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Flu Guide: What to Know This Season

Is It a Cold, the Flu or COVID-19?

Doctors answer the question on the minds of those feeling ill right now

Man sneezing into his elbow. Laptop and tissues are in front of him.

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En español | It’s understandable if every cough or sniffle brings on fears of COVID-19. After all, the symptoms associated with familiar illnesses, like a cold and the flu, can also be signs of a coronavirus infection. With reports of breakthrough infections among people who are vaccinated — and the spread of the delta variant, which is more contagious than earlier forms of the coronavirus — many people are on edge.  

That’s why, if you’re feeling sick in the next few months, experts stress the importance of contacting your health care provider, who can determine whether you should be tested for COVID-19 and/or the flu. If you have the flu, you could be treated with an antiviral medication, which works best when taken within two days of your becoming ill. If you have COVID-19, you’ll want to watch for serious symptoms and, of course, follow Centers for Disease Control and Prevention (CDC) guidelines for quarantining to avoid infecting others. 

It’s important to get tested even if you’ve been vaccinated against COVID-19 because you could have a breakthrough infection (even though most cases are mild). The symptoms can be especially hard to differentiate from those associated with a cold or flu. “If you have cold symptoms, you’re not going to know what kind of infection you have,” says Gregory A. Poland, M.D., a professor of medicine and head of the Vaccine Research Group at Mayo Clinic in Rochester, Minnesota.  

In addition to getting the COVID-19 vaccine, be sure to get the flu shot, which may actually lower your susceptibility to the coronavirus while cutting your chances of getting the flu by 40 to 60 percent.

In a study published in Scientific Reports, Mayo Clinic researchers found that people who had received certain vaccines, such as the high-dose flu shot for those over 65, had a lower risk of contracting COVID-19. “Any vaccine causes your immune system to rev up its response,” says Jennifer Johnson, a Mayo Clinic Health System family medicine physician in North Mankato, Minnesota. 

She notes that activating your immune system this way could make it “quicker to respond to COVID.” Other researchers had similar findings. In a study published in the American Journal of Infection Control, people who received the flu shot were less likely to test positive for COVID-19. Those who did contract the coronavirus were less likely to need hospitalization or mechanical ventilation. And if they were hospitalized, they had a shorter stay than those who did not have the flu shot. 


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The flu shot also seems to provide protection against COVID-19 complications, such as sepsis, stroke and deep vein thrombosis (a potentially life-threatening blood clot), according to a new study published in PLoS One. In the study, COVID-19 patients who received the flu shot were less likely to visit the emergency department and be admitted to a hospital intensive care unit.

Here’s what doctors know so far about the differences among the three infections and the symptoms to watch for.

A quick comparison of telltale signs

There are no hard-and-fast rules about COVID-19 symptoms because the coronavirus tends to affect people differently. Here’s what experts know now:

  • A sudden loss of smell or taste may be the first sign of COVID-19. This symptom seems to occur later and is less pronounced in those with the flu or a cold. However, some doctors are seeing this symptom less frequently in people who have the delta variant.  

  • Flu symptoms tend to strike abruptly, whereas those associated with COVID-19 start out mild and gradually worsen.

  • People who have the flu may be more likely to experience nasal congestion and a productive cough than those with COVID-19. A cough associated with the coronavirus tends to be dry. People who have the delta variant, however, may be less likely to experience a cough. 

  • Shortness of breath is common in people with the flu and COVID-19, but not in those with a cold.

  • Weakness is more common in people with COVID-19 versus the flu.

  • Nausea, vomiting and diarrhea are more common in COVID-19 patients than in those with the flu.

Common cold symptoms

The common cold — which is brought on by around 200 viruses that lead to respiratory illnesses — causes mild symptoms that develop over three to seven days. They include a runny nose, congestion, a sore throat and a cough. Sneezing is common, and some people may experience fatigue and/or a low-grade fever (99.5 to 100 degrees). Nasal congestion may disrupt the airflow in your nose, temporarily interfering with your ability to smell or taste.


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Flu symptoms

Flu is caused by infection with influenza viruses, which differ from year to year. Symptoms, which tend to strike suddenly, include fever (typically, 101 degrees or higher), chills, muscle or body aches, cough, sore throat, runny nose, congestion, headache, fatigue, weakness and shortness of breath. People who have the flu may be more likely to experience nasal congestion and a productive cough than those with COVID-19, according to a study published in the Journal of Infection. Shortness of breath seems to be more common in those with the flu, according to a study published in Microbes and Infection. Usually, symptoms appear one to four days after infection, according to the CDC.

COVID-19 symptoms

The coronavirus, called SARS-CoV-2, is responsible for COVID-19. Symptoms, which may appear two to 14 days after infection, start out mild and worsen about seven days after they begin. Many of the symptoms can resemble those of a cold and the flu, such as fever, chills, cough, shortness of breath or difficulty breathing, fatigue, sore throat, runny nose, congestion, muscle pain, body aches, headache and weakness.

There appear to be subtle differences, though. For instance, a cough associated with COVID-19 tends to be dry, rather than phlegmy, says osteopath Brian Clemency, an associate professor in the department of emergency medicine at the University of Buffalo in New York. Also, weakness is more common in people with COVID-19, according to the Journal of Infection study. Signs of illness tend to be more pronounced in adults over 50. “They tend to be weaker and more tired, and they may experience more severe shortness of breath,” says Mark Perazella, M.D., a professor of medicine and nephrology at Yale University School of Medicine in New Haven, Connecticut. He adds that they are also more likely to experience changes in their mental status, such as confusion, dizziness and brain fog.

A symptom that tends to occur in people with the coronavirus — and may be the first warning sign of the illness — is a sudden change in, or loss of, smell or taste. “This symptom seems to occur earlier and is more pronounced in people with COVID-19,” Clemency says. However, there’s some preliminary evidence that people who have the delta variant may not experience this symptom.  

“Our thinking is that people breathe in COVID-19 through the nose and it infects the olfactory tissue,” says Andrew Lane, M.D., professor of otolaryngology and head and neck surgery at Johns Hopkins Medicine in Baltimore. And research shows that it may take longer for COVID-19 patients to recover their ability to smell and taste, compared with those suffering from the flu (about seven days versus three days).

Fever also seems to be common in people with COVID-19. “The reason fever often shows up as the first symptom in COVID-19 patients is that the virus gets stuck in the upper respiratory tract for a long time,” Kuhn says. “Later, the virus drops into the lungs, and then coughing starts.” The influenza virus, on the other hand, infects the respiratory tract and moves into the lungs more quickly. Despite the similarities between COVID-19 and the flu, people with the former tend to experience a greater variety of symptoms (as well as health problems). 

Despite the similarities between COVID-19 and the flu, people with the former tend to experience a greater variety of symptoms (as well as health problems). In a study published in the Journal of Infection, researchers found that nausea, vomiting and diarrhea were about twice as common in COVID-19 patients as in those with the flu.

Other signs of COVID-19 include blotchy purple or red rashes, which can appear on the hands, forearms, legs and toes and may signal blood clots in arteries or veins. Research shows that people with the virus are also more likely to experience blood clots in the lungs and kidney failure.

Could You Have RSV?

In June, the CDC issued a health advisory that cases of respiratory syncytial virus (RSV) were rising in the southern United States. Typically, infections occur in the fall, winter and spring. However, since March, there’s been an increase in infections, possibly because many people stopped wearing masks and social distancing.

Though most adults who are diagnosed with RSV typically have mild or no symptoms, some develop pneumonia or heart failure. Every year in the U.S., RSV leads to 177,000 hospitalizations and 14,000 deaths in people 65 or older. Treatment involves managing symptoms, which include the following:

  • runny nose
  • sore throat
  • cough
  • headache
  • fatigue
  • fever

These symptoms also resemble those associated with COVID-19, so tell your doctor if you’re feeling ill. If you test negative for COVID-19, your doctor may test you for RSV. 

Editor’s note: This article was updated in September 2021 with new information.

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