The US Centers for Disease Control and Prevention (CDC) on 11 August 2026 issued a Health Alert Network advisory warning that patients receiving B cell‑depleting or B cell‑modulating medications are at increased risk of severe arboviral neuroinvasive disease, including encephalitis and aseptic meningitis. The advisory highlights particular concern about medicines that target the CD20 antigen, commonly known as anti‑CD20 monoclonal antibodies.
Who is at risk and why
Arboviruses are viruses spread by biting arthropods, most frequently mosquitoes and ticks, and are most active from late spring through early autumn. While most infections are asymptomatic, symptomatic disease can present as a febrile illness or as neurologic disease such as aseptic meningitis, encephalitis or acute flaccid myelitis.
The CDC notes that medications that deplete or modulate B cells — which are used to treat B cell malignancies, many autoimmune conditions and to prevent transplant rejection — can impair immune responses to these viruses. Among the drugs singled out in the advisory are currently approved anti‑CD20 monoclonal antibodies:
- rituximab and biosimilars
- ocrelizumab
- ofatumumab
- ublituximab
- obinutuzumab
- ibritumomab tiuxetan
Severity and outcomes reported
Published case reports of arboviral neuroinvasive disease in patients taking anti‑CD20 monoclonal antibodies have shown a high case fatality. The CDC summarises that overall mortality in these reports was about 40%, and most survivors experienced substantial morbidity.
The incubation period after an arthropod bite is typically days to weeks, but the advisory notes it can be longer in immunocompromised persons. The CDC also warned that the United States experienced an early and strong start to the West Nile virus season in 2026, and that many weeks of arbovirus transmission still remained at the time of the advisory.
Clinical and public‑health advice
The CDC’s advisory was issued to notify clinicians, public‑health authorities and the public so that
- clinicians can maintain a higher index of suspicion for arboviral infection in patients on these therapies;
- public‑health practitioners can target prevention messages and surveillance; and
- patients and caregivers can be counselled about bite‑avoidance measures.
Specifically, the advisory emphasises measures to protect against mosquito and tick bites as a key way to reduce risk for these vulnerable patients.
| Key clinical points | Details from CDC advisory |
|---|---|
| Who is vulnerable | Patients receiving B cell‑depleting or B cell‑modulating medicines, especially anti‑CD20 mAbs |
| Typical presentation | Acute febrile illness or neurologic disease (meningitis, encephalitis, acute flaccid myelitis) |
| Incubation period | Typically days to weeks; may be longer in immunocompromised patients |
| Reported mortality in cases on anti‑CD20 mAbs | Approximately 40% in published reports |
What patients and clinicians should consider
Clinicians should discuss the increased risk with patients who are receiving B cell‑targeted therapies and counsel on personal protection measures. The CDC advisory stresses prevention of mosquito and tick bites as the most practical intervention.
- Use EPA‑registered insect repellents and wear long sleeves and trousers when outdoors.
- Avoid outdoor activity at peak mosquito times where possible.
- Ensure screens on windows and doors are intact and consider permethrin‑treated clothing or gear when appropriate.
- Perform regular tick checks after possible exposure and remove ticks promptly and correctly.
Health professionals should also consider arboviral testing and early notification of public‑health authorities when clinically indicated.
This advisory underscores the need for heightened vigilance for severe arboviral disease among immunocompromised patients during arbovirus season. Patients on B cell‑depleting or B cell‑modulating medications who develop fever, severe headache, confusion, neck stiffness, focal neurologic signs or new limb weakness should seek urgent evaluation at a clinic or hospital — do not attempt to self‑diagnose.
Clinicians and patients wanting the full details can consult the CDC Health Alert Network advisory dated 11 August 2026 for guidance on diagnosis, reporting and prevention.
