Atlanta—If they have not already, pharmacists are likely to get a lot of questions this winter on how to tell the difference between influenza and COVID-19 infections.

The simple answer is testing, although testing for the flu cannot be done at home. However, there are other important distinctions that pharmacists can share with questioners.

The CDC advised that COVID-19 is believed to spread more easily than flu and is more likely to cause severe illness. "Compared to people with flu, people infected with COVID-19 may take longer to show symptoms and may be contagious for longer periods of time," it added.

And that is where the importance of testing comes in. "You cannot tell the difference between flu and COVID-19 by the symptoms alone because they have some of the same signs and symptoms," the CDC advised. "Specific testing is needed to tell what the illness is and to confirm a diagnosis. Having a medical professional administer a specific test that detects both flu and COVID-19 allows you to get diagnosed and treated for the specific virus you have more quickly."

Early treatment can reduce the risk of severe cases. In addition, testing can reveal if a patient has both flu and COVID-19 at the same time; that is rare, but it happens and can result in more severe disease than for someone infected with only one of the viruses, according to the guidance.

Still, both COVID-19 and flu can have varying degrees of symptoms, ranging from no symptoms (asymptomatic) to severe symptoms. Common symptoms that COVID-19 and flu share include:

• Fever or feeling feverish/having chills
• Cough
• Shortness of breath or difficulty breathing
• Fatigue (tiredness)
• Sore throat
• Runny or stuffy nose
• Muscle pain or body aches
• Headache
• Vomiting
• Diarrhea (more frequent in children with flu, but can occur in any age with COVID-19)
• Change in or loss of taste or smell, although this is more frequent with COVID-19.

In terms of incubation periods, someone with the flu might experience symptoms anywhere from 1 to 4 days after infection. With COVID-19, however, symptoms can occur anywhere from 2 to 5 days, and up to 14 days after infection, the CDC advised.

In addition, COVID-19 patients tend to be contagious for a longer time than flu patients. Specifically, public health officials say that those infected with the flu virus are potentially contagious for about 1 day before they show symptoms, although it is believed that flu is spread mainly by symptomatic people.

"Older children and adults with flu appear to be most contagious during the first 3-4 days of their illness, but some people might remain contagious for slightly longer periods," according to the CDC website. "Infants and people with weakened immune systems can be contagious for even longer."

With COVID-19, on average, infected people can begin spreading the SARS-CoV-2 virus about 2 to 3 days before their symptoms begin, although infectiousness peaks 1 day before their symptoms begin. What makes it especially difficult to control is that even those with symptomless infections can spread the virus.

Once symptoms begin, patients are considered contagious for about 8 days.

The CDC noted that COVID-19 and influenza are similar in that they can spread from person to person who are near or in close contact with one another. Both are spread mainly by large and small particles containing a virus that are expelled when ill people cough, sneeze, or talk. It added, "These particles can land in the mouths or noses of people who are nearby and possibly be inhaled into the respiratory tract. In some circumstances, such as indoor settings with poor ventilation, small particles containing virus might be spread longer distances and cause infections."

It also is possible to spread both of the viruses by touching another person and then transmitting the virus by touching their own mouth, nose, or eyes. COVID-19, however, is considered generally more contagious and more likely to be involved in superspreading events than the flu, according to the information.

Both COVID-19 and flu illnesses can result in severe illness and complications in those who are older, those with underlying medical conditions, or pregnant women, although COVID-19 appears more likely to result in hospitalization and death among those who were initially healthy.

In addition, some COVID-19 patients can go on to develop post-COVID conditions or multisystem inflammatory syndrome.

The two respiratory infections have some similar complications, including:

• Pneumonia
• Respiratory failure
• Acute respiratory distress syndrome (fluid in the lungs)
• Sepsis (a life-threatening illness caused by the body's extreme response to an infection)
• Cardiac injury (e.g., heart attacks and stroke)
• Multiple-organ failure (respiratory failure, kidney failure, shock)
• Worsening of chronic medical conditions (involving the lungs, heart, or nervous system or diabetes)
• Inflammation of the heart, brain, or muscle tissues
• Secondary infections (bacterial or fungal infections that can occur in people with flu or COVID-19).

Diarrhea is more common in young children with flu than in adults with flu.

Blood clots in the veins and arteries of the lungs, heart, legs, or brain are an additional complication of COVID-19, the CDC advised.

Approved treatments differ for COVID-19 and influenza, although, in both cases, patients at higher risk of complications or who have been hospitalized should receive recommended treatments and supportive medical care to help relieve symptoms and complications.

Prescription influenza antiviral drugs are FDA approved to treat flu but are not authorized for treatment of COVID-19.

For COVID-19, antiviral treatment is recommended for nonhospitalized patients at increased risk for severe cases, as well as for inpatients who have severe illness. With both COVID-19 and flu, treatment should be sought as soon as possible in those who need it, the CDC emphasized.

The content contained in this article is for informational purposes only. The content is not intended to be a substitute for professional advice. Reliance on any information provided in this article is solely at your own risk.