Cases of patients testing positive for both flu and COVID-19 have emerged in California and Tennessee as experts warn of a “twindemic” this winter.

“If you have ever received a flu vaccine in your life, this is the year to do it,” said Dr. Dr. Mia Taormina, an infectious disease specialist with DuPage Medical Group, The Chicago Tribune reported. “We are very concerned about the possibility of co-infection with influenza and COVID.”

In California, Solano County announced Thursday that its first resident had tested positive for COVID-19 and seasonal influenza at the same time. The patient is under age 65, the county said in a news release.

The case appears to be one of California’s first reported flu and COVID-19 co-infections this flu season.

Information about the interplay between influenza and COVID-19 remains limited because the latter is a novel virus, but both are respiratory diseases that weaken the immune system, especially in older adults, and each can result in hospitalization in severe cases.

“Symptoms of the flu can be like early symptoms of COVID-19, meaning people with flu symptoms may require a COVID-19 test and need to stay home from work and isolate while awaiting their results,” said Dr. Bela Matyas, Solano County’s health officer, in a statement.

Matyas and other health leaders continue to urge people to get their flu shots this year, and to do so as soon as possible, because it can take two weeks for the flu vaccine to become effective.

In Tennessee, Maury Regional Medical Center has admitted its first patient with both COVID-19 and the flu, WKRN reported.

“When you get influenza, which is a respiratory illness and you get COVID, which also impacts the respiratory tract, it only makes sense that those individuals are going to have a serious respiratory component with shortness of breath, potential for respiratory failure is very high in co-infection,” said Martin Chaney, chief medical officer at the hospital.

Story continues

Robert Redfield, director of the U.S. Centers for Disease Control and Prevention, has said the nation is heading for “the worst fall, from a public health perspective, we’ve ever had,” MarketWatch reported.

“We’re going to have COVID in the fall, and we’re going to have flu in the fall. And either one of those by themselves can stress certain hospital systems,” Redfield said, according to the publication.

“Here we are entering the cold-flu season,” said Dr. Russell Faust, medical director of Oakland County in Michigan, The Oakland Post reported. “Now it’s cold-flu-COVID season. It’s a real problem.”