A person has tested positive for both the flu and the coronavirus in Solano County, California, the county health department said.

Such co-infection is so far rare and is the first such case to be reported in the county but not the country. At least two such cases have been reported, in Texas and Northern California.

Experts have warned that if Covid-19 cases remain high as flu season begins, the U.S. could face a “twindemic,” with both viruses circulating at high levels.

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Much of that concern has been focused on health care systems, which could be overwhelmed with a surge of both Covid-19 patients and flu patients.

Oct. 1, 202002:04

If both viruses begin circulating at high levels, it raises the risk of being infected with both at the same time.

“With the likelihood of both Covid-19 and seasonal flu activity this winter, contracting either disease may weaken your immune system and make you more susceptible to the other disease,” Dr. Bela T. Matyas, Solano County Health Officer, said Thursday in a statement.

Little information was provided about the Solano County patient, except that the individual is under 65. The first confirmed case of community spread of Covid-19 in the United States was reported in the county in February.

In March, a Texas doctor reportedly fell ill with both viruses at the same time. And a research letter published in April in the Journal of the American Medical Association noted a case of flu-coronavirus co-infection in Northern California.

It’s not yet clear how common co-infections will be.

“We very much hope that it will not be,” Dr. Tara Bouton, an assistant professor of medicine at Boston University School of Medicine.

Dr. Peter Chin-Hong, a professor of medicine and infectious disease specialist at the University of California, San Francisco, predicted it’s only a matter of time before there are similar cases.

How common it will be “depends on what the flu season is going to look like,” Chin-Hong said. “There’s nothing that would preclude co-infection. It’s not like Covid comes in and takes out all the flu.”

It’s also not yet clear if patients with co-infection of the flu and Covid-19 will fare worse, Bouton said. “However, there are a few preliminary studies suggesting that this may in fact be the case.”

That’s “all the more reason why people need to do everything they can to prevent that scenario, including getting vaccinated against flu, wearing a mask when appropriate social distancing is not possible, and washing hands,” she said.

Matyas, the Solano County Health Officer, had similar advice.

“Getting a flu vaccine this year is more important than ever, and flu vaccines are the best way to protect yourself, your family, and the community from becoming seriously ill with the flu,” he said.

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Flu activity in the U.S. remains low, according to the Centers for Disease Control and Prevention, though that is not uncommon this early into flu season.

Covid-19 activity remains high in the U.S. There were more than 90,000 new cases reported Thursday, according to an NBC News tally, and more than 500,000 new cases have been reported in the past week.

More than 9 million cases have been reported in the U.S. since the start of the pandemic, and more than 225,000 people have died.

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Akshay Syal contributed.

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