First, you may notice a slight sniffle. Then you’re fatigued and feverish. As we move through another season of winter viruses, it’s natural to wonder, “Is this flu or COVID?”

“Both flu and COVID are respiratory illnesses caused by viruses, and they share some similar symptoms,” said Kathleen Cameron, former senior director of the National Council on Aging (NCOA) Center for Healthy Aging. “But there are important differences in how the viruses behave, spread and affect older adults.”

The flu and COVID vary in several key areas: cause and how they spread, symptoms and how quickly they appear, how long you’re contagious, treatment options and possible complications. 

CAUSE & TRANSMISSION

• Flu: Caused by influenza viruses, mainly influenza A and influenza B, which are responsible for seasonal flu outbreaks each year.

• COVID: Caused by the SARS-CoV-2 virus. Since 2019, multiple variants and subvariants have circulated, and the dominant strains change over time. 

Flu and COVID spread in similar ways. Both are mainly transmitted through respiratory droplets and smaller particles released when an infected person coughs, sneezes, talks or breathes. These droplets can be inhaled by people nearby, usually within a few feet.

It’s also possible—though less common—to get infected by touching a surface recently contaminated with the virus, then touching your eyes, nose or mouth. Evidence suggests that COVID spreads more easily than the flu.

 SYMPTOMS

Both flu and COVID mainly affect the respiratory system. Their symptoms overlap quite a bit, which is why it can be hard to tell them apart without testing.

The chart on the right highlights common symptoms of flu, COVID and the common cold.

How soon do symptoms start?

• Flu: Symptoms usually begin one to four days after infection.

• COVID: Symptoms often start two to five days after infection but can appear up to 14 days later.

Because flu and COVID symptoms overlap so much, testing is the only reliable way to know which one you have.

If you develop symptoms—especially if you were recently around someone who was sick—contact your doctor or take a rapid home COVID test. Many pharmacies also carry home flu tests and combination flu/COVID tests. The sooner you confirm what you have, the sooner you can start treatment and avoid exposing others.

INFECTIOUS PHASE

• Flu: People are usually most contagious during the first three days of symptoms, but some may be contagious longer.

• COVID: People begin infecting others one to two days before symptoms start and remain contagious for about eight to 10 days after symptoms begin. Some people never develop noticeable symptoms yet can still pass the virus to others.

COMPLICATIONS

Both flu and COVID can be mild, but either can lead to serious complications, especially in older adults and people with chronic health conditions.

• Flu: Complications are more likely in certain groups, including adults age 65 and older and people with chronic illnesses. Serious complications include pneumonia, inflammation of the heart (myocarditis) and brain (encephalitis). Flu can also worsen conditions such as asthma, heart disease or diabetes

• COVID: COVID can cause pneumonia, blood clots, septic shock and organ damage or failure. In severe cases, it can lead to acute respiratory distress syndrome (ARDS), a life-threatening condition in which the lungs cannot provide enough oxygen to the body.

A condition called long COVID can also develop after a COVID infection, even if the initial illness was mild. With long COVID, symptoms such as fatigue, brain fog, shortness of breath, sleep problems and changes in taste or smell may continue or emerge many months after the acute infection. 

TREATMENT

With both flu and COVID, early treatment is vital to reducing the severity of symptoms and preventing complications.

• Flu: Prescription antiviral drugs such as Tamiflu, Relenza, Rapivab and Xofluza can shorten how long you are sick and lower your risk of serious complications if started within about 48 hours of symptom onset.

• COVID: The main outpatient treatments include prescribed oral antivirals such as Paxlovid and Molnupiravir. Remdesivir is a three-day antiviral treatment given via IV in a hospital setting for severe cases of COVID. 

For both the flu and COVID, managing symptoms is important. Getting enough rest, drinking plenty of fluids and eating well will support your recovery. 

HOW TO PROTECT YOURSELF

Older adults face a higher risk of serious complications, so prevention is especially important. 

• Keep up with recommended vaccines. The Centers for Disease Control & Prevention (CDC) recommends a flu vaccine every year for everyone age 6 months and older, with rare exceptions. For adults 65 and older, certain “high-dose” or adjuvanted flu vaccines may provide better protection. CDC also recommends an updated COVID vaccine for most adults, especially adults 65 and older, people with weakened immune systems and others at higher risk for severe illness. You can receive flu and COVID vaccines at the same visit if you prefer. If you tend to have soreness or side effects, you might choose to space them out.

• Practice good hygiene: Wash your hands often with soap and water, avoid touching your face and sanitize high-touch surfaces regularly.

• Wear a mask: During peak flu season or when COVID and flu levels are high in your area, consider wearing a well-fitting mask in crowded indoor spaces like buses, airports, clinics or large gatherings. 

• Stay home when you’re sick: If you’re feeling ill, staying home gives your body the chance to recover and helps prevent spreading germs to others. Delay in-person visits, especially with older friends, family or anyone with underlying health conditions, until you’re feeling better.

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