Flu and Covid-19 are both contagious respiratory illnesses, but they are caused by different viruses. While Covid-19 is caused by infection with a new coronavirus, called SARS-CoV-2, flu is caused by infection with influenza viruses. The US Centers for Disease Control and Prevention (CDC) believes it is likely that flu viruses and the virus that causes Covid-19 will both be spreading in the fall and winter. It is also possible for a person to have the flu, as well as other respiratory illnesses, and Covid-19 at the same time, says the agency. Health experts are, however, still studying how common this can be.
Key similarities and differences between Covid-19 and flu
Flu and coronavirus can both result in serious illness, including illness resulting in hospitalization or death. However, Covid-19 seems to spread more easily than the flu and causes more serious illnesses in some people. Flu viruses can cause mild to severe illness. Covid-19 can also take longer before people show symptoms and people can be contagious for longer.
Another important difference is there is a vaccine to protect against the flu, but there is currently no vaccine against Covid-19. The best way to prevent infection is to avoid being exposed to the virus. While prescription influenza antiviral drugs are Food and Drug Administration (FDA)-approved to treat flu, there are currently no drugs or other therapeutics approved by the FDA to prevent or treat coronavirus.
Signs and symptoms
Some of the symptoms of flu and coronavirus are similar, making it hard to tell the difference between them based on symptoms alone. Diagnostic testing may be needed to help confirm a diagnosis.
According to the CDC, Both can have varying degrees of signs and symptoms, ranging from no symptoms (asymptomatic) to severe. Common symptoms include fever or feeling feverish/chills, cough, shortness of breath or difficulty breathing, sore throat, tiredness/fatigue, muscle pain or body aches, runny or stuffy nose, and headache. Some people may have vomiting and diarrhea, but this is more common in children than adults. Other signs and symptoms of Covid-19, different from flu, may include a change in or loss of taste or smell.
For both illnesses, 1 or more days can pass between a person becoming infected and when he or she starts to experience illness symptoms. More specifically, if a person has Covid-19, it could take them longer to develop symptoms than if they had the flu. Typically, a person with coronavirus develops symptoms 5 days after being infected, but symptoms can appear as early as 2 days after infection or as late as 14 days after infection, and the time range can vary. In the case of flu, however, a person typically develops symptoms anywhere from 1 to 4 days after infection.
Both the flu virus and coronavirus may be spread to others by people before they begin showing symptoms, with very mild symptoms or who never developed symptoms. “While Covid-19 and flu viruses are thought to spread in similar ways, Covid-19 is more contagious among certain populations and age groups than flu. Also, Covid-19 has been observed to have more super-spreading events than flu. This means the virus that causes Covid-19 can quickly and easily spread to a lot of people and result in continuous spreading among people as time progresses,” explains the CDC.
For both coronavirus and flu, it is possible to spread the virus for at least 1 day before experiencing any symptoms. But there are some differences. A person infected with Covid-19 can spread the virus for about 2 days before experiencing signs or symptoms and remain contagious for at least 10 days after signs or symptoms first appeared. If someone is asymptomatic or their symptoms go away, it is possible to remain contagious for at least 10 days after testing positive for coronavirus.
Most people with the flu are contagious for about 1 day before they show symptoms. Older children and adults with flu appear to be most contagious during the initial 3-4 days of their illness but many remain contagious for about 7 days. Infants and people with weakened immune systems can be contagious for even longer.
Both Covid-19 and influenza can cause complications, including respiratory failure, acute respiratory distress syndrome (fluid in lungs), pneumonia, sepsis, cardiac injury (heart attacks and stroke), multiple-organ failure, worsening of chronic medical conditions that involve the lungs, heart, nervous system or diabetes, inflammation of the heart, brain or muscle tissues, and secondary bacterial infections. Additional complications from coronavirus can include blood clots in the veins and arteries of the lungs, heart, legs, or brain, as well as MIS-C.
People at high-risk for severe illness
Those at highest risk include older adults, people with certain underlying medical conditions, and pregnant people. The risk of complications for healthy children is higher for flu compared to coronavirus. But infants and children with underlying medical conditions are at increased risk for both. There is another difference. Young children are at higher risk of severe illness from the flu. In the case of Covid-19, school-aged children are at higher risk of a multisystem inflammatory syndrome in children (MIS-C), a rare but severe complication of coronavirus.
Can a test detect both Covid-19 and the flu?
Yes, the CDC has developed a test that will check for A and B type seasonal flu viruses and SARS CoV-2. The FDA has granted it an Emergency Use Authorization. The new test will not replace other tests. It will be used by US public health laboratories, where it will help supplement and streamline surveillance for flu and coronavirus. Initial test kits were sent to public health laboratories in early August 2020. “Testing for these viruses at the same time will give public health officials important information about how flu and Covid-19 are spreading and what prevention steps should be taken. The test will also help public health laboratories save time and testing materials, and to possibly return test results faster,” explains the agency.
Can a flu vaccine protect against Covid-19?
No, it cannot. However, flu vaccination has many other important benefits: they have been shown to reduce the risk of flu illness, hospitalization, and death. There is also no evidence that getting a flu vaccination increases a person’s risk of getting sick from a coronavirus, like the one that causes Covid-19.
How to safely get a flu vaccine if there’s Covid-19 spread in your community?
The CDC recommends practicing everyday preventive actions and following its guidelines while going to get a vaccine, including the use of face masks, social distancing, and hand hygiene. A person can safely get a flu vaccine at multiple locations such as the doctor’s office, health departments, and pharmacies. You can check the VaccineFinder.org to find where flu vaccines are available near you.
“Ask your doctor, pharmacist, or health department if they are following CDC’s vaccination pandemic guidance. Any vaccination location following CDC’s guidance should be a safe place for you to get a flu vaccine,” suggests the agency.