Human Adenovirus 21 is a serotype of species B human adenoviruses (genus Mastadenovirus) within the Adenoviridae family. This non‑enveloped virus has an icosahedral capsid and a linear double‑stranded DNA genome around 34 kilobases. Isolates of HAdV‑21 were first reported in the 1950s and are now recognised as an emerging cause of acute respiratory disease. The virion is environmentally stable and resistant to many disinfectants, enabling transmission by respiratory droplets, contaminated surfaces and close contact.
Structure and replication
Human adenovirus 21 has a diameter of about 90 nm with distinct penton base, hexon and fiber proteins that mediate cell binding and entry. The fiber knob interacts with host complement regulatory proteins such as CD46 and with integrins to trigger endocytosis. Following uncoating, the viral genome translocates to the host nucleus where it uses host and viral polymerases to transcribe early and late genes, assemble structural proteins and replicate DNA. Newly assembled virions accumulate in the nucleus until the cell undergoes lysis, releasing thousands of progeny. HAdV‑21 is divided into subtypes, including 21a and 21b, which differ in their fiber genes and can show variable virulence. The virus is resistant to lipid solvents, heat and desiccation, contributing to its ability to persist on surfaces and in aerosols.
Clinical impact and epidemiology
Outbreaks of human adenovirus 21 have been associated with febrile pharyngitis, bronchitis and severe pneumonia. Cases often present with high fever, cough, sore throat and radiographic evidence of lung infiltrates; conjunctivitis, diarrhea and cystitis are less common. The virus can cause serious disease in infants, the elderly and individuals with underlying conditions or weakened immunity. Numerous clusters have been reported among children, military trainees and long‑term care facilities, and some outbreaks have had high hospitalization and mortality rates. Diagnosis relies on polymerase chain reaction, antigen detection or culture of respiratory specimens. There is no specific antiviral therapy; management is supportive, with cidofovir reserved for severe infections in immunocompromised patients. Strict infection control and surveillance are key to preventing transmission. Adenovirus 21 is an uncommon but important pathogen that underscores the diversity of adenovirus‑mediated respiratory disease. Its ability to cause severe pneumonia and its environmental resilience warrant ongoing monitoring and prompt supportive care when infections occur. Related Terms: respiratory infection, adenovirus, pneumonia, serotype 7, serotype 14