Severe COVID-19 can rouse other viruses lying dormant in the body — and the pattern may help explain the still-mysterious condition known as long COVID.
Researchers at Boston Children’s Hospital and 15 other US institutions tracked 1,154 COVID-19 patients over a year in the IMPACC study, collecting more than 200,000 samples and over a billion data points. Published August 5, 2026 in Nature.
What viral latency means
They found severe infection reactivated long-dormant viruses — including Epstein-Barr virus, herpes simplex 1 and cytomegalovirus, 11 viruses in all.
These are not new infections. Herpesviruses establish lifelong latency: after the initial illness, the virus persists in specific cell types with its genome intact but largely silent, producing few or no viral particles.
Most adults carry several. Epstein-Barr virus infects the large majority of the world’s population, usually in childhood and usually without notable illness, and thereafter remains permanently in memory B cells.
Reactivation — the virus resuming replication — happens periodically, often without symptoms. Cold sores are the visible version.
The surprising trigger
The bigger surprise was the cause. Rather than stemming from a weakened immune system, reactivation appeared to be driven by inflammation — the intense, body-wide inflammatory response of severe illness — even in people with otherwise healthy immune systems.
The conventional model treats latency as actively enforced by immune surveillance: T cells continually suppress reactivating virus, and when immunity falters — in transplant recipients, in advanced HIV — the virus escapes. That model is well supported in those settings.
Inflammation driving reactivation in immunocompetent people is a different mechanism. The inflammatory signalling molecules that dominate severe illness also activate transcription factors, and some of those factors bind viral genomes and switch dormant virus back on. The virus is not escaping suppression; it is being actively awakened.
That reframes how doctors think about why dormant viruses wake up during serious infection.
The virus family that stood out
Reactivation of Anelloviridae — a family of very common, usually harmless viruses — was prominently associated with long-term physical disability and long COVID.
Anelloviruses are unusual and underappreciated. Nearly everyone carries them, they have never been linked convincingly to any disease, and their levels vary with immune status — rising when immunity is suppressed, falling when it is intact.
That property makes them interesting as a readout rather than as a cause. Anellovirus levels behave as a running measure of immune competence, so elevated levels may indicate that something remains wrong with immune regulation months after the acute illness.
Why that would matter for long COVID
Long COVID has resisted objective definition, and that is its central clinical problem. Diagnosis rests on symptoms — fatigue, cognitive difficulty, exercise intolerance — that are real, disabling and unmeasurable.
Without a biological marker, patients struggle to be believed, trials cannot select participants reliably, and treatment response cannot be assessed.
A measurable viral signal associated with the condition would address that directly, even if the viruses turn out not to cause the symptoms. A marker need only track the condition reliably to be useful.
What the study design supports
“This association with long COVID is an interesting finding, as millions around the world suffer from this chronic condition,” said senior author Ofer Levy.
The scale is what gives the work weight. Tracking 1,154 patients prospectively for a year, sampling repeatedly, means viral reactivation was observed as it happened rather than inferred afterwards — and the temporal ordering, reactivation preceding persistent symptoms, is established rather than assumed.
The limits
The findings are associations, not proof of cause.
Three explanations remain compatible with the data: reactivated viruses drive long COVID; reactivation and long COVID both result from the same underlying immune disturbance; or reactivation is a marker of severity that predicts poor recovery generally.
Why this pattern appears beyond COVID
Inflammation-driven viral reactivation is unlikely to be specific to one virus, and that generality is part of what makes the finding significant.
Severe illness of many kinds — sepsis, major trauma, extensive surgery, burns — produces the same intense systemic inflammatory response. Herpesvirus reactivation has long been observed in intensive-care patients and has generally been attributed to the immunosuppression that accompanies critical illness.
If inflammation is a direct trigger rather than a consequence of weakened surveillance, that reframes a familiar observation across critical care medicine.
It also connects to a broader question about post-illness syndromes. Persistent fatigue and cognitive difficulty following severe infection are described after several pathogens, not only after COVID, and Epstein-Barr virus in particular has been repeatedly implicated in chronic fatigue conditions without the relationship ever being resolved. A mechanism explaining why severe inflammation reactivates dormant virus offers a common thread for a set of conditions that have resisted explanation individually.
Distinguishing them requires an intervention — suppressing reactivation with antivirals and seeing whether long COVID rates fall. The study points toward new ways to diagnose and potentially treat by targeting viral reactivation. Research news, not medical advice.