Determining death rates is especially challenging in the midst of a pandemic, while figures are necessarily fluid. Fatality rates based on comparing deaths, which are relatively easy to count, to infections, which are not, almost certainly overestimate the true lethality of the virus, epidemiologists say. Health officials and epidemiologists have estimated there are five to 10 people with undetected infections for every confirmed case in some communities, and at least one estimate suggests there are far more.

On top of that, deaths lag infections. The thousands of people with Covid-19 who died this week in the United States were most likely infected as far back as a month ago. So as the number of new cases reported begins to fall in hard-hit places like New York City, the death rate will almost certainly rise.

“To know the fatality rate you need to know how many people are infected and how many people died from the disease,” said Ali H. Mokdad, a professor of health metrics sciences at the Institute for Health Metrics and Evaluation. “We know how many people are dying, but we don’t know how many people are infected.”

In fact, even the number of people dying is a moving target. Covid-19 deaths that happen at home appear to be widely underreported. And New York City increased its death count by more than 3,700 on Tuesday, after officials said they were now including people who had never tested positive for the virus but were presumed to have died of it.

But the missing data on deaths in the deaths-to-infections ratio is still almost certain to be dwarfed by the expected increase in the denominator when the total number of infections is better understood, epidemiologists say. The statistic typically cited by mayors and governors at Covid-19 news conferences relies on a data set that includes mostly people whose symptoms were severe enough to be tested.

Epidemiologists call it “severity bias.” It is why the fatality rate in Wuhan, China, where the outbreak began, was reported to be between 2 percent and 3.4 percent before it was revised to 1.4 percent, and it may yet be lower.

One intriguing case study for epidemiologists looking for the true fatality rate is the Diamond Princess cruise ship, which became a kind of natural experiment when nearly all of its 3,711 passengers and crew members were tested for the coronavirus after an outbreak on board.