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Immunization Science

Effectiveness over time of varicella vaccine

The article

Effectiveness Over Time of Varicella Vaccine. Vázquez M, LaRussa PS, Gershon AA, Niccolai LM, Muehlenbein CE, Steinberg SP, and Shapiro ED. JAMA 2004(291):851-855.

The question

How effective is varicella (chickenpox) vaccine over time? Is effectiveness affected by age at the time of vaccination?

The study

Researchers identified 339 children 13 months of age or older with chickenpox from twenty pediatric practices in southern Connecticut and compared each of these children with 2 children of the same age who had not had chickenpox. Then they analyzed their medical records to see which children were vaccinated against varicella, at what age they were immunized, and for the severity of their chickenpox illness.

The findings

The effectiveness of the chickenpox vaccine at preventing all chickenpox symptoms during the first year after vaccination was 97%, which decreased to 86% in the second year after vaccination and to 81% in years 7 to 8 after vaccination.

The vaccine's effectiveness in year 1 was substantially lower if the vaccine was administered at younger than 15 months (73%) than if it was administered at 15 months or older (99%) but the effectiveness was not much different for subsequent years. Chickenpox was much milder in vaccinated than unvaccinated children regardless of when they were immunized, even after 8 years.

“This study indicates that at least through the first 8 years after vaccination, the overall effectiveness of live, attenuated varicella vaccine remains good, although breakthrough varicella is not rare,” the researchers wrote. “Most vaccinated children who develop chickenpox have mild disease, regardless of their age at the time of vaccination or the time since vaccination; at least up to 7 to 8 years after vaccination (the vaccine's effectiveness against moderate to severe disease is excellent throughout the period of the study).”

The relevance/bottom line

Chickenpox vaccine was licensed in 1995 and therefore had only been in routine use in the U.S. for up to 7 to 8 years at the time of this study. Although the vaccine’s effectiveness to protect against all chickenpox symptoms decreases after the first year, it is still protective after 8 years. Further studies will be needed to assess whether the vaccine remains effective for a longer period.

NNii’s Comment

This study is reassuring in that protection against chickenpox appears to be long lasting; children who were immunized but who later developed chickenpox had very 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.