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Many countries' coronavirus curves are flattening, at least for now.

Yet somehow, the global case-fatality rate has increased significantly since March, when it was around 3.4%. The rate was 5.8% on Tuesday, according to tallies from the World Health Organization, and it hovered around 7% from mid-April through May.

But it seems testing has not increased enough to result in a significant downward trajectory.

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Some epidemiologists say that because death rates are so heavily influenced by testing and delays in reported cases and deaths, they're simply not a reliable measure of the virus's toll over time.

In general, the more cases that are included in the data including people with mild or no symptoms the lower the death rate.

In that sense, case-fatality rates "are more a measure of how much testing and case finding you do," John Edmunds, a professor of infectious-disease modeling at the London School of Hygiene & Tropical Medicine, told Business Insider.

South Korea, which has tested more than 1 million people, far outpaced other countries in early case detection and contact tracing. While US labs waited for weeks for instructions on how to fix faulty test kits in February, South Korea was testing tens of thousands of people.

Limited testing in other countries, like Sweden and the US, make their case counts inaccurately low. In the US, experts think we need to multiply the official confirmed case count by 10 to get an accurate estimate of true infections nationwide.

When countries miss many mild cases, deadly cases seem like a higher proportion of infections than they really are. Countries like the US and Sweden, therefore which have case-fatality rates of 5.7% and 10.3%, respectively could be inflating the global death rate.

"Mortality will spuriously spike when cases are decreasing," William Hanage, an epidemiologist at Harvard's TH Chan School of Public Health, told Business Insider. "The reason is that you are seeing the deaths from when the epidemic was expanding, or at least more intense than it is now, while your total case counts are up to the present."

The fact that death tolls lag behind case counts can briefly create a very high death rate. That's what seems to have happened in April and May. (And limited testing can magnify that effect.)

That math suggests the virus has killed roughly 1% of the people who tested positive four weeks ago. But again, since that doesn't include many people with mild or asymptomatic cases, the true proportion of people who die after being infected is probably much lower.

That's far lower than what case-fatality rates suggest, but it's still 13 times higher than the death rate of the seasonal flu.

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Scientists predicted that the coronavirus death rate would fall over time, but instead it doubled. Here's why - Business Insider India