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Immunization Science

Are two doses of varicella vaccine more effective?

The article

Ten year follow-up of healthy children who received one or two injections of varicella vaccine. Kuter B, Matthews H, Shinefield H, Black S, Dennehy P, Watson B, Reisinger K, Kim LL, Lupinacci L, Hartzel J, Chan I, and The Study Group For Varivax. The Pediatric Infectious Disease Journal 2004;23(2):132-137

The question

Are two doses of varicella (chickenpox) vaccine more effective against chickenpox than one dose of the vaccine?

The study

This study included a total of 2,216 children 12 months to 12 years of age with a negative history of varicella; 1,114 received 1 injection of varicella vaccine, and 1,102 received 2 injections. The two injections were given 3 months apart from each other.

The researchers followed the children for varicella, any varicella-like illness or zoster (shingles) and any exposures to varicella or zoster on a yearly basis for 10 years after vaccination. They also measured yearly the persistence of varicella antibody for 9 years.

The findings

A total of 60 children who received 1 injection and 17 who received 2 injections of vaccine had a breakthrough case of varicella (usually mild chickenpox in vaccinated persons). Among the children who developed a breakthrough case of varicella, the majority had mild disease (less than 50 lesions).

In both groups most breakthrough cases occurred between Years 2 and 5 after vaccination. There were no breakthrough cases of varicella in Years 7 to 10 in the recipients of two injections, but cases continued to occur in recipients of one injection during those years.

Both the one and two dose regimens protected all children from having more than 300 lesions and fever of 102°F or more.

Children who received two injections were three times at less risk of developing varicella during the 10-year observation period than those who received one injection. Antibodies to the varicella virus and protection against varicella persisted for 9 years in all children.

The relevance/bottom line

Administration of either one or two injections of varicella vaccine to healthy children results in long term protection against most varicella disease. The results of this study suggest that the use of a two dose regimen could provide an increased rate of protection.

NNii’s comment

Vaccine protection against chickenpox was long lasting. Two immunizations separated by 3 months provided greater protection than those that had one immunization. Most children who were immunized but who later developed chickenpox had mild episodes.

The questions that arise from this and other recent studies are what is the ideal immunization strategy for chickenpox; whether there should be a second dose of varicella vaccine for all children, whether varicella vaccination should be given after 15 months of age, and how contagious this so-called breakthrough disease is for unvaccinated children.