MRI scans suggest that mild SARS-CoV-2 infections are unlikely to cause lasting damage to the heart, researchers report in a new UK study.

An analysis of data from 74 healthcare workers who had tested positive for COVID-19 but experienced mild or no symptoms revealed that six months after infection, these patients had no differences in heart structure and function from 75 seronegative controls, according to the report published in JACC: Cardiovascular Imaging.

The findings show “that if you are a healthy adult of working age and have had mild, non-hospitalized COVID, cardiac effects six months later are rare,” said Dr. James Moon, a professor at University College London and Barts Heart Center in London.

“COVID-19 often scars the heart if you are hospitalized with (the virus), particularly if you develop blood troponin elevation, a heart damage marker,” Dr. Moon said in an email. “This is not a study of those patients. This is a mild disease study with a crucial design aspect: we recruited our exposed healthcare workers in March 2020. Subsequently around 20 percent of them developed COVID and we looked at these. This removes a big bias from the study.”

The study doesn’t prove that mild cases won’t cause heart damage, Dr. Moon noted.

“We found no lingering cardiac damage,” he said. “That is not to say some people cannot get heart damage, but the study constrains the maximum rate of that in the community to 4 percent, which is very reassuring. However, is it a) ‘workforce representative’—not the elderly, not those with high co-morbidity, and b) 4 percent means that there may be thousands of subjects with long COVID—but from a population perspective, after mild disease, this is very reassuring.”

To take a closer look at the effect of SARS-CoV-2 infection on the heart when the disease is mild, Dr. Moon and his colleagues turned to the COVIDsortium study, a prospective study of 731 healthcare workers from three London hospitals starting early in the first COVID-19 wave. Participants underwent weekly PCR and serology testing over 16 weeks with symptom ascertainment.

Comorbidities were low in the cohort—18 percent were smokers, 13 percent had BMI greater than 30 kg/m2, 11 percent had asthma, 7 percent had hypertension, 2 percent had diabetes, 1 percent had rheumatological disease, and 1 percent had cancer.

Overall, 21 percent tested positive for SARS-CoV-2 and the disease was mild in 99 percent, with 25 percent symptomatic and only two were hospitalized (for two days). There were no deaths. Those in the larger study were invited to participate in a cardiovascular nested substudy. In total, 74 seropositive participants signed on.

The researchers collected blood samples to test for high-sensitivity troponin percent and N-terminal pro-B-type natriuretic peptide and hematocrit. Cardiac MRI scans were performed to evaluate heart structure and function.

The 74 seropositive participants in the substudy and 75 age-, sex- and ethnicity-matched seronegative controls had an average age of 37. There were no differences between the seropositive and seronegative groups in cardiac structure (left ventricular volumes, mass and atrial area), in function (ejection fraction, global longitudinal shortening and aortic distensibility), tissue characterization, or biomarkers.

The new study is “definitely interesting,” said Dr. Saman Setareh, a senior cardiovascular fellow at Mount Sinai Heart in New York City. “Even though this is a very promising study and the authors should be congratulated on it, we have to be cautious (in interpreting it),” Dr. Setareh said. “It’s a small study with relatively young participants who had no comorbidities.”

“There is a lot that is unknown, and we are constantly learning more and more about COVID-19 long-term effects,” he added. “At Mount Sinai Heart, early on, a lot of young athletic patients who had just mild COVID-19 were barely able to exercise anymore. Part of that could be deconditioning.”

Dr. Setareh points to a recent study involving NBA players and staff. “In that study, 6 percent had measurable cardiovascular effects,” he said. “We have to look at patients more long-term. This study suggests that the likelihood of long-term effects on young patients with no comorbidities is low.”