Thousands of medical workers in the U.S. have been sickened by the coronavirus – but the official numbers fall short of the virus' true toll, according to a new report from the Centers for Disease Control and Prevention.

The recent surge of COVID-19 cases and deaths in parts of the U.S., paired with a dire shortage of medical supplies such as masks, have prompted concerns that the country's physicians, nurses and other health care workers are being left unprotected and even more vulnerable to infection as they battle the virus on the frontlines. And as the coronavirus spreads in more communities across the U.S., the number of health care workers who fall ill is "expected to rise," the new report says.

As of April 9, at least 9,282 health care workers in the U.S. had COVID-19 and at least 27 had died, according to the new report. However, that number is almost certainly an undercount because researchers only knew whether a patient was a health care worker in 16% of all U.S. coronavirus cases – which had reached nearly 460,000 by April 9 – leaving 84% of cases unaccounted for in this report.

"We know that health care workers are not only getting infected, but becoming critically ill and dying from COVID-19," says Dr. Alison Haddock, an assistant professor at the Baylor College of Medicine and an emergency physician in Houston. "Tracking it is the only way that we're going to be able to prevent it in the future."

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While incomplete, the new figures offer the clearest picture yet on how many health care workers in the U.S. have been infected. Two weeks ago, U.S. News contacted all 50 state health departments or coronavirus response teams to ask how many health care workers had COVID-19. Many were not tracking that data, and others weren't publicly reporting it. The CDC also did not respond to a U.S. News request for data.

The new report shows that among infected health care workers with data available, 73% were women and the median age was 42, "reflecting these distributions among the (health care personnel) workforce." About 72% of these health care workers were white and 21% were black.

More than half of these workers had only been exposed to the virus in a health care setting, but it's unclear whether those infections were caused by contact with a patient, visitor or other health care personnel, or whether these health care workers were wearing personal protective equipment, the report says.

"The PPE shortages are very challenging – we're reusing things that we've never had to reuse before across the country," Haddock says. "And so I think it's important to know if that's putting us at higher risk or not."

The majority of infected workers showed common symptoms like fever, cough or shortness of breath, though the report notes that health care workers with "mild or asymptomatic infections might also have been less likely to be tested, thus less likely to be reported."

While most of the infected health care workers were not hospitalized, 184 were admitted to an intensive care unit and 27 died. Just 6% of the patients were 65 or older, but 10 of the deaths were among these older adults and 38% of those infected had an underlying medical issue, the report found, tracking with other data that shows the virus is most dangerous for older and sicker people.