Varicella (chickenpox) is an acute, highly contagious disease caused by varicella-zoster virus (VZV), a member of the herpesvirus family. Only one serotype of VZV is known, and humans are the only reservoir. Following infection, the virus remains latent in neural ganglia and in about 10-20% of cases it is reactivated it is reactivated to cause herpes zoster, or shingles, generally in persons over 50 years of age or immunocompromised individuals.
VZV transmission occurs via droplets, aerosols, or direct contact with respiratory secretions, and almost always produces clinical disease in susceptible individuals. While mostly a mild disorder in childhood, varicella tends to be more severe in adults. It may be fatal, especially in neonates and in immunocompromised persons. In temperate climates most cases occur before the age of 10. Varicella is characterized by an itchy, rash usually starting on the scalp and face and initially accompanied by fever and malaise. The rash gradually spreads to the trunk and extremities. The vesicles gradually dry out and crusts appear which then disappear over a period of one to two weeks.
The infection may occasionally be complicated by pneumonia or encephalitis (inflammation of the brain), at times with serious or fatal consequences. Shingles is a painful rash that may occasionally result in permanent damage to the nerves or visual impairment. It is relatively common in HIV-infected persons, sometimes with fatal consequences.
Current varicella vaccines are attenuated vaccines based on the Oka VZV strain that has been modified through sequential propagation in different cell cultures. The attenuated strain is grown in cell culture, purified and lyophilized. Reconstituted vaccine is injected subcutaneously. Varicella vaccine has also been included in a combination vaccine with mesles mumps rubella (MMRV). A vaccine which contains higher level of the virus has also been developed for the prevention of shingles in the elderly.
Varicella Vaccine Standardization
Requirements for varicella vaccine (live) were adopted by the Expert Committee on Biological Standardization in 1984. They were updated in 1993to include requirements for the use of cell cultures in production.
No WHO reference materials for varicella vaccine are currently available.