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 new coronavirus could infect somewhere between 1.5 and 3.5 people without effective containment measures.
That would make the 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 HIV or hepatitis, which spread
only through direct contact with the bodily fluids of an infected person. If each person infected with the new coronavirus infects two to three others, that may be enough to sustain and accelerate an outbreak if nothing is done to reduce it.
Compare that with a less contagious virus, like the seasonal flu. People with the flu tend to infect 1.3 other individuals, on average.
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.
The time it takes for symptoms to appear after a person is infected can be vital for prevention and control. Known as the incubation period, this time can allow health officials to quarantine or observe people who may have been exposed to the virus. But if the incubation period is too long or too short, these measures may be difficult to implement.
As the coronavirus outbreak continues to spread across China, a flurry of early research is drawing a clearer picture of how the pathogen behaves and the key factors that will determine whether it can be contained.
As the coronavirus outbreak continues to spread across China, a flurry of early research is drawing a clearer picture of how the pathogen behaves and the key factors that will determine whether it can be contained.
Some illnesses, like influenza, have a short incubation period of two or three days. SARS, however, had an incubation period of about five days. In addition, it took four or five days after symptoms started before sick people could transmit the virus. That gave officials time to stop the virus and effectively contain the outbreak, said Dr. Allison McGeer, an infectious disease specialist at Mount Sinai Hospital in Toronto, who was at the front lines of the Canadian response to SARS.
Officials at the Centers for Disease Control and Prevention estimate that the new coronavirus has an incubation period of 2 to 14 days. But it is still not clear whether a person can spread the virus before symptoms develop or whether the severity of the illness affects how easily a patient can spread the virus.
“That concerns me because it means the infection could elude detection,” said Dr. Mark Denison, an infectious disease expert at Vanderbilt University in Nashville, Tennessee.
This is one of the most important factors in how damaging the outbreak will be, and one of the least understood.
