Flu, COVID or Allergies: Can You Tell Which One You Have?

We're about to nose-dive into "flu season," which actually includes the spread of respiratory viruses like influenza, COVID-19 and RSV (also known as respiratory syncytial virus).

Though it's too early to make a call on how severe the flu will be this year (most flu activity typically peaks between December and February, according to the US Centers for Disease Control and Prevention), we can expect both COVID-19 and flu cases to go up around the holidays, when more people are gathering indoors.

In some good news, RSV will likely be "getting back to its normal levels this season versus last, so it will not be as severe as last year," said Dr. Amesh Adalja, an expert in infectious diseases and a senior scholar with the Johns Hopkins Center for Health Security. Last year saw an early and harsh spike in RSV cases as it rebounded post-pandemic. 

"This is also going to be coupled with the fact that there are vaccines for older individuals, as well as vaccines for pregnant women and monoclonal antibodies for infants, that will have some impact on severity," Adalja said in an email. In addition to COVID and flu vaccines, adults and babies with the highest risk of severe illness from RSV have new tools to protect themselves this year.

But how do you tell which virus you have? And how do you know if it's allergies instead? This is an important distinction to make, given that viruses that cause the flu, COVID and RSV are all contagious, while allergies aren't contagious and don't require the same social-distancing precautions. (And yes, some people still experience allergy symptoms during the winter.) 

Here's what to know about flu and COVID symptoms, as well as some clues to look for regarding whether you're sick with a virus rather than allergies (or vice versa). For more, here's our guide on who needs a flu, COVID or RSV vaccine -- and when they should get one -- as well as a guide for how to order more free COVID tests.

Read more: Your Decongestant Might Be Ineffective, FDA Panel Says 

Best Over-the-Counter Vitamins and Supplements Proven to Shorten a Cold See at Cnet

Flu vs. COVID symptoms -- is there a difference? 

Not really, according to Adalja. 

"There is no real way to distinguish COVID-19 from influenza based on clinical symptoms because they overlap so much," he said. Common COVID and flu symptoms include sore throat, fever, fatigue, muscle aches, cough and more. The big COVID symptom, loss of taste and smell, has become much less common than during the peak of the pandemic. 

There may be a slight difference in how soon your symptoms show up after getting infected with the flu vs. COVID, though, according to the CDC. Flu symptoms may appear one to four days post infection, vs. two to five post-COVID infection, and some people may take up to two weeks to show COVID symptoms, per the CDC. But because the average window to develop symptoms is similar for both viruses, you likely won't be able to tell based on this criteria either. 

Both COVID and the flu cause the most severe illness and lead to complications in the same populations: adults in their 60s and up and younger people with certain medical conditions. If you're planning on spending time with someone at high risk of any virus, it's especially important that you err on the side of caution and wait until you're no longer sick to see them face-to-face. 

If you are at higher risk, you should speak with your doctor about getting a real diagnosis so you can get the appropriate treatment. Flu has different treatment than COVID-19, but antivirals for either should be started as soon as possible.  

Read more: Flu, COVID and RSV: Your Guide to What Vaccines You Need and When 

What about RSV?

RSV is a common virus that causes only mild symptoms in most adults, such as a runny nose, decreased appetite and fever. But in older adults and young babies, it can cause severe symptoms, like pneumonia or trouble breathing.

Vaccines for RSV are now available to older adults and to pregnant people, to protect newborns once they're born. Some infants may also receive a monoclonal antibody as protection against RSV. 

Quick tips to help check whether it's allergies or a virus 

"It's very common that people who have allergies, they think they're having a series of viral infections," Dr. Geoff Rutledge, chief medical officer at HealthTap, told CNET last spring. And on the other end, some people have reported COVID-19 symptoms, from newer versions of the virus, that are typically linked to allergies, including pink eye. That can make it even trickier to know what you're down with.

However, there are some symptoms and clues that are "very suggestive" you're experiencing a virus versus seasonal allergies, or vice versa, Rutledge said.

If you have a fever, it's not allergies

While "fever" is in the name, hay fever does not actually cause a fever, according to the American College of Allergy, Asthma and Immunology. However, if you have a sinus infection or other bacterial infection, that may cause a fever and can be caused "sometimes secondarily" from allergies, according to Rutledge. 

If you're really itchy, it might be allergies

"Allergies typically cause itching of either the eyes, nose or top of mouth, which is not usually common in viral infections," Dr. Jennifer Bourgeois, pharmacy expert at SingleCare, told CNET in an email last spring. It's important to note that, while it's hard to pin symptoms down to specific variants of COVID-19, there have been some reports of more cases of conjunctivitis or pink eye with the newer versions of the virus. Viruses and bacteria can also cause pink eye, which in turn can cause itching around the eye.

According to the Cleveland Clinic, other symptoms of allergic rhinitis besides itching include congestion; sneezing; headaches and sinus pain; dark circles under the eyes; increased mucus; postnasal drip (mucus draining down the back of your throat); trouble breathing; and fatigue or generally feeling bad. 

Take a COVID-19 test 

This one's pretty simple, but the best way to find out whether you have COVID-19 or something else is by taking an at-home test, or a more accurate lab test in a doctor's office. This might be especially important if you plan on being around a person who's older or otherwise at higher risk of serious illness from COVID-19. Also: that "expired" COVID-19 test you have leftover from last winter might not be expired after all. 

Read more: More Free COVID Tests: Here's How to Order Them Through the Mail 

Take the antihistamine test 

For people who experience seasonal allergies, but aren't entirely sure if that congested feeling is from the same cold your friend had or the familiar allergies that usually kick in this time of year, Rutledge suggested what he calls a "therapeutic trial" of taking an over-the-counter antihistamine to see if that clears up your symptoms. 

"There are specific treatments that only work for allergies," he said, and antihistamines are one of them. Histamine is what your body releases when you have an allergy and your immune system is activated. Antihistamines work by blocking the effects of histamine and helping many allergy symptoms. On that note, if you have sinus pain or sneezing and your symptoms improve after you take a histamine, it might be safe to say you have allergies.

COVID, flu and RSV treatments 

Antiviral medications like Paxlovid may be prescribed for older adults and younger people who have a higher risk of getting severe illness from COVID-19 (for Paxlovid to work, you need to start the medication within the first few days of your symptoms appearing). 

There are also flu treatments available for people at higher risk. Like COVID, it's important for people who are at risk to start antiviral medication for the flu as soon as possible. 

What are some effective allergy treatments?

In terms of allergies, what kind of medication you need depends on which symptoms you have. 

"Each medication is designed to treat specific symptoms, so it's important to find the correct over-the-counter medicine in order to manage and relieve your symptoms," Bourgeois said. For example, antihistamines like Claritin or Zyrtec are standard allergy medications, but you could also find relief from a decongestant. 

In a pinch, a fever-reducing medication like ibuprofen might also work for allergies because they have anti-inflammatory properties, Bourgeois said.

You might even try stopping allergy symptoms before they start with a steroid nasal spray, according to Bourgeois. They do the work before you're exposed to the allergens in the air.

"It is best to begin the steroid nasal spray a couple of weeks before the allergy season that triggers your symptoms and continue throughout the duration of the season, as it's typically not required to use the steroid nasal spray all year long," she said.

If you choose to do a nasal rinse after symptoms and mucus sets in, Bourgeois referred to the CDC's guide for safely carrying it out with sterile material. 

Many people will manage allergy symptoms with help from their primary doctor or the right medication, Rutledge said, and they won't need further medical help. But if you're still not getting relief from your symptoms, and you've done some detective work into what's causing your allergies, then you might be a good candidate for in-clinic allergy testing or a follow-up appointment with a specialist to get to the source, according to Rutledge.