Human adenovirus 21 (HAdV‑21) is a member of species B of the Adenoviridae family. These viruses are non‑enveloped, double‑stranded DNA viruses with a 70–100 nm icosahedral capsid. More than 100 antigenic types are known, but HAdV‑21 is relatively rare compared with types 3 or 7. It infects epithelial cells of the respiratory tract and can persist in lymphoid tissues.
Biology and pathogenesis
HAdV‑21 was first isolated in 1956. Like other adenoviruses, it replicates in the nucleus of infected cells, producing large amounts of virus before cell lysis. Transmission occurs through respiratory droplets or contact with contaminated surfaces. Infection begins in the upper respiratory tract, causing symptoms such as fever, sore throat and cough, and the virus may spread to the lower airways. Studies of paediatric respiratory illness show that a small proportion of adenovirus infections are caused by HAdV‑21, yet some cases progress to severe lower‑respiratory disease requiring hospitalization. Molecular analyses divide HAdV‑21 into at least two subtypes (21a and 21b) with differences in the hexon gene, and both have circulated in military recruits and civilian populations.
Outbreaks and clinical impact
Although less frequently encountered than some other adenovirus types, HAdV‑21 has been associated with outbreaks of acute respiratory disease in several settings. It can cause severe pneumonia, myocarditis and fatal infections, particularly in young children, elderly patients and individuals with compromised immunity. Nosocomial transmission has been reported in transplant units where immunosuppressed patients developed prolonged viraemia and respiratory distress. In community outbreaks, HAdV‑21 spreads efficiently in crowded environments such as military training camps, similar to HAdV‑14 and HAdV‑7. Supportive care remains the primary treatment, although cidofovir may be considered in severe cases. HAdV‑21 infections are uncommon but highlight the need for surveillance. Laboratory identification depends on PCR and sequencing because clinical features overlap with other viral pneumonias. Vaccines against adenovirus types 4 and 7 are used in military recruits but do not cover HAdV‑21. In summary, human adenovirus 21 is a species B virus that occasionally causes severe respiratory infections. Molecular epidemiology studies have identified distinct subtypes and emphasised its potential for nosocomial spread. Continued monitoring and infection control are important to limit outbreaks. Related Terms: Human Adenovirus 7, Human Adenovirus 14, Human Adenovirus 11, Human Adenovirus 55