Flu and COVID-19 are today two respiratory and viral infections whose most serious cases are preventable through vaccination. The flu virus can cause between three and five million cases of seriously ill people a year and more than 200,000 deaths from respiratory infection worldwide. In the case of COVID-19, which has generated a global public health emergency since December 2019, it has already affected more than 242 million people and caused the death of 4.9 million. Although some symptoms are similar, with the advance of the pandemic, the 5 main differences that exist between both infections and their complications and sequelae have been clarified.

Multiple scientific studies have already shown that vaccination against the virus that causes influenza reduces the risk of having a more serious evolution of the COVID-19 disease, reduces hospitalization and the number of admissions to intensive care units. It has also been found that a person can acquire infection with the coronavirus and the flu virus simultaneously.

As some of the symptoms of the flu, COVID-19, and other respiratory diseases are similar, the Centers for Disease Control and Prevention (CDC for its acronym in English) of the United States has clarified that it is not possible to establish the difference based solely on symptoms. “It is necessary to carry out a screening test to determine what disease it is and confirm the diagnosis,” they recommended.

Is it flu or COVID-19?: 5 main differences that distinguish these infections today — The swabs in the nose allow to do the test to know if a person has COVID-19 (EFE / Bienvenido Velasco)

In addition, they warned that “people can be infected by the influenza virus and the virus that causes COVID-19 at the same time, and have symptoms of both influenza and COVID-19,” according to the CDC. During the COVID-19 pandemic, approximately between 7% and 11% of cases of coinfections have been registered in patients with COVID-19, in which the influenza virus was one of the most frequent.

Here are the 5 differences to consider according to the CDC and the Mayo Clinic in the United States:

1- Differences in signs and symptoms

Both COVID-19 and the flu can have varying degrees of signs and symptoms, ranging from no symptoms (these are called asymptomatic cases) to severe symptoms. The most common symptoms of both COVID-19 and the flu include: fever or feeling feverish, having chills, cough, trouble breathing (or feeling short of breath), fatigue or tiredness, sore throat, runny nose, or nose stuffy.

Also in both infections, people may have muscle pain and body aches, headache, vomiting and diarrhea, and changes or loss of taste or smell. Nevertheless, in the case of the flu, loss of smell or taste is rare. The symptoms of diarrhea and vomiting occur more in cases of children with the flu and not so much in adults.

2- How long it takes for symptoms to appear

With the flu, people usually have symptoms within 1 to 4 days of being infected. Instead, People develop symptoms of COVID-19 after about 5 days after being infected, but symptoms can manifest between 2 and 14 days after being infected.

3- For how long people can infect

In both the case of COVID-19 and the flu, it is possible to spread the virus at least 1 day before experiencing any of the symptoms. But a person with the coronavirus can spread the virus longer than if they have the flu virus.

Most people with the flu virus can be contagious for about 1 day before they develop symptoms. Older children and adults with the flu appear to be most contagious during the first 3-4 days of illness, but many people remain contagious for about 7 days, according to the CDC. Babies and people with weakened immune systems can be contagious even longer.

Regarding COVID-19, it is still being investigated for how long a person can spread the coronavirus. According to the CDC, “It is possible that people can spread the coronavirus for approximately 2 days before manifesting signs or symptoms (or possibly before) and remain contagious for at least 10 days after the first appearance of signs or symptoms.”

In addition, they warned: “A person who is asymptomatic or whose symptoms disappear can continue to infect for at least 10 days after his positive result in the detection test for COVID-19. People hospitalized with severe cases of the disease and people with weakened immune systems can be contagious for 20 days or more.”

4- How are they different in terms of transmission

Both the coronavirus and the flu virus can be transmitted from person to person between people who are in close contact with each other from two meters apart. Both diseases are spread mainly through large and small particles that contain the virus that is expelled when people who are infected (even without symptoms) cough, sneeze or speak.

Those particles of different sizes can end up in the mouth or nose of those who are nearby and possibly be inhaled and reach their lungs. In some circumstances, such as in confined spaces with poor ventilation, small particles – called aerosols – could spread beyond 2 meters and cause infections.

Although most of the spread occurs through inhalation, it can happen that a person becomes infected by touching another (for example, by shaking the hand of someone who has the virus on their hand) or by touching a surface or object with the virus, and then touching your mouth, nose, or eyes.

Although the virus that causes COVID-19 and influenza viruses are believed to spread in similar ways, “the virus that causes COVID-19 is generally more contagious than flu viruses,” according to the CDC. Additionally, COVID-19 has been observed to cause many more super-spreaders than flu. This means that the virus that causes COVID-19 can spread more quickly and easily to many, many people and cause continuous spread between people as time progresses.

5- They are distinguished in complications and sequelae

Both COVID-19 and influenza can cause complications, such as pneumonia, respiratory failure, acute respiratory distress syndrome (fluid in the lungs, sepsis, heart injury (for example, heart attacks and strokes, multiple organ failure (respiratory failure, respiratory failure, shock), and worsening of chronic conditions (involving the lungs, heart, nervous system, or diabetes).

Also, patients with both infections can suffer from inflammation of the heart, brain, and muscle tissues, and secondary infections (bacterial or fungal infections that can occur in people who have already been infected by the flu or COVID-19).

But differences have been detected. In the case of the flu, secondary bacterial infections are more frequent in cases of influenza than in cases of COVID-19. Diarrhea is more common in young children with the flu than in adults with the flu.

Instead, In COVID-19 cases, there are complications such as blood clots in the veins and arteries of the lungs, heart, legs, or brain. Multisystemic Inflammatory Syndrome can develop in children (MIS-C) and in adults (MIS-A), which was described by different scientists as of April last year.

In addition, COVID-19 can leave sequelae, which today are known as Prolonged Covid Syndrome or Post Covid. It is a range of 55 symptoms that can last for weeks or months after being infected by the virus that causes COVID-19 for the first time or appear weeks after infection. Long-term COVID-19 can occur in anyone who has had COVID-19, even if the illness was mild or had no symptoms.