What is the recommended course of action for a 2-year-old child who received the first dose of varicella (chickenpox) vaccine at 12 months and is now due for the second dose?

Administer the Second Dose of Varicella Vaccine Now

This 2-year-old child who received the first varicella vaccine dose at 12 months should receive the second dose immediately at this visit, as the minimum 3-month interval between doses has been met. 1

Recommended Action

• Give the second dose of varicella vaccine at the current 2-year visit since 12 months have elapsed since the first dose, which far exceeds the minimum 3-month interval required for children aged 12 months through 12 years. 1, 2

• Either single-antigen varicella vaccine (Varivax) or combination MMRV vaccine (ProQuad) may be used, as both are approved for children aged 12 months through 12 years. 1, 2

• If other vaccines (measles, mumps, rubella) are also due, the combination MMRV vaccine is preferred over separate injections when all components are indicated and none are contraindicated. 1

Minimum Interval Requirements

• The Advisory Committee on Immunization Practices (ACIP) establishes a minimum interval of 3 months between the first and second varicella vaccine doses for children aged 12 months through 12 years. 1, 2

• This 3-month minimum interval recommendation is based on the design of clinical studies that evaluated two-dose varicella vaccination schedules in this age group. 1, 2

• If the second dose is inadvertently administered between 28 days and 3 months after the first dose, it is considered valid and does not need to be repeated, though 3 months remains the recommended minimum. 1, 2

Clinical Rationale for Two-Dose Schedule

• Two doses of varicella vaccine provide 98-99% protection compared to only 76-94% protection after a single dose. 2, 3, 4

• Children who receive two doses are 3.3-fold less likely to develop breakthrough varicella compared to those who received only one dose. 2

• After two doses, >99% of children achieve protective antibody levels against varicella, compared to 76-85% after a single dose. 2

• The effectiveness of two doses versus one dose has been demonstrated at 98.3% (95% CI: 83.5%-100%) in real-world studies, with odds of developing varicella being 95% lower for two-dose recipients. 4

Timing Considerations

• While the routine recommendation is to administer the second dose at age 4-6 years (before school entry), the second dose may be given at any earlier age as long as the 3-month minimum interval is maintained. 1, 2

• The standard 4-6 year timing is strategically chosen because varicella incidence and outbreaks are higher among elementary school-aged children, but earlier administration (as in this case at age 2 years) is fully acceptable and provides earlier protection. 1, 2

Common Pitfall to Avoid

• Do not delay the second dose unnecessarily if the child presents after the minimum interval has passed—there is no advantage to waiting until age 4-6 years if the family is present and willing to vaccinate now, and earlier completion provides better protection during the preschool years. 1, 2