By comparison, influenza — known as the common flu — has infected as many as 45 million Americans since October and killed as many as 46,000, according to estimates from the Centers for Disease Control and Prevention. President Trump invoked the large number of flu deaths each year in a tweet on Monday.

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Both the coronavirus and influenza are respiratory illnesses. Both have similar symptoms. Both are contagious. Both can be deadly.

So why the nationwide coronavirus frenzy?

The key difference between the novel coronavirus and influenza, said Melissa Nolan, an infectious disease epidemiologist at the University of South Carolina, is this: We know what to expect from the flu.

Covid-19, the disease the coronavirus causes, has plunged us into the unknown. We don’t know how severe an outbreak might be and how many people it would kill. We don’t know if we might be forced into quarantine for two weeks. We don’t even know if we will know if we get it.

The numbers only tell part of the story — largely because flu season has been going on longer than the coronavirus has existed and because researchers simply don’t have enough data yet to accurately assess which is more deadly.

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Further complicating health officials’ ability to control the spread of the coronavirus is that, unlike influenza, there is no vaccine for it.

Though it remains to be seen whether the virus can be contained worldwide, deliberate quarantines and other health and travel precautions can slow its growth and help ensure hospitals aren’t overwhelmed with coronavirus patients — especially at the same time health-care workers are trying to manage seasonal influenza.

What are the symptoms?

The known symptoms for influenza and covid-19 are nearly identical: fever, cough, body aches, fatigue and at times vomiting and diarrhea. Both illnesses can manifest in mild or severe ways or even cause death, according to Lisa Lockerd Maragakis, senior director of infection prevention at Johns Hopkins Health System.

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As of now, there are “no unique clinical signatures” that distinguish the onset of covid-19 from influenza, Nolan said. Research on the known cases so far suggests that the vast majority, some 80 percent, are mild. Health officials have urged those experiencing mild symptoms to self-manage from home with over-the-counter cold and flu aids, rather than crowding hospital emergency and waiting rooms.

Those experiencing more severe symptoms, such as trouble breathing, lethargy or a fever that won’t break, should call a doctor. Both the coronavirus and influenza can cause pneumonia, an infection of the lungs that can be life-threatening in infants, children and people over 65.

Who is at risk?

So far, it seems those most at risk for severe complications from covid-19 are the elderly and those with compromised immune systems or underlying health conditions.

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The same goes for influenza, though young children appear to be more susceptible to the flu than they are to the coronavirus, data shows. Children younger than 5 years old, especially those under the age of 2, are at a high risk of developing serious health complications from the flu, according to the CDC.

Which virus is more deadly?

That’s a difficult question to answer for many reasons. First, health officials are not comparing analogous data sets between the viruses. They have years of influenza data but just months of covid-19 numbers — which are evolving by the day. It’s possible the estimated death rates for the coronavirus might be inflated because a considerable number of people likely have or had the virus but were not diagnosed because their symptoms were too mild to see a doctor.

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“There is also a number of asymptomatic infections we don’t know about,” Nolan said.

World Health Organization Director General Tedros Adhanom Ghebreyesus said at a news briefing March 3 that the global case fatality rate for the coronavirus is believed to be about 3.4 percent, higher than the 2.3 percent reported in a China CDC study released in February. But a study published in the New England Journal of Medicine last week found a death rate of 1.4 percent among a group of 1,099 patients, suggesting the rate could be lower than those reported by the WHO and Chinese officials.

“By comparison, seasonal flu generally kills far fewer than 1 percent of those infected,” Tedros said of the global flu caseload during the news briefing.

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Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, has said that the mortality rate for seasonal flu is 0.1 percent.

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“You know, the risk is this: We have a brand-new virus in a completely naive population on Earth; we’re all susceptible to it. Everyone is potentially infectable with this virus,” said Christopher Mores, a global health professor at George Washington University. “Even with a high transmission rate and the low case fatality rate, that still becomes a massive number of ill and fatal cases.”

As The Post’s Joel Achenbach and Laurie Mcginley reported, data from China shows that covid-19 has a significantly higher case fatality rate than seasonal flu. A recent World Health Organization report by an international team of researchers, including from China, found that the case fatality rate for covid-19 patients over the age of 80 in China was 21.9 percent.

What about vaccinations?

For decades, the flu shot has been a critical preventive measure to inoculate the nation’s most vulnerable populations. Though just 62.6 percent of adolescents and 45.3 percent of adults in the United States got the vaccine last flu season, according to the CDC, it still provides front-line health-care workers protection while they treat ill patients, a shield the nurses, doctors, home health aides and other medical workers involved in the coronavirus containment do not have right now.

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There will be no coronavirus vaccine for at least another year, health officials have said, which is why governments across the globe have been scrambling to prevent and track the infection as it inches toward a possible pandemic.