(Tribune News Service) -- Flu season is unpredictable in any year, depending on such factors as how many people get the vaccine and how well it matches circulating strains of the virus.

This year, throw in the wild card of COVID-19.

Physicians say if we stick with the social distancing and mask-wearing to prevent COVID-19, those precautions could reduce spread of flu in the bargain. That theory is supported by lower-than-usual flu numbers so far this year in some parts of the southern hemisphere, where flu season is nearing its end.

But amid uneven compliance with these preventive measures in much of the United States — whether due to pandemic fatigue or a misguided belief that the crisis is exaggerated — infectious-disease specialists warn that we could face a double whammy of COVID-19 and flu this fall and winter. Among those urging vigilance is Jeanne Marrazzo, a director of the Infectious Diseases Society of America and a physician researcher at the University of Alabama at Birmingham.

“The big concern this year, of course, is that we are going to see what could be a perfect storm,” she said. “We really can’t be complacent about this.”

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The difficulty is not just that flu and COVID-19 will be circulating at the same time, said John Zurlo, chief of the division of infectious diseases at Thomas Jefferson University. It is hard to tell one disease from the other.

Both illnesses can be characterized by fever, aches, and shortness of breath. Among the few distinguishing features is the sudden loss of smell experienced by some COVID-19 patients. Flu can impair the sense of smell, too, but in that case the culprit is a stuffy nose, whereas in COVID-19 the reason is temporary damage to olfactory cells, Zurlo said.

Another way to tell one illness from the other is laboratory testing. The Jefferson health system and others plan to use “multiplex” tests that will analyze a patient’s nasal swab for evidence of both viruses. Such tests might be used more for older patients who are at greater risk of complications from the flu, he said. The results would help physicians determine whether to administer an antiviral drug, which can reduce the severity of flu if given soon enough.

If there are delays in testing or if supplies run short, the fallback option would be to treat any severe respiratory illness as a potential case of COVID-19, Marrazzo said. Patients with either illness should isolate in any case, but if there is a suspicion of COVID-19, health departments may wish to notify others with whom infected people have come into contact.

And above all, get a flu shot, she said. That reduces the risk of flu for the individual and lessens the burden on the health care system.

“If there’s ever a year that you need to get your flu vaccine and get your kids vaccinated, this is the year,” she said.

Yet at some point in the coming weeks, many of us will get a scratchy throat, a stuffy nose, or other symptoms that come with cooler weather. Is it COVID-19? Flu? Maybe just allergies? The accompanying chart may help you tell the difference.

©2020 The Philadelphia Inquirer.

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