The reproduction number—R0 or “r naught”—simply refers to the number of additional people that an infected person typically makes sick. Maia Majumder, ...Boston Children’s Hospital and Harvard Medical School, has been tracking exactly that.

Her preliminary results indicate a transmissibility rate for the new coronavirus ranging from 2.0 to 3.1 people. That’s higher than influenza—1.3 to 1.8—but similar to SARS, which has a basic reproduction number in the 2 to 4 range. So, coronaviruses are slightly more prone to spreading between people.

The case-fatality ratio—or death-to-case ratio—is the number of people killed by disease divided by the number of people who catch it. Seasonal influenza, despite being considered a global scourge, technically kills a relatively small proportion of its cases, with a case-fatality ratio around 0.1 percent. The reason the flu is an annual public health emergency is because it infects boatloads of people—35.5 million in the U.S. across 2018 and 2019, which led to 490,000 hospitalizations and 34,200 deaths. That’s why health officials perpetually recommend that people receive a flu shot.

The case-fatality ratio also helps explain why public health agencies send up alerts over emerging outbreaks of coronaviruses. SARS had a case-fatality rate of 10 percent, about 100 times higher than influenza, and the rate for the new coronavirus is currently near 3 percent, which is on par with the 1918 Spanish influenza pandemic.