Each year, from around October through May, international health coverage is primarily focused on the flu—and with good reason: The flu is notoriously infectious and can cause up to 45 million illnesses each year, according to the Centers for Disease Control and Prevention. But this year, the new coronavirus outbreak (aka, COVID-19) is dominating the headlines, causing hundreds of thousands of Americans to fall ill since the start of the outbreak.

Both viruses—influenza and coronavirus—are highly infectious diseases with similar symptoms, transmission, and prevention methods, but is one necessarily worse than the other? Here, infectious disease experts across the US compare all aspects of both illnesses, to determine whether coronavirus is any worse than the flu—or vice-versa.

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Both seasonal flu viruses (which include influenza A and influenza B viruses) and COVID-19 are contagious viruses that cause respiratory illness. 

According to the CDC, the flu typically manifests itself in symptoms such as fever, cough, sore throat, muscle aches, headaches, runny or stuffy nose, fatigue and, occasionally, vomiting and diarrhea—typically, the latter two are more common in children. Those flu symptoms often come on suddenly after an incubation period of two to five days. 

While the majority of people infected with the flu will be fully recovered in about two weeks, for some people (most often those with compromised immune systems), the flu can lead to complications like pneumonia. According to recent data from the CDC, this year just 1% of people suffering from the flu have been hospitalized.

As for COVID-19, the CDC describes a wide range of symptoms, such as fever; cough; shortness of breath or difficulty breathing; chills or shaking with chills; muscle pain; headache; sore throat; and loss of taste or smell, coming on two to 14 days after exposure to the virus. One study, involving just 100 people with the virus, published January 30 in the The Lancet, noted that the most common symptoms were fever, cough and shortness of breath, with approximately 5% reporting sore throat and runny nose, and just 1-2% reporting diarrhea, nausea and vomiting. 

If the symptoms sound extremely similar, that's because they are, according to Manisha Juthani, MD, a Yale Medicine infectious disease specialist. “Based on symptoms alone, COVID-19 and flu are very difficult to distinguish,” says Dr. Juthani.

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The first and most major difference between the flu and novel coronavirus, is how little we know about the latter—which plays into into the absence of treatment and vaccines for COVID-19. "People often compare the flu to COVID-19, but we have a vaccine to prevent and medications to treat the flu. Right now, we have no [approved] medications or vaccine for COVID-19," says Dr. Juthani.

The influenza vaccine, for example, was first licensed for use in civilians in 1945—and now, doctors recommend all people over six months old get the flu shot each year, ideally as early as the end of October. Treatment for the flu is also an option, with antiviral medications like Tamiflu, which can shorten the amount of time you're sick and prevent flu complications like pneumonia. But, for Tamiflu to work effectively, it needs to be taken with in the first 48 hours of the onset of symptoms—and most flu cases are so mild they may not even need treatment other than rest and symptom management.

Scientists are currently working on a coronavirus vaccine. But as Jeremy Brown, MD, director of the Office of Emergency Care Research at the National Institutes of Health and author of Influenza: The Hundred-Year Hunt to Cure the Deadliest Disease in History, previously told Health, the entire vaccine process—from early development through human testing—can take a lot of time and cost a lot of money. Fortunately, in working on a vaccine for the current coronavirus, researchers may discover clues that may help treat or even prevent future outbreaks.

At this point, the best options for keeping yourself infection-free is following the CDC’s recommendations for all respiratory illnesses. That includes washing your hands often with soap and water for at least 20 seconds; not touching your eyes, nose and mouth with unwashed hands; maintaining social distancing; avoiding close contact with people who are sick; staying home when you are sick; wearing a cloth mask, and disinfecting frequently touched objects and surfaces.

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In both the flu and coronavirus, the main method of transmission appears to be from person to person via respiratory transmission—essentially by coming in close contact (within six feet) with respiratory droplets from the coughs and sneezes of infected people. The flu and coronavirus also have similar periods of time when people are asymptomatic but still contagious. “It appears that with both viruses, people may be able to transmit the virus before they are symptomatic,” explains Dr. Juthani. 

However, the latest evidence suggests COVID-19 is much more contagious and spreads more rapidly than the flu.

“In public health we measure something called the R0 (pronounced 'R-naught') which is the average number of people infected by a person with the virus,” says Dr. Brown. He goes on to explain that the R0 for influenza is about 1.3, meaning about 1.3 people get the virus from every ten who have the infection. Measles, on the other hand, is extremely contagious, with an R0 of 12-18. (But, of course, we have a measles vaccine.) As for COVID-19? A recent CDC analysis of confirmed cases in China puts the Ro for SARS-CoV-2 at 5.7. That's up from a previous estimate of 2.2, according to the study, released ahead of its slated publication in the journal Emerging Infectious Diseases.

An R0 of 5.7 means that 82% of the population must be immune to the virus—through vaccination or prior infection—to achieve so-called herd immunity, the study authors note. At 2.2, only 55% of the population would need to be immune to stop transmission.

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