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Chickenpox Vaccine: Timing Matters

During the recommended administration window, when is the best time to give the vaccine?

The incidence of chickenpox has declined with widespread use of live-attenuated varicella vaccine in infants (Journal Watch Feb 22 2002). To assess the vaccine's effectiveness, researchers actively monitored reports of disease in 20 Connecticut pediatric practices and ultimately identified 339 children (age range, 13 months to 16 years) with clinically diagnosed chickenpox who tested positive for varicella-zoster virus. These cases were matched with 669 controls without symptomatic chickenpox.

Varicella vaccine had been received by 36% of cases and 70% of controls. The vaccine's overall effectiveness (87%) remained almost unchanged after adjustment for potential confounders. However, effectiveness declined significantly over time: It was 97% during the first year after vaccination but only 81% during years 7 to 8. Within the first year after vaccination, effectiveness was significantly lower if the vaccine had been administered before age 15 months (73%) than if it had been given at or after that age (99%). Chickenpox was significantly more severe in unvaccinated children than in vaccinated children.

Comment: Varicella vaccine currently is recommended for children at age 12 to 18 months. Not surprisingly, it is more effective if administered later within that age window. As with many vaccines, protection declines with time. I believe that experts ultimately will recommend a second dose of varicella vaccine.

— Howard Bauchner, MD

Published in Journal Watch General Medicine March 2, 2004

Citation(s):

Vázquez M et al. Effectiveness over time of varicella vaccine. JAMA 2004 Feb 18; 291:851-5.

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