1. How contagious is the virus?
It seems moderately infectious, similar to SARS.
The scale of an outbreak depends on how quickly and easily a virus is transmitted from person to person. While research has just begun, scientists have estimated that each person with the Wuhan coronavirus could infect somewhere between 1.5 and 3.5 people without effective containment measures.
That would make the new virus roughly as contagious as SARS, another coronavirus that circulated in China in 2003 and was contained after it sickened 8,098 people and killed 774. Respiratory viruses like these can travel through the air, enveloped in tiny droplets that are produced when a sick person breathes, talks, coughs or sneezes.
These droplets fall to the ground within a few feet. That makes the virus harder to get than pathogens like measles, chickenpox and tuberculosis, which can travel a hundred feet through the air. But it is easier to catch than H.I.V. or hepatitis, which spread only through direct contact with the bodily fluids of an infected person.
If each person infected with the Wuhan coronavirus infects two to three others, that may be enough to sustain and accelerate an outbreak, if nothing is done to reduce it.
Here’s how that works. In the animation below, a group of five infected people could spread the virus to about 368 people over just five cycles of infection.
Compare that with a less contagious virus, like the seasonal flu. People with the flu tend to infect 1.3 other individuals, on average. The difference may seem small, but the result is a striking contrast: Only about 45 people might be infected in the same scenario.
But the transmission numbers of any disease aren't set in stone. They can be reduced by effective public health measures, such as isolating sick people and tracking individuals they’ve had contact with. When global health authorities methodically tracked and isolated people infected with SARS in 2003, they were able to bring the average number each sick person infected down to 0.4, enough to stop the outbreak.
Health authorities around the world are expending enormous effort trying to repeat that.
So far, the number of cases outside China has been small. But in recent days, cases have turned up in several countries, including the United States, with people who have not visited China. And the number of cases within China has accelerated, far surpassing the rate of new SARS cases in 2003.
2. How deadly is the virus?
It’s hard to know yet. But the mortality rate is probably less than 3 percent, much less than SARS.
This is one of the most important factors in how damaging the outbreak will be, and one of the least understood.
It’s tough to assess the lethality of a new virus. The worst cases are usually detected first, which can skew our understanding of how likely patients are to die. About a third of the first 41 patients reported in Wuhan had to be treated in an I.C.U., many with symptoms of fever, severe cough, shortness of breath and pneumonia. But people with mild cases may never visit a doctor. So there may be more cases than we know, and the death rate may be lower than we initially thought.
At the same time, deaths from the virus may be underreported. The Chinese cities at the center of the outbreak face a shortage of testing kits and hospital beds, and many sick people have not been able to see a doctor.
“There’s still a lot of uncertainty about what this virus is like and what it is doing,” said Dr. Allison McGeer, an infectious disease specialist at Mount Sinai Hospital in Toronto, who was at the frontlines of the Canadian response to SARS.
Early indications suggest the mortality rate for this virus is considerably less than another coronavirus, MERS, which kills about one in three people who become infected, and SARS, which kills about one in 10. All of the diseases appear to latch on to proteins on the surface of lung cells, but MERS and SARS seem to be more destructive to lung tissue. As of Jan. 31, fewer than one in 40 of the people with confirmed infections had died. Many of those who died were older men with underlying health problems.
Here’s how the new coronavirus compares with other infectious diseases:
Pathogens can still be very dangerous even if their fatality rate is low, Dr. McGeer said. For instance, even though influenza has a case fatality rate below one per 1,000, roughly 200,000 people end up hospitalized with the virus each year in the United States, and about 35,000 people die.
3. How long will it take to develop a vaccine?
A vaccine is still a year away — at minimum.
A coronavirus vaccine could prevent infections and stop the spread of the disease. But vaccines take time.
After the SARS outbreak in 2003, it took researchers about 20 months to get a vaccine ready for human trials. (The vaccine was never needed, because the disease was eventually contained.) By the Zika outbreak in 2015, researchers had brought the vaccine development timeline down to six months.
Now, they hope that work from past outbreaks will help cut the timeline even further. Researchers have already studied the genome of the new coronavirus and found the proteins that are crucial for infection. Scientists from the National Institutes of Health, in Australia and at least three companies are working on vaccine candidates.
“If we don’t run into any unforeseen obstacles, we’ll be able to get a Phase 1 trial going within the next three months,” said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.
Dr. Fauci cautioned that it could still take months, and even years, after initial trials to conduct extensive testing that can prove a vaccine is safe and effective. In the best case, a vaccine may become available to the public a year from now.
