(CNN)The deadly coronavirus may have circulated in the United States as early as December, about a month earlier than believed by the US Centers for Disease Control and Prevention, according to researchers with UCLA.
Their study, published last Thursday in the Journal of Medical Internet Research, found a statistically significant increase in clinic and hospital visits by patients who reported respiratory illnesses as early as the week of December 22.
The first known case of Covid-19 in the US was thought to be a patient in Washington who had visited Wuhan, China, according to the CDC. The case was reported in January.
Dr. Joann Elmore and colleagues looked through nearly 10 million medical records from the UCLA Health system, including three hospitals and 180 clinics.
Elmore said she started the search after receiving a number of emails from anxious patients in March through her clinic's patient portal at UCLA. Patients kept asking if the cough they had in January could have been Covid-19.
"With the outpatients, I found a 50% increase in the percentage of patients coming in complaining of a cough. It came out to over 1,000 extra patients above the average of what we would typically see," Elmore told CNN.