A woman wears a protective mask near the Chinatown section of New York City on January 23, 2020. Timothy A. Clary | AFP | Getty Images

Infectious disease specialists and scientists say the new coronavirus that's shuttering companies across mainland China may be more contagious than current data shows. Emerging in Wuhan, China, about a month ago, the virus has spread from about 300 people as of Jan. 21 to close to 21,000 and killed more than 420 — with the number of new cases growing by the thousands every day. "The rapid acceleration of cases is of concern," Dr. Mike Ryan, executive director of the World Health Organization's emergencies program, said at a news conference last week before the agency declared a global health emergency.

Chinese scientists worry the respiratory illness, which world health officials say likely came from a fish market, has mutated to adapt to its new human hosts far more quickly than SARS. Data on the virus is changing by the day, and some infectious disease specialists say it will take weeks before they can see just how contagious it is. What they're seeing so far is concerning and leading U.S. and international scientists to believe the virus is more contagious than the current data shows, according to interviews with epidemiologists, scientists and infectious disease specialists. The disease is spreading quickly. China's health minister, Ma Xiaowei, told reporters last month that there is evidence it's already mutated into a stronger variation that is able to spread more easily among humans. World health officials know the respiratory disease is capable of spreading through human-to-human contact, droplets carried through sneezing and coughing, and germs left on inanimate objects. The illness is cable of spreading before symptoms show, and about 20% of patients become severely ill, leading to pneumonia and respiratory failure, health officials say. "[The] continued increase in cases and the evidence of human-to-human transmission outside of China are, of course, most deeply disturbing," WHO Director-General Tedros Adhanom Ghebreyesus said during a news conference at the organization's Geneva headquarters last week. "Although the numbers outside China are still relatively small, they hold the potential for a much larger outbreak."

The 'R naught'

The so-called R naught of the disease, a mathematical equation that shows how many people will get sick from each infected person, is around 2.2, according to a report last week from the New England Journal of Medicine. That means two or more people will catch the virus from a person who already has it, making it more infectious than the seasonal flu and the 1918 Spanish flu pandemic, which had an R naught of about 1.8 and killed at least 50 million people across the world. The current R naught of the new virus is lower than the 2003 SARS outbreak, which had an R naught of between 2 and 5. World health officials caution that it may take months before the true R naught is known as more coronavirus cases come to light. Measles, one of the most contagious viruses in the world, has an R naught around 12 to 18. While the new coronavirus is milder, by comparison, what physicians have seen so far is still concerning, said Yanzhong Huang, a public health researcher at the Council on Foreign Relations and director of the Center for Global Health Studies at Seton Hall University.

"If the R naught is higher than 1, it will spread and it will be contagious," Huang said in a phone interview with CNBC. "Without any containment measures, technically it can spread to the whole population." Additionally, the virus's current transmission number may be underestimated by scientists who currently have very limited data, according to Huang. While the new virus appears to be less lethal than the 2003 SARS outbreak, which sickened 8,098 people and killed almost 800 over nine months, it is spreading significantly faster. It took the new virus less than a month to surpass the number of SARS cases. "A relatively mild virus can cause a lot of damage if a lot of people get it," WHO's Ryan said last week. "And this is the issue at the moment. We don't fully understand it."

Researchers from Lancaster University in England estimated the virus's R naught may be closer to 3.1, saying "current clinical and epidemiological data are insufficient to understand the full extent of the transmission potential of the epidemic" and the outbreak comes "at a time when there is a substantial increase in travel volume" due to the Lunar New Year holiday. Researchers at the University of Hong Kong say more than 75,000 individuals could already be infected with the virus, much higher than the official total. Raymond James analyst Chris Meekins believes infections are likely above 100,000. "Based on the current trajectory of the virus, [the R naught] should be higher than SARS," Huang said.

Country control

It's also possible the R naught estimate could decrease or differ, depending on how well other countries contain local outbreaks, according to Marc Lipsitch, a professor of epidemiology at the Harvard School of Public Health. The U.S. is imposing mandatory quarantines on citizens returning from Wuhan and is denying entry to many others who've recently been in China. Vaccinations are effective as well, though there are currently no proven therapies for the new coronavirus. Health officials in 2003 were able to reduce SARS' infections by keeping patients in isolation, among other measures.

"Most other infections that we know [have a higher R naught] than that but we control them with vaccines so we don't notice that they are so high," said Lipsitch, who added it may take weeks before scientists have a firm grasp on how contagious the new virus truly is. The virus could also begin to lessen in some parts of the world as spring and summer approach and the weather gets warmer. University of Hong Kong researchers estimate the disease will peak in April. However, Lipsitch said there is evidence of transmission before symptoms. If that turns out to be the case, isolating individuals will not be effective in curbing transmission, he said. "It'll be group-level interventions like keeping people away in bulk rather than in individual cases," he said.

Containment efforts