COVID-19 vs. Influenza (the flu)What’s the difference?

It’s flu season. While the exact timing and duration of flu seasons varies, it usually begins in October and peaks between December and February. Elevated activity can last until May. With the co-presence of COVID-19, we need to be especially careful because it is possible to get both at the same time, with the impact on health potentially significant.

 The first step in protecting yourself is to get a flu shot. It’s something we should do every year, but it is especially important this year. The CDC estimates that flu vaccines are 40% to 60% effective. If you’ve had your shot and start to feel ill, you may be less likely to wonder about what it is, and more prone to take steps to determine if it’s COVID-19 sooner. Even a few days can make a difference in your health, as well as cut down on the time you may expose others to coronavirus.

How are COVID-19 and influenza viruses similar?

COVID-19 and influenza viruses (flu) have a similar disease presentation. They both cause respiratory disease, which presents a wide range of illness, from being asymptomatic, to mild, to severe, and even possibly death. Symptoms common in both include:

  • Fever
  • Cough
  • Shortness of breath or difficulty breathing
  • Tiredness
  • Sore throat
  • Runny or stuffy nose
  • Muscle aches
  • Headache
  • Nausea or vomiting, but this is more common in children than in adults

Both viruses are also transmitted by contact, droplets and fomites (e.g. traces on clothes, utensils, and furniture). As a result, the same public health measures, such as hand hygiene and good respiratory etiquette (wearing a mask, coughing into your elbow or a tissue) are important actions all can take to prevent infection. 

How are they different?

Cause. COVID-19 as caused by a new coronavirus called SARS-CoV-2, while influenza is caused by influenza A and B viruses.

Speed of transmission. Flu has a shorter incubation period (time from infection to symptoms). COVID-19 virus is estimated to be 5-6 days, while for the flu, the serial interval is 3 days. This means that influenza can spread faster than COVID-19.

Transmission. Transmission in the first 3-5 days of illness is a major driver for influenza. In contrast, while we are learning that there are people who can shed COVID-19 virus 24-48 hours prior to symptom onset, at present, this does not appear to be a major driver of transmission.

Mortality. COVID-19 appears to have a higher mortality rate than influenza, especially seasonal influenza. While exact data on COVID-19 will take more time to fully understand, indications are that the mortality rate in the US is approximately 3.0%. For seasonal influenza, mortality is usually well below 1.0%.