Workers in protective suits prepare to disinfect a tram due to coronavirus disease (COVID-19) concerns in Prague, Czech Republic, March 12, 2020. REUTERS/David W Cerny

(Reuters) - Italy and South Korea, two countries outside China that have been hard hit by the new coronavirus, have reported dramatically different numbers of deaths as a proportion of cases. Why are these not comparable?

Calculating mortality rates during a disease epidemic is fraught with pitfalls.

This is in part because the numbers of deaths and patients constantly change. In part it’s because not every single case will be detected - some will be so mild they never get noticed. And in part it’s because there can be a significant time lag between when someone is infected and when or if they die.

“What you can safely say is that if you divide the number of reported deaths by the number of reported cases, you will almost certainly get the wrong answer,” said John Edmunds, a professor at the center for mathematical modeling of infectious diseases at the London School of Hygiene & Tropical Medicine.

That’s why World Health Organization (WHO) officials - who said last week that 3.4% of the people worldwide confirmed as having been infected with the new coronavirus had died - were careful not to describe that as a mortality rate or death rate.

“In an unfolding epidemic, it can be misleading to look at the naïve estimate of deaths so far divided by cases so far,” said Christl Donnelly, a disease specialist at Oxford University and Imperial College London. “This is due to the delay from the time it takes for individuals to progress from being diagnosed as cases to dying.”

With COVID-19 - the infection caused by the new coronavirus - the time between onset of disease and death is fairly significant, at around two to three weeks or more.

So the sum involved would need to compare the number of deaths at a given point with the actual case numbers from some weeks beforehand.