A deadly, drug-resistant fungus emerging in the US gained ground faster and picked up yet more drug resistance amid the COVID-19 pandemic, researchers at the Centers for Disease Control and Prevention reported Monday.

The yeast Candida auris has been considered an "urgent threat"—the CDC's highest level of concern—since it was first reported in the US in 2016. The yeast lurks in health care settings and preys upon vulnerable patients, causing invasive infections with a fatality rate of between 30 to 60 percent.

In 2019, before the pandemic began, 17 states and Washington, DC, reported a total of 476 clinical cases. But in 2020, eight additional states reported cases for the first time, with the national clinical case count jumping 59 percent to 756. In 2021, 28 states were affected, with the clinical case count nearly doubling to 1,471. Asymptomatic cases detected through patient screening also jumped amid the pandemic, tripling from 1,310 cases in 2020 to 4,041 cases in 2021. The data appeared Monday in the Annals of Internal Medicine.

“The rapid rise and geographic spread of cases is concerning and emphasizes the need for continued surveillance, expanded lab capacity, quicker diagnostic tests, and adherence to proven infection prevention and control,” Meghan Lyman, a CDC epidemiologist and lead author of the study, said in a statement.

But its spread wasn't the only alarming aspect of the yeast's pandemic activities. It also became more drug-resistant. Before the pandemic, six patients developed infections resistant to first-line antifungal drugs, echinocandins. But, in 2021 alone, there were 19 such cases. Similarly, before the pandemic, there were only four reports of pan-resistant infections; that is, the fungus was resistant to all available drugs. In 2021, there were seven patients with pan-resistant infections.

Before the pandemic, cases of echinocandins-resistance and pan-resistance appeared to develop independently in patients amid ongoing treatment—in other words, they were isolated cases that didn't come about by human-to-human transmission. But in the 2021 cases, there was evidence of two outbreaks of echinocandins-resistant and pan-resistant strains with human-to-human transmission.

"The timing of this increased C. auris spread and findings from public health investigations suggest it may have been exacerbated by pandemic-related strain on the health care and public health systems," Lyman and colleagues concluded in the study.

The CDC researchers suspect that shortages of staff and personal protective equipment may have contributed to the problem, as well as changes in patient movements and increased use of antimicrobial drugs. They also speculated that while specific infection control measures tightened during the pandemic, others may have fallen by the wayside, such as environmental disinfection, which could have exacerbated the spread. They called for renewed efforts to clamp down on transmission. "C. auris remains an ongoing health threat in the United States," they wrote. The alarming uptick during the pandemic "provides motivation to refocus on public health fundamentals to prevent illness and save lives."