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Chickenpox is highly contagious and very common, with a secondary attack rate in households of 85 – 90%. It is primarily a disease of young children and is most common in school aged children between 5 and 9 years of age when approximately 50% of cases occur. Only 10% of cases occur in adolescents older than 15 years of age and it is estimated that 95% of people have been infected by 30 years of age.
The most common complication of chickenpox is secondary bacterial infection with Group A Streptococcus Pyogenes and Staphylococcus Aureus that occurs in 5 to 10% of children. Bacterial infection should be suspected whenever redness and swelling occur around a pox lesion that is enlarged in size. Because cellulitis may follow a local infection, prompt empirical therapy with oral antibiotics such as cloxacillin or cephalexin is indicated. Otitis media may also complicate chickenpox and has been reported to occur in about 5% of children. More serious complications of chickenpox are rare and hospitalization is indicated in only 1 in 5,000 cases. Severe complications include pneumonia, encephalitis, cerebellar ataxia, Reye’s syndrome, Guillain – Barre syndrome, nephritis, carditis, arthritis, orchitis, uveitis, thrombocytopenia, hemorrhagic chicken pox and purpura fulminans.
Consolidated experience for 10 years shows that about 70-80% of vaccinees will have complete protection and the remaining vaccinees will have partial protection against clinical illness. The vaccine is apparently 95% effective against severe varicella, the disease that predisposes an individual to the frequent complications of bacterial superinfection. It is not yet known however whether rarer complications to varicella, such as pneumonia encephalitis or hepatitis will be prevented. In Japan, where 20 year follow-up studies have been conducted, there is evidence for persistent immunity and at this time it appears that there is no evidence for waning post-vaccination protection. The concern is of course that if immunity wanes over time, these individuals will become fully susceptible to chickenpox and the effect of vaccination might have a negative outcome by shifting the disease from children to adults.
Zoster may occur in individuals vaccinated but the reported cases have been mild and the rate has been less than in age-matched control patients after natural varicella infection. The choice between having a latent natural virus or latent vaccine virus in the ganglia causing zoster seems to favor the latter.
Current recommendations are to use VZIG (Varicella zoster immune globulin) alone or in combination with a acyclovir in high risk patients. These would include such patients as immuno-compromised children and susceptible newborns (infants born to mothers who developed varicella within 5 days of delivery or 2 days after delivery). Adolescents and adults are at greater risk for developing severe chicken pox and should be treated with oral acyclovir at the first sign of varicella lesions.
Key References:
CPS Statement, Chickenpox: Prevention and treatment, Can J Paed, 1994; 1:88-93
Plotkin S, Varicella Vaccine, Pediatrics, 1996; 97: 251-252
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