Here’s the latest on the novel coronavirus outbreak in Wuhan

Five travel-related cases have been confirmed in the United States, according to officials at the Centers for Disease Control and Prevention. Arizona, Washington, and Illinois have each reported one case, and California has reported two cases. All of the cases had connections to Wuhan, the capital city of the central Hubei province, where the outbreak erupted.

The CDC said in a press conference Monday, January 27, that there are 105 other people in 26 states who have been identified as having possible travel exposure to the virus. Of those 105, 32 have already tested negative for the virus. Results for the remaining 73 people are pending.

At this time, the public health risk from this virus to the American public is low, Dr. Nancy Messonnier told reporters Monday. Dr. Messonnier is the director of the CDC’s National Center for Immunization and Respiratory Diseases.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health, put things more bluntly. In a radio interview Sunday, he said:

“The American people should not be worried or frightened by this. It’s a very, very low risk to the United States.”

What we know about the virus so far

The virus—which is being referred to as the 2019 novel coronavirus, or 2019-nCoV—is a member of the large coronavirus family. Coronaviruses, named for the crown-like halo of spikes on their outer surfaces, cause a wide range of respiratory and gastrointestinal infections in humans and animals. Several strains of coronavirus already circulate in people, causing common, mild-to-moderate upper-respiratory-tract infections (common colds). Other family members largely circulate in animals, such as bats.

In several instances, coronaviruses have jumped from animal species to humans, which can occur when wild animals and people are in close, unhygienic conditions. Species jumps can result in severe disease in people. Such is the case for the coronaviruses that cause SARS (Severe Acute Respiratory Syndrome)—which was linked to viruses in bats, masked palm civets, and raccoon dogs—and MERS (Middle East Respiratory Syndrome)—which was linked to viruses in bats and dromedary camels.

Like SARS and MERS, 2019-nCoV is suspected to have jumped from animals to people in Wuhan. The outbreak was initially linked to a live animal market in the city, which contained a range of animals, including chickens, bats, marmots, snakes, and other wild animals. A majority of the early cases has direct exposure or a link to the market. In fact, early reports found that many of the cases were in people who worked at the market.

However, a new report published in The Lancet describing 41 early cases in the outbreak indicate that the earliest identified person sickened had no epidemiological links to the market. The case was in a man who had a laboratory-confirmed 2019-nCoV infection with onset of symptoms on December 1, 2019. None of the man’s family became ill and, intriguingly, he had no identifiable ties to any of the other cases in the outbreak. The significance of this and the ultimate source of the outbreak remain unknown.

Investigations into the origins of SARS and MERS have determined that bats seem to be a source (aka a reservoir) for different coronaviruses. But again, no animal source has yet been confirmed for this new coronavirus strain. (There have been reports claiming that the virus came from snakes, but experts are skeptical of the suggestion.)

What we know about the infections so far

In people, the virus appears to be able to cause asymptomatic cases, as well as illnesses that range from mild respiratory infections to life-threatening pneumonia and respiratory distress, according to preliminary clinical data.

The incubation period—the time between an exposure and the onset of symptoms—appears to be between two and 14 days. Common symptoms are fever, cough, chest tightness, and trouble breathing.

The mortality rate is estimated to be around 3 percent, and the rate of severe illness has hovered around 20 percent. But both of those calculations may easily change as the outbreak continues, more testing is performed, and any additional mild cases are detected.

The majority of cases so far are in older adults, mainly men and many with underlying health conditions that make them more susceptible to disease. But there are reports of cases in healthy people, as well as children and infants. The youngest confirmed case has been reported to be in a 9-month-old girl in Beijing.

There have also been reports of asymptomatic cases in several places. For instance, in a second case report published in The Lancet, researchers identified an asymptomatic case in a 10-year-old boy. The case was discovered in a cluster of infections in a family. The boy’s asymptomatic case was only caught at his parent’s insistence that he be examined and tested. Both of the boy’s parents and three of his grandparents had contracted the virus after the family traveled to Wuhan and visited other sick relatives in a local hospital.

The discovery of asymptomatic cases has led to concerns that infected people with no symptoms may be unknowingly spreading the virus and thwarting outbreak control efforts, such as quarantines. However, it is unclear if this is happening, and experts suspect that, if it is, it may only be a limited source of new infections.

What we know about transmission so far

So far, experts suspect that 2019-nCoV mainly spreads through respiratory droplets—sprayed from things like coughs and sneezes—that can then enter the nose, mouth, or eyes of an uninfected individual to cause an infection. It’s still unclear when during an incubation period an infection a person is contagious, though.

The virus appears to mainly be spreading among people who have close contact with each other, such as between family members and from patients to medical staff, and not, say, strangers passing in an airport.

This is a positive sign in terms of outbreak control, as the NIH’s Anthony Fauci described:

It does not seem to be as efficient in the persistent sustained transmission from human to human… without a doubt it can spread from one human to another. What it doesn’t seem yet to be doing as efficiently—certainly not like influenza, which spreads very efficiently in a sustained way—this does not do so. Which means that, just like SARS, we have the possibility—with good public health measures—of hopefully getting control of it.

Can face masks protect someone from being infected?

Not completely. Surgical masks, which many people are using to try to protect themselves, may help to keep an uninfected person from inhaling infectious droplets and may help to keep an infected person from spreading the infection. But masks are far from foolproof. The virus can still enter the nose, eyes, or openings around the mask, particularly if it is not on properly or tightly.

The R₀ question

There’s been a lot of discussion and speculation online about the 2019-nCoV outbreak’s R₀ (pronounced R-naught), the so-called basic reproduction number of an infectious agent. An R₀ is a complex calculation meant to represent the average number of new infections generated by any one infected person in a completely susceptible population. Generally, an R₀ of one or less indicates that an outbreak will peter out; values greater than one suggest that the outbreak will continue. These numeric values are sometimes used as a shorthand for an infectious agent’s infectiousness, but they are often misrepresented and misinterpreted.

For one thing, it’s important to keep in mind that R₀ is not an intrinsic property of an infectious agent. It’s a descriptive calculation of an outbreak that can change with changing circumstances and control measures, such as viral mutations or effective quarantines.

There are already many different, preliminary estimates of 2019-nCoV outbreak’s R₀. The World Health Organization has reported a preliminary estimate of R₀ as 1.4 to 2.5. For a point of reference, the highly infectious measles virus has an R₀ of 12 to 18.

The R₀ estimates are extremely preliminary and likely to change continually during the outbreak and as new control measures are put into place.