Is it long flu or long COVID? Both show lingering symptoms

As doctors and health care providers learn more about the ongoing health challenge of long COVID, infectious disease experts say we may have been seeing this trend all along — with the flu.

The lingering symptoms following a COVID-19 infection, deemed long COVID, have put an extraordinary stress on the health care system as providers tend not only to those hospitalized the first time but also those who take months or longer to recover.

But what feels like a novel challenge may have already been adding pressure to clinics across the country for years, according to a study published this month in The Lancet Infectious Diseases.

“I think we’ve ignored this for a long time, and the COVID-19 pandemic kind of put it in the spotlight,” Dr. Ziyad Al-Aly, study author and clinical epidemiologist at Washington University in St. Louis, told Live Science. Just like with COVID-19, “flu can lead to long-term problems.”

Long flu began pre-pandemic

The study, supported by health care databases from the U.S. Department of Veterans Affairs, analyzed the cases of more than 80,000 people who had been hospitalized from a COVID-19 infection between March 1, 2020, and June 30, 2022, and compared the cases with more than 10,000 hospitalization cases of seasonal flu from Oct. 1, 2015, to Feb. 28, 2019.

In the 18 months following the patients’ initial admittance to the hospital, the researchers found both COVID-19 patients and influenza patients had lingering and significant symptoms that resulted in rehospitalization or other treatment.

During the “post-acute” phase of illness, the researchers found patients had a “higher burden of health loss” in the months after the more severe part of their illness than they did when their illness was at its peak.

“Both (COVID-19) infection and seasonal influenza result in a higher burden of health loss in the post-acute phase than their respective acute phases,” the researchers said.

This means the rates of death, hospital admission, intensive care unit admission and 94 other health outcomes remained high in the 18 months after infection for both the flu and COVID-19, according to the study.

This also means cases of what could be called “long flu” existed years before the COVID-19 pandemic began, Al-Aly said in a Washington University news release.

The difference in severity of the two lingering illnesses is significant, however.

“The excess burden per 100 people admitted to hospital for COVID-19 versus seasonal influenza should be interpreted in the context of a two to three times greater number of people being admitted to hospital for COVID-19 versus seasonal influenza in the USA during the same period,” researchers said in the study.

The lingering symptoms of the flu (cough, shortness of breath, lung disease, etc.) remained mostly within the pulmonary, or lung, system of the body, according to the study.

The long-term symptoms of COVID-19, on the other hand, affected multiple body systems, ranging from the heart to the intestines, the researchers said.

Differences between COVID, flu

From symptoms alone, it can be difficult to differentiate between the two respiratory illnesses dominating cold season.

Both illnesses can cause fever, cough, shortness of breath, fatigue, runny nose and muscle aches, according to the Centers for Disease Control and Prevention.

What’s different is the time after exposure it takes to feel these symptoms.

Someone exposed to the flu will typically experience symptoms one to four days later if they have been infected.

Those exposed to COVID-19 could take two to 14 days to show symptoms, the CDC reports.

Vaccines are available and recommended for both viruses.