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.
And this season, vigilance can be a good thing, since “early intervention leads to a better outcome,” says Peter Kuhn, a professor of medicine and engineering at the University of Southern California in Los Angeles.
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. In addition, 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 preliminary study, 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,” explains Jennifer Johnson, DO, 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.” Another benefit: If you've had the flu shot and develop flulike symptoms, your doctor may suspect COVID-19 more strongly and recommend testing.
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
- 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.
- 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.
- 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.
A closer look at each infection
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.
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 and a sore throat also seem 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 Centers for Disease Control and Prevention (CDC).
A new 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 resemble those of 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 Brian Clemency, DO, an associate professor in the department of emergency medicine at the University of Buffalo in Buffalo, 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 more frequently 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 observes.
"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 explains. “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.
Researchers at the University of Southern California reviewed early studies of COVID-19 patients and those with other illnesses, including influenza, and found that fever appeared earlier in COVID-19 patients than in flu patients, but a cough appeared earlier in flu patients. 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). For instance, some coronavirus patients have only gastrointestinal symptoms, says Nehal Galal, DO, assistant professor of medicine at Columbia University Vagelos College of Physicians and Surgeons and a primary care physician in New York City. 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. Recent research shows that people with the virus are also more likely to experience blood clots in the lungs and kidney failure.