As the US passes 1 million fatalities from COVID-19, a new analysis from the World Health Organization reveals how dramatically governments have undercounted deaths related to the pandemic.

The agency announced on May 5 that the pandemic claimed approximately 14.9 million lives in its first two years—more than twice the number of confirmed deaths reported by countries around the world. The WHO estimate includes people who died of COVID-19 as well as deaths that the novel coronavirus caused indirectly. 

Data on COVID-19 fatalities is tricky to make sense of, both in the US and globally. However, accounting for these deaths is important to gauge the full impact of the pandemic, experts say. 

“Understanding mortality is so critical,” says Albert Ko, an infectious disease physician and epidemiologist at the Yale School of Public Health. “Having these numbers will be important for decision makers to know how many lives could be saved in the future.” 

Analyzing mortality data reveals which groups are most at risk from COVID-19 due to their age, health status, race, or other characteristics. This information can help policymakers and public health organizations determine how best to protect vulnerable populations going forward. Mortality rates in different states or countries can also reveal the ability of control measures or vaccination campaigns to avert deaths.

There are several ways to measure the pandemic’s death toll. One is the number of deaths reported to national organizations such as the Centers for Disease Control and Prevention. This approach has a few drawbacks. Deaths tend to be undercounted in areas where COVID-19 tests aren’t widely available or health infrastructure is limited. 

“Those places that have the weaker surveillance or reporting systems are likely also the places that may have the highest death rates,” Ko says. And it’s often difficult to figure out “who actually died of COVID, versus those people who may have died of cancer or heart disease or trauma but were infected at the same time,” he adds.

Some governments are also more forthcoming with COVID-19 mortality data than others. Aleksei Raksha, a demographer who quit Russia’s state statistics service, recently told the New York Times that COVID-19 was frequently omitted as the primary cause of death in official reports.

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Another challenge is that the criteria used to define COVID-19 deaths isn’t necessarily consistent from one location to the next, says Justin Lessler, an epidemiologist at the University of North Carolina Gillings School of Global Public Health. It can even change over time; Massachusetts initially counted all deaths that occured after a positive test as COVID-related, but has since narrowed its criteria

“That presents challenges in the sense that you’re not making apples-to-apples comparisons when you’re looking across various jurisdictions and areas,” Lessler says.

Unlike Britain, which can rely on its National Health Service, the US and other countries that lack centralized healthcare systems face additional difficulties. “The fragmentation of information has certainly been a challenge in the United States,” Ko says. 

In such patchwork systems, death counts can jump quite a bit from one week to the next as some jurisdictions lag behind others in reporting data. “You can generally assume if there’s a two- or threefold increase in how many deaths are reported on a given day that it’s probably not the result of a lot of people dying then,” Lessler says. “It probably has something to do with reporting.” 

A more reliable way to assess COVID-19 deaths is a metric called “excess mortality.” Based on records from past years, researchers calculate how many people would be expected to die in a given time and place. The difference between the total number of deaths that occurred and this estimate is the excess mortality. Scientists often use this approach to understand the full impact of crises, including diseases and disasters such as Hurricane Maria.

In its new report, the WHO used excess mortality to conclude that nearly 15 million people died worldwide because of the pandemic in 2020 and 2021. Another excess mortality estimate published in March by an international team of researchers in the Lancet put the number of deaths even higher—at 18.2 million. 

Excess mortality doesn’t just capture fatal COVID-19 cases. It also includes deaths that occurred because hospitals became overwhelmed, or because people were afraid to seek treatment for other issues or schedule routine screenings. Furthermore, the estimates account for deaths that COVID-19 prevented. The early pandemic’s containment measures resulted in fewer traffic accidents and decreased the transmission of other diseases such as influenza. 

The WHO found that 84 percent of the fatalities came from Southeast Asia, Europe, and the Americas, with the US among the top five countries with the most excess deaths. For places with sparse mortality data, the agency used other measures such as the country’s COVID-19 containment efforts and population demographics to estimate excess deaths.  

Researchers will continue to fine-tune their estimates of the pandemic’s death toll. A century after the 1918 flu pandemic, Lessler notes, scientists are still attempting to unravel the mystery of how many people actually died. “We have much better data and much better information to go on now, and there aren’t going to be the same massive uncertainties for this pandemic as for that one,” he says. “[But] I think there will be debates and accountings and attempts to understand it for years to come.” 

However, Lessler and Ko predict, the message conveyed by the excess mortality estimates is unlikely to change.

“The bottom line is the number of people who died because of either direct or indirect effects of COVID is much larger than what was counted by reported deaths,” Ko says.