There is new evidence that COVID-19 can have lasting effects on heart health, which may go undetected in patients who assume they have recovered from the infection.
Two studies from Germany, published Tuesday in the journal JAMACardiology, show how the virus can linger in the heart for months, even without producing symptoms.
The first study included 100 coronavirus patients from the University Hospital Frankfurt COVID-19 Registry. Most were otherwise healthy adults in their 40s and 50s.
All had MRIs of their heart two to three months after they were diagnosed with the virus, when many seemed to have fully recovered. Those images were compared to people who'd never had COVID-19.
Out of those 100 COVID-19 patients, 78 still had visual signs that the virus had an impact on the heart. Sixty of those patients had signs of ongoing inflammation of the heart muscle.
"That's really compelling," Dr. Clyde Yancy, chief of cardiology in the department of medicine at Northwestern Medicine in Chicago, told NBC News. "It indicates that months after exposure to COVID-19, we can still detect evidence of a heart that's not completely normal."
The problem may not lead to physical symptoms, but could indicate risk for further heart damage.
"Once the heart muscle has been injured, there is the potential for progressive injury," Yancy wrote in an editorial accompanying the studies.
Because the virus is so new, it's not yet known what long-term cardiovascular risks come with COVID-19.
No pre-existing conditions would have explained the damage, the study authors said, and only a third had been hospitalized with COVID-19. The rest were able to remain at home throughout the course of their illness.
"Our findings may provide an indication of potentially considerable burden of inflammatory disease in large and growing parts of the population," the study authors wrote.
The second study included 39 autopsies of people who'd died of COVID-19. Those patients tended to be older, in their 80s. Researchers found evidence of the virus in the heart tissue in 24 of the 39 patients.
What's more, five of those patients had signs the virus was actually replicating in the heart tissue.
"We know the virus is making its way into the heart muscle, and seems to cause an inflammatory response that we don't fully understand yet," Dr. Matthew Belford, an interventional cardiologist at Wake Forest Baptist Health in Winston-Salem, North Carolina, said. Belford was not involved in either study.
Doctors have known for some time that COVID-19 can wreak havoc on the circulatory system, potentially leading to dangerous blood clots. A study published in the journal Cell in March showed that the virus infiltrates the body by binding to a type of receptor on cells called ACE2.
ACE2 receptors are often found in endothelial cells, located in the lining of the heart and blood vessels. One of the main jobs of the cells is to help control blood clotting and platelets.
It remains unclear how long such damage will persist.
The research "gets my attention," Yancy said.
"It makes me say, 'We are not done yet,'" he said. "We must respect COVID-19."
"There is still much that we don't know," he added. "We have to stay on guard."