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Coronavirus

What are the differences between COVID and flu? What to know as influenza season nears

COVID-19 and the flu — both illnesses that attack the respiratory system but are caused by different viruses — are expected to spread simultaneously beginning this fall. And yes, you can be infected with both at the same time.

It’s difficult to predict how the flu season will fare this year, but some experts say it may be worse and begin earlier than in 2020 when COVID-19 preventive measures kept other respiratory viruses, including the flu, at bay. Flu season typically runs October to May.

This year, however, we have more tools at our disposal. Coronavirus vaccines have been widely available since April, and now, COVID-19 boosters are expected to roll out for all Americans beginning the week of Sept. 20, pending recommendations and authorizations from federal health officials.

Flu shots are also available as they are every year, and are still recommended for everyone 6 months and older, with some exceptions, in addition to COVID-19 vaccines for those aged 12 and older.

Because both illnesses share many similarities, health officials say it’s best to take a test to get a proper diagnosis if you feel sick throughout the flu season and beyond. The flu is caused by one of the many influenza viruses spreading during a given year, while COVID-19 is caused by SARS-CoV-2 (coronavirus).

Doctors and scientists are still learning about the coronavirus and how it behaves, but here is how COVID-19 and the flu compare given the “best available information,” according to the Centers for Disease Control and Prevention.

How do symptoms of COVID-19 and the flu compare?

The coronavirus and flu can cause similar symptoms, mild or severe, and both can cause asymptomatic illness, meaning infected people may not experience or show symptoms.

Some symptoms the viruses share include fever, cough, shortness of breath, sore throat, runny or stuffy nose, muscle pain, headache, vomiting, diarrhea, fatigue and loss of taste or smell.

People with COVID-19 tend to lose their sense of taste or smell more frequently than people who come down with the flu.

How do COVID-19 and the flu spread?

Both illnesses are spread between people via respiratory droplets that shoot into the air after infected people cough, sneeze or talk. These droplets contain tiny particles of virus that can land in nearby people’s mouths or noses and float in the air for extended periods of time.

In areas with poor ventilation, droplets can spread further than the 6 feet many health experts consider the standard for airborne infections.

Both the coronavirus and flu can also be spread via fomite transmission, or when people touch objects, such as door knobs or others’ hands, that may contain virus particles and then touch their face.

Although both illnesses are spread similarly, the coronavirus appears to be more contagious and tends to be involved in more superspreading events, the CDC says.

“This means the virus that causes COVID-19 can quickly and easily spread to a lot of people and result in continual spreading among people as time progresses,” the agency says.

How long can symptoms of COVID-19 and flu last?

It can take one or more days after exposure to either the coronavirus or flu before a person starts to feel symptoms, but it may take longer for those with COVID-19 to experience sickness — if they feel anything at all.

People with the flu can experience symptoms anywhere from one to four days after infection, whereas people with COVID-19 typically come down with symptoms about five days after infection, but the timing can range from two to 14 days.

How long are you contagious with COVID-19 versus the flu?

Generally, people with COVID-19 can spread the coronavirus for a longer time compared to the amount of time flu-ridden people can spread the influenza virus circulating at the time.

But how long people can spread the coronavirus is still under investigation, the CDC says.

A recent study found that coronavirus-infected people are most contagious two days before and three days after their symptoms appear. They can remain contagious for at least 10 days after symptoms show up, but people hospitalized with severe disease, as well as those with weakened immune systems, can spread the coronavirus for 20 days or longer.

Those with the flu are contagious for about one day before symptoms appear, but older children and adults are most infectious during the first three to four days of their sickness, according to the CDC.

Most people can spread the flu for about seven days. Babies and immunocompromised people can be contagious for longer.

What are the complications of COVID-19 versus the flu?

Just like symptoms, COVID-19 and the flu share many complications post infection, including pneumonia, heart injury, inflammation of the brain or heart, acute respiratory distress syndrome and sepsis.

People with the flu typically recover in a few days, though the process can take up to two weeks. Secondary bacterial infections are more common after having influenza compared to the coronavirus.

Blood clots and multisystem inflammatory syndrome (MIS-C), which occurs mostly in children but can manifest in adults, are more common complications associated with COVID-19.

People can also develop long COVID-19 — when coronavirus symptoms last for weeks or months after a person’s infection subsides. It can happen to anyone, regardless of how mild or severe one’s case was.

Who’s at high risk for severe COVID-19 or flu?

Older adults, pregnant people and those with certain medical conditions, including cancer, diabetes, HIV and chronic kidney disease, are at high risk of a severe COVID-19 or flu case.

The main difference is that COVID-19 generally causes more severe illness in some people, resulting in hospitalization or death even in healthy and young people.

This story was originally published September 14, 2021, 2:18 PM.

Follow more of our reporting on Full coverage of coronavirus in Washington

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Katie Camero is a McClatchy National Real-Time Science reporter. She’s an alumna of Boston University and has reported for the Wall Street Journal, Science, and The Boston Globe.
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