WASHINGTON (AP) — U.S. health officials offered a reality check Tuesday about the scary new virus from China: They’re expanding screenings of international travelers and taking other precautions but for now, they insist the risk to Americans is very low.

“At this point Americans should not worry for their own safety,” Health and Human Services Secretary Alex Azar told reporters Tuesday.

China has confirmed more than 4,500 people with the respiratory illness, which in severe cases can cause pneumonia, with dozens more counted in other countries. In the U.S. so far, there are five confirmed patients, all of whom had traveled to the hardest-hit part of China — and no sign that they have spread the illness to anyone around them.

Still, “this is a very fast-moving, constantly changing situation,” Azar added.

Flanked by the government’s top infection specialists, Azar listed the biggest unanswered questions of the outbreak and tried to tamp down some of the fear and speculation provoked by China’s rising toll:

® How deadly is this new virus? China’s death toll has passed 100 but the first patients counted in an outbreak “are naturally the most severe cases” and “skew our understanding,” Azar cautioned. Over time, if doctors find many more people had just a mild, cold-like illness, the death rate will change.

® How easily does it spread? One way to measure that is an estimate of how many people could catch an infection from one contagious patient. Some reports have suggested that number might be between 1.5 and 3.5 for the new coronavirus, but Azar stressed it’s too soon to know. For comparison, one patient with measles could spread it to 12 to 18 others, he added.

® What about silent carriers? Reports from China suggest some people may have spread the virus before showing symptoms. And Germany on Tuesday said a man with the virus near Munich never traveled to China or had close contact with anyone showing symptoms. Instead, he may have been infected by a coworker from China who briefly visited for a company training session and didn’t report feeling ill until her flight home. Later authorities confirmed three additional cases from the German company, all connected to the first.

Some viruses, such as the flu, can spread before symptoms are obvious. But there’s no evidence it’s happened with the new virus in the U.S., where health officials are checking contacts of the sick. And epidemics are driven by the openly sick, said Dr. Anthony Fauci, infectious disease chief of the National Institutes of Health.