Varicella (chickenpox)

Our analysis of varicella (chickenpox).

CDC’s Pink Book varicella chapter

Manufacturer’s Inserts and efficacy statements:

VariVax (live virus)– (since no placebo-controlled trial was completed using the current Varivax vaccine the manufacturer uses statistics from a different trial using similar dosage)
                          1 dose regimen- approximately 83%
                          *0.3-3.8% show breakthrough chickenpox episode
                          2 dose regimen- approximately 98%
                          *similar breakthrough episodes.

*Studies have shown that the efficacy of the varicella vaccine is closer to 44%. This result has been noted in both the Journal of Infectious Diseases and The New England Journal of Medicine.

Results: Varicella developed in 25 of 88 children (28.4 percent) between December 1, 2000, and January 11, 2001. The index case occurred in a healthy child who had been vaccinated three years previously and who infected more than 50 percent of his classmates who had no history of varicella. The effectiveness of the vaccine was 44.0 percent (95 percent confidence interval, 6.9 to 66.3 percent) against disease of any severity and 86.0 percent (95 percent confidence interval, 38.7 to 96.8 percent) against moderate or severe disease. Children who had been vaccinated three years or more before the outbreak were at greater risk for vaccine failure than those who had been vaccinated more recently (relative risk, 2.6 [95 percent confidence interval, 1.3 to 5.3]).

Additionally, of serious concern, is the relationship between chickenpox exposure and immunity to shingles (herpes-zoster). Exposure to varicella boosts immunity to herpes-zoster: implications for mass vaccination against chickenpox presents data to this effect and states that the lack of exposure to chickenpox may have consequences for us down the road.

We present data to confirm that exposure to varicella boosts immunity to herpes-zoster. We show that exposure to varicella is greater in adults living with children and that this exposure is highly protective against zoster (Incidence ratio=0.75, 95% CI, 0.63–0.89). The data is used to parameterise a mathematical model of varicella zoster virus (VZV) transmission that captures differences in exposure to varicella in adults living with and without children. Under the ‘best-fit’ model, exposure to varicella is estimated to boost cell-mediated immunity for an average of 20 years (95% CI, 7–41years). Mass varicella vaccination is expected to cause a major epidemic of herpes-zoster, affecting more than 50% of those aged 10–44 years at the introduction of vaccination.

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