At the height of the COVID-19 pandemic, the onset of a fever, cough, sore throat, or sniffle may have led to frantic online searches for “Is this COVID-19 or the flu?”

Even as the Public Health Emergency for COVID-19 has ended,1 it’s still a good idea to understand the similarities and differences between COVID-19 and influenza (the flu) and how to help protect yourself from getting sick. The information may be especially valuable because as spikes in COVID-19 cases occur, including the introduction of new variants, overlap with flu season (October to May) could result in what is referred to colloquially as a “twindemic,” or both viruses co-circulating.2

What are the Similarities Between Flu and COVID-19?

Before exploring the differences, it’s worth understanding why it can be hard to distinguish between the two illnesses: Both COVID-19 and influenza are contagious infections affecting the respiratory system that can share several symptoms, including fever, cough, runny nose, sore throat, body aches along with vomiting, and diarrhea.3

Since COVID-19 and the flu might be hard to tell apart based just on the symptoms, testing may be needed to provide an accurate diagnosis. This, in turn, can help your healthcare provider decide how to treat the illness.3

Similar Spread Patterns Between Flu and COVID-19

Both viruses spread through close contact with someone who is infected. COVID-19 and the flu are transmitted through small and large virus-containing particles that move through the air when we cough, sneeze or talk. These particles can land in mouths and noses or can be inhaled into the respiratory tract, spreading infection.3

It’s possible for someone to be infected with the flu or COVID-19 and spread either virus before they show any symptoms or when they have mild symptoms. This is also true if someone is asymptomatic, meaning they never develop or show symptoms.3

Surface transmission or touching a surface or object that has the virus on it and then touching your mouth, nose or eyes is a less common way of transmitting the COVID-19 and influenza virus, but it’s still possible.3,4

Different Viruses, Different Patterns

While COVID-19 and the flu share several similar symptoms and can be spread in some of the same ways, two different viruses cause these illnesses. As a result, some key differences distinguish them.

For instance, COVID-19 is thought to be more contagious and to cause more severe illnesses.3 COVID-19 also has higher likelihood of “super spreader” events than the flu.3

Typically, those with the flu are contagious for about one day prior to showing symptoms and are most contagious the first three to four days of their illness. The risk of individuals spreading COVID-19 peaks one day before symptoms begin and those diagnosed are considered contagious for up to eight days after their symptoms appear.3

COVID-19, depending on the variant, may also have a few unique telltale symptoms, such as loss of taste and smell,5 lingering symptoms,6 red/swollen eyes,7 and skin rashes.8

There are also significant differences between when these two illnesses occur and how they mutate.

“Influenza is seasonal whereas SARS-CoV-2 has (so far) been a constant presence, or ‘endemic,’” explains Santiago Lopez, U.S. Senior Medical Director, Medical and Scientific Affairs Lead at Pfizer. “There is some data suggesting peaks when it’s more prevalent, which mimics a seasonality, but at this juncture it is classified as endemic.”

He continues: “The rate of changes to SARS-CoV-2 is significantly higher than that of influenza. The flu does have seasonal ‘strains’ that are somewhat different from one another, but SARS-CoV-2 has much more drastic ‘variants’.”

What kinds of complications may occur?

“There are risks of developing complications for both flu and COVID-19, including severe illness or long-term complications,” says Verna Welch, Global Senior Medical Director, Influenza Medical and Scientific Affairs Lead at Pfizer. In addition to the signs and symptoms traditionally associated with the flu, she explains, it has also been linked to several other diseases like pneumonia, ear infections, sinus infections, and possibly meningitis.9

Futher, the Centers for Disease Control and Prevention (CDC) describes a range of non-infectious diseases that are also associated with the flu, including pneumonia, ear and sinus infections, inflammation in the muscles, brain, or heart, and potentially multiple organ failure.9

Similarly, a more severe case of COVID-19—dubbed “severe COVID -19”—can require hospitalization, a ventilator, or intensive care and can lead to higher risk of death. Severe COVID-19 infections are more common in older adults and those with underlying medical conditions such as cancer, diabetes, or heart disease.10

Some people who are diagnosed with COVID-19 develop serious complications, including blood clots in the veins and arteries of the lungs, heart, legs, or brain. Multisystem inflammatory syndrome is a rare but severe complication of COVID-19 that can occur in children and adults who develop COVID-19.3

For some who contracted the COVID-19 virus, there is potential fortheir symptoms to last much longer due to a phenomenon known as “long COVID ”.11

“Often these post-viral illnesses present like chronic fatigue syndrome with symptoms such as fatigue, brain fog, and post-exertional malaise,” Dr. Welch explains. “This too isn’t unique to COVID-19 and has been observed with flu and other viruses.”

“Prevention is key,” she emphasizes. “The best way to not get long COVID, long flu, or experience post-infection illnesses in general is to not get the illnesses in the first place.”

How are COVID-19 and the flu treated?

Treatment typically isn’t needed for mild flu symptoms, according to the CDC. In more severe cases or for those at high risk of complications, over-the-counter and FDA-approved prescribed medicines can be taken to alleviate flu symptoms, make the illness less severe, and reduce the risk of complications.12

Medications are also available to treat mild to moderate COVID-19 in certain patients, including those at high risk for serious infection, hospitalization, or death.13,14

Tips for Preventing COVID-19 and the Flu

The similarities between the flu and COVID-19 mean that the strategies for preventing illness can also be similar:

Getting vaccinated: Vaccines are available to potentially help lower the risk of serious illness from influenza and COVID-19.3 The CDC recommends an annual flu vaccine and staying up-to-date with COVID-19 vaccines for everyone over six months old (with rare exceptions) to potentially help reduce the risk of serious illness, hospitalization, and death.15,16

But what does staying “up to date” mean?

“While vaccination for the flu is annual, we don’t yet know at what frequency we’ll need to vaccinate for SARS-CoV-2,” says Lopez. “We’re currently creating vaccines as often as new variants require them without a specific schedule.”

Reducing the risk: Wearing a mask, frequent hand washing, and limiting contact with others if you feel sick are key strategies to potentially help reduce the risk of contracting or spreading the flu or COVID-19.17

Understanding the similarities and differences between influenza and COVID-19 and how to protect your health could potentially help lower the chances of getting sick.