The new coronavirus outbreak has made headlines in recent weeks, but there’s another viral epidemic hitting countries around the world: flu season. But how do these viruses compare, and which one is really more worrisome?

So far, the new coronavirus, dubbed 2019-nCoV, has led to more than 20,000 illnesses and 427 deaths in China, as well as more than 200 illnesses and two deaths outside of mainland China. But that’s nothing compared with the flu, also called influenza. In the U.S. alone, the flu has already caused an estimated 19 million illnesses, 180,000 hospitalizations and 10,000 deaths this season, according to the Centers for Disease Control and Prevention (CDC).

That said, scientists have studied seasonal flu for decades. So, despite the danger of it, we know a lot about flu viruses and what to expect each season. In contrast, very little is known about 2019-nCoV because it’s so new. This means 2019-nCoV is something of a wild card in terms of how far it will spread and how many deaths it will cause.

“Despite the morbidity and mortality with influenza, there’s a certainty … of seasonal flu," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in a White House press conference on Jan. 31. “I can tell you all, guaranteed, that as we get into March and April, the flu cases are going to go down. You could predict pretty accurately what the range of the mortality is and the hospitalizations [will be],” Fauci said. “The issue now with [2019-nCoV] is that there’s a lot of unknowns.”

Scientists are racing to find out more about 2019-nCoV, and our understanding of the virus and the threat it poses may change as new information becomes available. Based on what we know so far, here’s how it compares with the flu.

Symptoms and severity

Both seasonal flu viruses (which include influenza A and influenza B viruses) and 2019-nCoV are contagious viruses that cause respiratory illness.

Typical flu symptoms include fever, cough, sore throat, muscle aches, headaches, runny or stuffy nose, fatigue and, sometimes, vomiting and diarrhea, according to the CDC. Flu symptoms often come on suddenly. Most people who get the flu will recover in less than two weeks. But in some people, the flu causes complications, including pneumonia. So far this flu season, about 1% of people in the United States have developed symptoms severe enough to be hospitalized, which is similar to the rate last season, according to data from the CDC.

With 2019-nCoV, doctors are still trying to understand the full picture of disease symptoms and severity. In a recent study of about 100 people with the virus, published Jan. 30 in the journal The Lancet, the most common symptoms were fever, cough and shortness of breath. Only about 5% of patients in that study reported sore throat and runny nose, and only 1-2% reported diarrhea, nausea and vomiting. Of the more than 20,000 cases reported in China so far, about 14% have been classified as severe, according to data from the World Health Organization (WHO) posted Tuesday (Feb. 4).

It’s important to note that, because respiratory viruses cause similar symptoms, it can be difficult to distinguish different respiratory viruses based on symptoms alone, according to WHO.

Death rate

So far this flu season, about 0.05% of people who caught the flu have died from the virus in the U.S., according to CDC data.

The death rate for 2019-nCoV is still unclear, but it appears to be higher than that of the flu. Throughout the outbreak, the death rate for 2019-nCoV has been about 2%. Still, officials note that in the beginning of an outbreak, the initial cases that are identified “skew to the severe,” which may make the mortality rate seem higher than it is, Alex Azar, U.S. secretary of the Health and Human Services, said during a news briefing on Jan. 28. The mortality rate may drop as more mild cases are identified, Azar said.

Virus transmission

The measure scientists use to determine how easily a virus spreads is known as the “basic reproduction number,” or R0 (pronounced R-nought). This is an estimate of the average number of people who catch the virus from a single infected person, Live science previously reported. The flu has an R0 value of about 1.3, according to The New York Times.

Researchers are still working to determine the R0 for 2019-nCoV. A study published Jan. 29 in the New England Journal of Medicine (NEJM) estimated an R0 value for the new coronavirus to be 2.2, meaning each infected person has been spreading the virus to an average of 2.2 people.

It’s important to note that R0 is not necessarily a constant number. Estimates can vary by location, depending on such factors as how often people come into contact with each other and the efforts taken to reduce viral spread, Live Science previously reported.

Risk of infection

The CDC estimates that, on average, about 8% of the U.S. population gets sick with the flu each season.

There are currently only 11 cases of 2019-nCoV in the U.S. Still, newly emerged viruses like 2019-nCoV are always of public health concern, according to the CDC. It’s unclear how the situation with this virus in the U.S. will unfold, the agency said. Some people, such as health care workers, are at increased risk for exposure to 2019-nCoV. But for the general American public, the immediate health risk from the virus is low at this time.

Pandemics

It’s important to note that seasonal flu, which causes outbreaks every year, should not be confused with pandemic flu, or a global outbreak of a new flu virus that is very different from the strains that typically circulate. This happened in 2009 with the swine flu pandemic, which is estimated to have killed between 151,000 and 575,000 people worldwide, according to the CDC. There is no flu pandemic happening currently.

The 2019-nCoV outbreak has not yet been declared a pandemic, as the majority of cases have occurred in China. But on Jan. 30, the WHO declared the 2019-nCoV outbreak a “public health emergency of international concern.” The declaration was primarily due to concern that the virus could spread to countries with weaker health systems.

Prevention

Unlike seasonal flu, for which there is a vaccine to protect against infection, there is no vaccine for 2019-nCoV. But researchers at the U.S. National Institutes of Health are in the early stages of developing one. Officials plan to launch a phase 1 clinical trial of a potential vaccine for 2019-nCoV within the next three months.

In general, the CDC recommends the following to prevent the spread of respiratory viruses, which include both coronaviruses and flu viruses: Wash your hands often with soap and water for at least 20 seconds; avoid touching your eyes, nose and mouth with unwashed hands; avoid close contact with people who are sick; stay home when you are sick; and clean and disinfect frequently touched objects and surfaces.

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