.

These researchers, like the CDC study, looked for antibodies in the blood that are used as evidence of coronavirus infection and can be detected as early as two weeks after a person is infected.

According to the researchers, nine study participants — five from Illinois and one from each of Massachusetts, Mississippi, Pennsylvania, and Wisconsin — were infected before any COVID-19 case was ever reported in those states.

According to Keri Althoff, an associate professor at the Johns Hopkins Bloomberg School of Public Health and the study’s lead author, one of the Illinois cases was infected as early as Christmas Eve.

Antibodies that neutralize SARS-CoV-2, the virus that causes COVID-19, can be difficult to distinguish 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 reduce false positive results, but some experts believe their 2019 positives could have been caused by other coronaviruses rather than the pandemic strain.

“While it is entirely plausible that the virus was introduced into the United States much earlier than is commonly assumed,” said William Hanage, a Harvard University expert on disease dynamics.

The NIH researchers have not yet followed up with study participants to see if any of them had traveled outside of the United States prior to becoming infected. They did notice, however, 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 the virus took seed,” Althoff explained. The new study indicates “it probably seeded in multiple places in our country,” she added.