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The U.S. may have underreported the number of people who died from the coronavirus pandemic by as many as 15,000 people as late as April 4, a newly published study suggests.

The research, which has not yet been peer-reviewed and is available to read on the Medrxiv repository, notes there was a "substantial variability" in the deaths reported because of COVID-19 and the "estimated burden of excess deaths due to [pneumonia and influenza]."

"Excess P&I deaths provide a conservative estimate of COVID-19 burden and indicate that COVID-19-related deaths are missed in locations with inadequate testing or intense pandemic activity," the researchers wrote.

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According to an investigation from the Washington Post and Yale University, there were 15,4000 "excess deaths" from March 1 to April 4. By that date, just 8,128 deaths were linked to COVID-19.

As of Tuesday morning, there were more than 211,000 deaths linked to COVID-19 around the world. Of those, more than 56,000 have occurred in the U.S.

Certain states were specifically mentioned, including New Jersey and New York (largely driven by New York City), as well as Georgia, Illinois and Washington for having a "notable increase in the proportion of total deaths due to pneumonia and influenza" than what would be expected based off the time of year and influenza activity.

"The increase in all-cause deaths in New York and New Jersey is 1.5-3 times higher than the official tally of COVID-19 confirmed deaths or the estimated excess death due to [pneumonia and influenza]," the researchers added.

"In some states, such as Florida and Georgia, the increase in deaths due to P&I preceded the widespread adoption of testing for the novel coronavirus by several weeks," the researchers explained. "As a result, the increase in P&I preceded the first reported COVID-19 deaths, and the excess P&I was greater than the number of reported COVID-19 deaths each week. In contrast, in Washington state, there was concordance between reported COVID-19 deaths, excess P&I deaths, and the increase in testing."

States and regions that have not yet seen a jump in deaths, such as Ohio and Texas, could start to see a rise in the "coming weeks" as P&I mortality "typically lags behind the increase in [influenza-like illness] visits, the researchers warned.

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The researchers acknowledged that "excess [pneumonia and influenza] deaths provide a conservative estimate of COVID-19 burden and indicate that COVID-19-related deaths are missed in locations with inadequate testing or intense pandemic activity."

The potential shortfall in reported deaths from COVID-19 may not just be an issue with U.S. data, but rather a global concern.

The Financial Times performed a similar experiment using global data and came to a similar conclusion, noting deaths could be 60 percent higher across 14 countries, including Italy, Spain and England.

Some of these deaths may be the result of causes other than COVID-19, as people avoid hospitals for other ailments," the FT wrote in its analysis. "But excess mortality has risen most steeply in places suffering the worst COVID-19 outbreaks, suggesting most of these deaths are directly related to the virus rather than simply side-effects of lockdown."

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There were 1,378 deaths from coronavirus on Monday, according to data compiled by Johns Hopkins University. The four-day average stood at 1,576, the lowest since the period between April 5 and April 8.

Nearly 60 percent of all coronavirus-linked deaths in the U.S. have occurred since April 14 and 9 of 10 deaths in the U.S. have occurred in April, according to Johns Hopkins.

As of Tuesday morning, more than 3 million coronavirus cases have been diagnosed worldwide, more than 988,000 of which are in the U.S., the most impacted country on the planet.

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