When Should You Get Tested for the Flu?

As the United States enters its second flu season during the COVID-19 pandemic, experts warn of the risk of a “twindemic” of the two respiratory illnesses.

The number of flu cases are steadily increasing as the flu season picks up steam. The U.S. saw 841 influenza-related hospitalizations in the week ending on December 4, nearly a double from the week prior, according to the Centers for Disease Control and Prevention (CDC).

The number of flu-related hospitalizations pale in comparison to the more than 55,000 hospitalized COVID-19 patients. However, since the two respiratory illnesses often present similar symptoms, it can be important to know which virus you are infected with early on to inform treatment.

William Schaffner, MD, medical director of the National Foundation for Infectious Diseases (NFID), told Verywell that COVID-19 and the flu can present in ways that are overlapping, "particularly in their milder forms."

If left untreated, a case of the flu could progress into pneumonia or another serious complication. This is especially true for older adults, those with chronic conditions, and unvaccinated people.

How to Know if You Have the Flu

If you are experiencing flu-like symptoms, it is first and foremost important to test for COVID-19. Especially as the new highly transmissible Omicron variant circulates, getting a COVID-19 diagnosis early will allow you to self-isolate and protect others from infection.

For a flu diagnosis, Schaffner recommends first reaching out to a primary care provider via a telehealth portal, email, or phone. Showing up to a doctor’s office or urgent care site puts others at risk of getting sick and it may be unnecessary unless you’re experiencing a medical emergency.

Peter Chin-Hong, MD, professor of infectious diseases at University of California, San Francisco, recommends taking a flu test if you tested negative for COVID-19. While uncommon, it’s possible to be infected with both COVID-19 and flu at the same time.

“I think a lot of people are stuck in limbo land with symptoms,” Chin-Hong told Verywell. “You keep doing a million BinaxNOW assays just because you want to convince yourself that you don't have COVID.”

Health providers can diagnose a case of the flu through a description of symptoms, a physical exam, or through a nasal or throat swab test.

There was an increase in the number of flu tests this year, driven in part by concern that this flu season would see a particularly high case rate, Chin-Hong said. In the last week of November, nearly 59,000 samples were tested for the flu, compared with 22,000 specimens during the same week in 2018.

Dual COVID-19 and flu tests make it possible to get tested for both viruses with a single swab. But these tests aren’t widely used and there’s room for at-home rapid tests to become as commonplace for flu as they are for COVID-19, Chin-Hong said.

“It's very tough to diagnose flu by symptoms only. Back in the pre-COVID days, it was probably a little bit easier to make a distinction between severe flu, classic flu, and other things” Chin-Hong said. “Now it's more complicated, because COVID can present like flu, so it's harder to tell without doing a test.”

Getting Treated for the Flu

Your doctor may prescribe an antiviral medication if you have the flu. There are four FDA-approved drugs, including Tamiflu (osteltamivir) and Relenza (zanamivir). These treatments will slow the progression of the illness to minimize the risk of hospitalization and severe outcomes.

Only about 44% of Americans are vaccinated against influenza in 2021, according to the CDC. For those who are unvaccinated, a flu can lead to serious adverse outcomes, including hospitalization and death if left untreated.

“Promptness is important if one wishes to benefit from treatment,” Schaffner said. “It's frequently the case that we don't present on day one, two, or three of our illness. We're down to day four or five before we present ourselves to the doctor and finally say, 'Oh, it's not going away. I guess I'd better call the doctor.' And by that time, the capacity to intervene with treatment is really substantially diminished.”

Getting tested and receiving treatment early in the course of illness is especially important for people at higher risk of complications or severe disease, like older people and those with underlying conditions.

Various strains of the influenza virus circulate each flu season. This season, a majority of the sampled specimens are H3N2, an influenza A strain.

This strain typically affects older adults the most. According to the “original antigenic sin” theory, the immune system learns to respond to viruses like the flu based on its first infection. H3N2 didn’t evolve until 1968, meaning most adults over the age of 50 likely had their first childhood experience with the flu before this strain began circulating, leaving them relatively vulnerable to H3N2.

Additionally, older adults tend to have weaker immune systems and have health conditions like diabetes, heart disease, and chronic obstructive pulmonary disease, which make it more complicated to treat the flu.

The majority of cases are among young people aged 5 to 24, though the number of cases among adults older than 25 are on the rise, according to the CDC.

“If you needed an additional reason to get your flu vaccine this year, the fact that A(H3N2) is sticking its nose out already ought to be good reason,” Schaffner said. “Even partial protection is better than no protection at all, particularly for those among us who are so frail—older persons and people with underlying illnesses."

Fortunately, this year’s flu vaccine contains antigens to protect against the H3N2-like virus. It also accounts for other strains that may become widespread or dominant later in the flu season.

“That's why the flu vaccine has four different types in it—not only to hedge bets, but to cover multiple strains that are probably circulating at the same time,” Chin-Hong added.

Mitigation efforts like proper hand-washing, disinfecting surfaces, staying home when you feel sick, and wearing a mask can minimize infection risk and keep vulnerable people safe.

"In the pre-COVID days, we were kind of living fatalistically and thinking that 'oh, well if you get it, you get it.' But there are a lot of things that you can do to help protect your family members, your friends, your loved ones, your circle,” Chin-Hong said.

“We're still in the beginning of flu," he added. "People can make a difference by getting the flu shot."

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.