The coronavirus that causes COVID-19 can spread to the heart and brain within days of infection and can survive for months in organs, according to a new study by the National Institutes of Health.

The virus can spread to almost every organ system in the ody, which could contribute to the ongoing symptoms seen in "long COVID" patients, the study authors wrote. The study is considered one of the most comprehensive reviews of how the virus replicates in human cells and persists in the human body. It is under review for publication in the journal Nature.

"This is remarkably important work," Ziyad Al-Aly, MD, director of the Clinical Epidemiology Center at the Veterans Affairs St. Louis Health Care System in Missouri, told Bloomberg News. Al-Aly wasn't involved with this study but has researched the long-term effects of COVID-19.

"For a long time now, we have been scratching our heads and asking why long COVID seems to affect so many organ systems," he said. "This paper sheds some light and may help explain why long COVID can occur even in people who had mild or asymptomatic acute disease."

The NIH researchers sampled and analyzed tissues from autopsies on 44 patients who died after contracting the coronavirus during the first year of the pandemic. They found persistent virus particles in multiple parts of the body, including the heart and brain, for as long as 230 days after symptoms began. This could represent infection with defective virus particles, they said, which has also been seen in persistent infections among measles patients.

"We don't yet know what burden of chronic illness will result in years to come," Raina MacIntyre, PhD, a professor of global biosecurity at the University of New South Wales, told Bloomberg News.

"Will we see young-onset cardiac failure in survivors or early-onset dementia?" she said. "These are unanswered questions which call for a precautionary public health approach to mitigation of the spread of this virus."

Unlike other COVID-19 autopsy research, the NIH team had a more comprehensive post-mortem tissue collection process, which typically occurred within a day of the patient's death, Bloomberg News reported. The researchers also used a variety of ways to preserve tissue to figure out viral levels. They were able to grow the virus collected from several tissues, including the heart, lungs, small intestine, and adrenal glands.

"Our results collectively show that while the highest burden of SARS-CoV-2 is in the airways and lung, the virus can disseminate early during infection and infect cells throughout the entire body, including widely throughout the brain," the study authors wrote.