In Hong Kong, universal varicella immunization with a two-dose schedule was implemented in July 2014, while the vaccinations had been accessible in the private market since 1996. The study utilized the screening technique to estimate dose-specific varicella vaccine efficacy (VE) among preschool children in Hong Kong prior to universal vaccination, using data from varicella notification and questionnaires on immunization coverage. The VE of a one-dose varicella vaccine against all notified varicella was predicted to be 69.4 percent and 93.4 percent, respectively. Researchers discovered that VE did not decrease with time. 

One dosage of varicella vaccine was 100 percent effective against complications, and one dose was 93.1 percent effective against hospitalization. The one-dose varicella vaccination provided inadequate protection, resulting in outbreaks of varicella. Second-dose varicella vaccine (given as a combination measles, mumps, rubella, and varicella vaccine) was initially planned for children when they reached primary one and was recently increased to 18 months of age under universal vaccination. Shortening the time between the first and second doses of varicella immunization could minimize outbreaks and breakthrough varicella in preschool.