Chickenpox (Varicella, Herpes Zoster)
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Varicella-zoster virus (VZV) is a highly contagious herpesvirus. Primary infection with the virus results in varicella (chickenpox), whereas reactivation from latency results in herpes zoster (shingles).
- Transmission occurs by droplet and airborne transmission of infectious respiratory secretions or direct contact with vesicles and respiratory secretions.
- Incubation 10 to 21 days (usually 14 to 16 days) after exposure; cases most contagious 12 days before the rash appears until skin lesions have fully crusted
- Immunocompromised patients may have longer or shorter incubation.
- Post–intravenous immunoglobulin (IVIG), incubation may be up to 28 days.
- The attack rate for susceptible household contacts exposed to varicella is 90%.
- Disease is more severe in immunocompromised persons, infants >3 months of age, adolescents, adults, persons with pulmonary disorders (asthma), persons with chronic skin disorders (eczema), and persons on oral and/or IV steroids or long-term aspirin therapy.
- Congenital varicella embryopathy: Risk is 1–2% when maternal primary VZV infection occurs before the 20th week of gestation.
- The incidence of chickenpox has declined by 90% since the introduction of universal varicella vaccination.
- The rate of complications from varicella has declined dramatically since the licensure of the varicella vaccine.
- Varicella vaccine
- Live attenuated vaccine (Oka strain)
- 2-dose series for routine immunization of all healthy, susceptible children, adolescents, and adults
- Immunogenicity: 76–85% of immunized children developed protective levels of humoral and cellular immunity after 1 dose, ~100% with 2 doses.
- Effectiveness after 1 dose of vaccine: 86%
- Effectiveness after 2 doses of vaccine: 98%
- >97% effective in preventing severe varicella
- 3.3 times less likely to have breakthrough disease when 2 doses of vaccine (as compared to 1 dose) were administered
- Herpes zoster can occur following varicella vaccination, but clinical severity of the zoster is milder and the risk of acquiring zoster following immunization is lower than following wild-type chickenpox.
- Anaphylaxis to vaccine components (e.g., neomycin, gelatin)
- Pregnant, immunocompromised, or <12 months of age
- HIV is an exception: Vaccine is recommended for HIV-positive children if CD4+ T-cell counts are ≥15%. Give doses 3 months apart.
- High-dose corticosteroid doses of ≥2 mg/kg/day or ≥20 mg/day of prednisone, or its equivalent, for ≥14 days are considered immunosuppressive doses: VZV vaccine should not be given until systemic corticosteroid therapy has been discontinued for at least 1 month.
- Postexposure prophylaxis
- If no contraindication to VZV vaccine: Administer VZV vaccine to susceptible hosts (first or second dose) within 3 days (possibly up to 5 days) of exposure.
- If contraindications to VZV vaccine exist: Consider passive immunization (see below).
- Give passive immunization if
- (i) No evidence of immunity in exposed person, (ii) probability that exposure will result in infection, and (iii) likelihood of complications of VZV in the exposed person due to risk factors
- Susceptible immunocompromised people, pregnant women, and neonates whose mothers develop varicella infection 5 days prior to 2 days after delivery should be especially considered for passive immunization upon exposure.
- Administer varicella zoster immune globulin (VariZIG®) or IVIG as per protocol within 10 days of exposure.
- If VariZIG® or IVIG is unavailable, some experts recommend postexposure prophylaxis with oral acyclovir (20 mg/kg q6h; max dose: 3,200 mg/24 h) or valacyclovir (20 mg/kg/dose q8h; max dose: 3,000 mg/24 h), beginning 7 to 10 days after exposure and continuing for 7 days.
- After primary infection, the virus establishes latency in dorsal root ganglia cells.
- Immunity from natural disease is usually lifelong, but symptomatic and asymptomatic reinfections do occur, boosting antibody levels.