"Case fatality rates have been very confusing," says Dr. Steven Lawrence, an infectious disease expert and associate professor of medicine at Washington University School of Medicine in St. Louis. "The numbers may look different even if the actual situation is the same."
So it's likely that the seemingly stark difference between Germany and Italy is misleading and will diminish as scientists get more data, Lawrence says.
But a much more likely scenario, he says, is that early in an outbreak, testing is limited to people who are so sick they wind up in the hospital. That means the only infections that get counted are in the people most likely to die. So the denominator is missing a huge number of infected people who survive, and that makes the virus appear much more deadly than it really is.