Severe COVID-19 linked to reactivation of some dormant viruses: Study

Severe COVID-19 may reactivate some dormant viruses that remain in the body after prior infections, a new study suggests.

Researchers from Boston Children's Hospital and collaborators from more than 20 biomedical research hospitals across the U.S. and in Ontario, Canada, followed 1,154 hospitalized COVID-19 patients and analyzed more than 200,000 biological samples collected over one year.

The team detected 11 different dormant viruses that reactivated within the first 40 days after hospital admission for SARS-CoV-2, the virus that causes COVID-19, according to the study, published in the journal Nature.

Reactivated viruses included several herpes viruses -- including Epstein-Barr virus, cytomegalovirus and herpes simplex virus-1 and Anelloviridae, a family of viruses that do not cause any proven human disease.

These viruses commonly remain inactive in healthy individuals but can reactivate during periods of illness or stress.

Reactivation of Anelloviridae, a virus carried by most adults, was associated with long-term physical disability and long COVID, a condition that occurs when patients still have symptoms at least four weeks after initial infection, the study found.

Researchers also found that Epstein-Barr virus and cytomegalovirus appeared to reactivate in response to inflammation rather than immune suppression, suggesting a previously unrecognized mechanism of viral reactivation during severe illness.

Dr. Ofer Levy, study co-author and director of the Precision Vaccines Program at Boston Children's Hospital, said that he was surprised by the study's findings.

"It was not previously known to this degree of detail and scale how viral reactivation (i.e. the "waking up" and replication of non-SARS-CoV-2 viruses that live in our body -- such as herpes viruses) might associate with COVID severity and outcome," he told ABC News in a statement.

The findings could help researchers better understand long COVID and support the development of future diagnostic tests and treatments.

Because the study only included hospitalized patients with severe COVID-19, the findings may not apply to people with mild or moderate infections.

Although the study demonstrated an association between viral reactivation and severe COVID-19, it did not establish that reactivated viruses cause long COVID or long-term disability.

Levy said the findings raise the possibility that reactivation of other viruses could contribute to COVID-19 disease, but that more studies are needed.

Dr. William Schaffner, a professor of preventive medicine at Vanderbilt University Medical Center in Nashville, who was involved in the study, told ABC News that the new study raises more questions than it answers and that more research is required to determine whether the results are noteworthy.

"The authors carefully do NOT attribute any disease (including long COVID) to [the reactivated viruses]," Schaffner told ABC News in a statement. "It is not clear whether they play any role in disease production; this may be an entirely coincidental event."

Levy also said more is needed to learn about anelloviruses, which have traditionally been considered harmless, and whether they may contribute to disease.

"Although the pandemic is over, SARS-CoV-2 continues to cause infections with both acute and long-term ("long COVID") consequences," Levy wrote. "It is estimated that up to 50,000 Americans died of COVID in 2025 [to] 2026 and up to 10 million Americans may have suffered long COVID during the prior year."

He went on, "Immunization of vulnerable populations such as older adults remains very important to avoid severe outcomes. Research into COVID continues as we try to find better ways of preventing disease."

Onaola Adedeji, MD, is a resident physician in Family Medicine with Obstetrics and Lifestyle Medicine and a member of the ABC News Medical Unit.