As coronavirus infections began appearing across the United States, in cities from Seattle to New York, Americans wondered how to measure this new threat against a more familiar foe: influenza.
President Trump, a self-described germophobe, has said he was amazed to learn that tens of thousands of Americans died from the flu each year. On several occasions, Mr. Trump has accused the news media and Democrats of exaggerating the dangers of the coronavirus.
“The flu kills people,” Mick Mulvaney, the acting White House chief of staff, said in February. “This is not Ebola. It’s not SARS, it’s not MERS. It’s not a death sentence.”
To many public health officials, that argument misses the point.
Yes, the flu is terrible — that’s exactly why scientists don’t want another contagious respiratory disease to take root. If they could stop the seasonal flu, they would. But there may yet be a chance to stop the coronavirus, or at least slow its spread.
In many ways, the flu is the best argument for throwing everything at the coronavirus. Here’s a closer look at the similarities and differences.
Which virus is deadlier?
The coronavirus seems to be more deadly than the flu — so far.
On average, seasonal flu strains kill about 0.1 percent of people who become infected. The 1918 flu had an unusually high fatality rate, around 2 percent. Because it was so contagious, that flu killed tens of millions of people.
Early estimates of the coronavirus death rate from China were about 2 percent. But a later report on 1,099 cases from many parts of China, published in The New England Journal of Medicine, found a lower rate: 1.4 percent.
In a recent speech, Dr. Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, asserted that the global case fatality rate for people infected with coronavirus was 3.4 percent, a startling figure.
W.H.O. officials later clarified that Dr. Tedros’s figure was a crude “snapshot” based on incomplete data and heavily skewed by the intensity of the initial outbreak in Wuhan, China.
The true death rate could turn out to be similar to that of a severe seasonal flu, below 1 percent, according to an editorial published in the journal by Dr. Anthony S. Fauci and Dr. H. Clifford Lane, of the National Institute of Allergy and Infectious Diseases, and Dr. Robert R. Redfield, director of the Centers for Disease Control and Prevention. But more recently, Dr. Fauci has cited the 1 percent estimate, emphasizing that it is 10 times the death rate from seasonal flu.
Even a disease with a relatively low death rate can take a huge toll if enormous numbers of people catch it. As of Friday, there were more than 135,000 coronavirus cases and nearly 5,000 deaths. In the United States, there have been more than 1,200 coronavirus cases and about 36 deaths.
But because of the lack of testing capacity in the United States, the true case count and number of deaths are not known for sure.
Which virus is more contagious?
So far, the new coronavirus seems to be more contagious than most strains of the flu, and roughly as contagious as strains that appear in pandemic flu seasons.
Each person with the coronavirus appears to infect 2.2 other people, on average. But the figure is skewed by the fact that the epidemic was not managed well in the beginning, and infections soared in Wuhan and the surrounding province. As an epidemic comes under control, the reproduction number, as it’s called, will fall.
By comparison, the figure for the seasonal flu is roughly 1.3. The reproduction number for the flu of 1918 was about the same as that of the new coronavirus, perhaps higher, but that was before modern treatments and vaccines were available.
In both flu and the illness caused by the coronavirus, people may be contagious before symptoms develop, making it difficult or even impossible to control the spread of the virus. Nobody knows yet how many people infected with the coronavirus have only very mild symptoms or none at all.
Who is most at risk from infection?
People who are older than 60, or have a weakened immune system or chronic illnesses like lung disease, heart disease or diabetes, have the highest risk of becoming severely ill if they contract the coronavirus or the flu. Each underlying illness adds to the risk.
Many people in the United States have an increased risk of becoming seriously ill if they are infected: about 60 percent of adults have at least one underlying health condition, and 40 percent have two or more underlying conditions. Approximately 25 million have diabetes, which can lower immunity.
Death rates among men infected with the coronavirus in China, particularly those in their late 40s and older, have exceeded those among women, a pattern not seen in the seasonal flu. The reason for the discrepancy is not known, although Chinese men do smoke more, often resulting in compromised lung function.
There seems to be another important difference: The flu appears far more dangerous to children, particularly very young ones, who can become severely ill. Children infected with the new coronavirus tend to have mild or no symptoms.
The flu is also especially dangerous for pregnant women, who can become severely ill from it. Whether the new coronavirus poses as serious a threat to pregnant women is not known.
Not everyone who becomes seriously ill fits the high-risk profile. In every infectious disease outbreak, there are unexplained cases that defy the statistics, such as severe illness striking a young, healthy person who would have been expected to become just mildly sick. The physician in China who was penalized for alerting colleagues to the outbreak there, Dr. Li Wenliang, died from the disease at age 34.
Which virus makes you sicker?
In the current season, there have been at least 34 million cases of flu in the United States, 350,000 hospitalizations and 20,000 flu deaths, according to the C.D.C. Hospitalization rates among children and young adults this year have been unusually high.
There would be even more illnesses and deaths if there were no flu vaccine. Most people recover in less than two weeks, and sometimes in just days.
By contrast, at least 90,000 people in the United States have been infected with the new coronavirus by late March, and there have been at least 1,400 deaths. There are no treatments or vaccines for the coronavirus, only supportive care for infected people.
Most cases of coronavirus infection are not severe, but some people do become quite sick. Data from the largest study of patients to date, conducted in China, suggests that of coronavirus patients receiving medical attention, 80 percent had mild infections, about 15 percent had severe illnesses, and 5 percent were critical. (But many of the mild infections included patients with pneumonia, experts later learned.)
The first symptoms, fever and cough, are similar to that of the flu, so the diseases can be hard to tell apart without a test to identify the virus. Pneumonia is common among coronavirus patients, even among those whose cases are not severe.
Experts think there may also be many people with no symptoms at all, or such mild ones that they never bother to seek medical attention. Because those cases have not been counted, it’s not possible now to know the real proportion of mild versus severe cases.
Antibody tests, which can determine whether someone has ever been infected, may eventually help to establish how many people had mild or asymptomatic coronavirus infections.
Can people become immune to the coronavirus?
After viral infections, people generally develop antibodies in their blood that will fight off the virus and protect them from contracting it again. It’s reasonable to assume that people who have had the new coronavirus will become immune to it.
But it is not known how long that immunity will last. With other coronaviruses, which cause the common cold, immunity can wane.
There are vaccines for the seasonal flu, of course, and these induce at least some immunity to influenza.
What treatments are available?
There is no approved antiviral drug for the coronavirus, though several are being tested. Doctors can recommend only the usual remedies for any viral illness: rest, medicine to reduce pain and fever, and fluids to avoid dehydration.
Coronavirus patients with pneumonia may also need oxygen, and a ventilator if breathing trouble worsens.
For the flu, however, there are four prescription medicines. All work best if they are taken within a day or two of when symptoms start.
They’re not miracle cures: They can lessen the severity of the illness and shorten its course by a day or so, and they may lower the risk of serious complications.
The drugs are also recommended for people who have been exposed to a flu patient, to try to prevent the illness.
The flu, like the coronavirus illness, can also cause pneumonia and breathing trouble. Anyone who becomes short of breath needs medical attention quickly.
Can I get vaccinated?
An experimental vaccine for the coronavirus may be ready for safety testing in humans soon, but will take much longer, at least a year or two, to become available for widespread use — if it works.
Flu vaccines, on the other hand, are widely available and generally 40 percent to 60 percent effective, which means they will reduce cases by that amount in a population that has been vaccinated, compared with one that has not.
The vaccine for the current season falls into that range, according to the C.D.C., which said in February that people who have not been vaccinated should still get the shot, because the flu season is ongoing.
Experts have been urging people to get the flu shot for all the usual reasons. But now there’s another: As the coronavirus spreads in the United States, hospitals will need all the beds, equipment and staff they can muster.
It will be important not to have those resources taken up by patients with flu that could have been prevented.
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Will the coronavirus go away when the weather warms?
Mr. Trump has said repeatedly that the coronavirus will retreat as weather warms, just as influenza does. In fact, because this is a new virus, there is no information about how the weather might affect it.
Even if the virus were to diminish in the spring, it might rebound later in the fall, as the weather cools. This is a pattern often seen in severe flu seasons.
Containment is becoming less likely, because of the contagiousness of the virus, the possibility that people can spread it before they have symptoms and the increasing number of outbreaks around the world.
Cases in California, New York, Oregon and Washington State without known links to overseas travel indicate the new coronavirus has already begun to circulate.
Reporting was contributed by Gina Kolata and Knvul Sheikh.