Causative Agent
Varicella-Zoster Virus (VZV)
Incubation Period
10 - 21 days
Infectious Period
Infectious 1 - 2 days before the onset of rash and 5 - 7 days after the appearance of vesicles until scabbing.
Transmission
Highly contagious (secondary household attack rate of susceptible individuals of >90%). Transmission is mainly by airborne route (respiratory secretions) and less often by direct contact with vesicular fluid of lesions.
Epidemiology
A total of 23,476 cases were reported in 2007 (overall incidence of 511.6/100,000 population). The age-specific incidence rate for 2007 was highest in the 5-9 year age group, which was 5 times that of the overall rate. The male to female ratio was1.4:1. Malays had the highest incidence rate among the three major ethnic groups of resident population. Cases occurred throughout the year. Outbreaks in institutional and child-care settings are common.
The age-specific immunity is lowest in those less than 6 years of age (34.5%) and increases steadily with age; 60.5% of those aged 7-12 years, 71% aged 13-17 years and 87.9% aged 18 years and above have VZV antibodies.