A new study published in the journal Nature has found that severe cases of COVID-19 can lead to the reactivation of dormant viruses already present in patients’ bodies. The research, led by Dr. Ofer Levy at Boston Children's Hospital and involving more than 20 biomedical hospitals across the United States and Ontario, Canada, followed 1,154 hospitalized COVID-19 patients and analyzed over 200,000 biological samples collected over a year. According to the study, 11 different dormant viruses were detected reactivated within the first 40 days after hospital admission for SARS-CoV-2 infection.

The viruses found to reactivate included several from the herpes family—such as Epstein-Barr virus, cytomegalovirus, and herpes simplex virus-1—as well as Anelloviridae, a family of viruses that do not cause any proven human disease. Reactivation of Anelloviridae, which is carried by most adults, was strongly associated with long-term physical disability and long COVID, a condition where symptoms persist for at least four weeks after the initial infection.

Researchers observed that some viruses, like Epstein-Barr and cytomegalovirus, seemed to reactivate in response to inflammation rather than from immune suppression, suggesting a new mechanism for viral reactivation during severe illness. “It was not previously known to this degree of detail and scale how viral reactivation might associate with COVID severity and outcome,” Dr. Ofer Levy said in a statement. He added, “Although the pandemic is over, SARS-CoV-2 continues to cause infections with both acute and long-term ('long COVID') consequences.”

The study was unique in its use of RNA measurements to detect actively replicating viruses, rather than relying solely on antibody responses. The research also found that virus reactivation may represent a broader phenomenon that should be considered in clinical settings beyond immune suppression.

The researchers plan to continue investigating how the immune system responds to these viruses during COVID-19 infection and to identify ways to prevent and treat viral reactivation. The study’s findings may help inform future strategies for diagnosing and treating both acute COVID-19 and long COVID.

While the study focused on severely ill, hospitalized patients, and results may not apply to those with mild or moderate COVID-19, it highlights the importance of ongoing research as the virus continues to impact millions. Up to 50,000 Americans died of COVID-19 from 2025 to 2026, and as many as 10 million suffered from long COVID, according to Dr. Ofer Levy. Immunization of vulnerable populations remains important to prevent severe outcomes.