Varicella
Part A Illness, Risk Factors and Prevention
1 Etiology:
Varicella (chickenpox) is caused by the Varicella-zoster virus (VZV) of the herpes virus Family. VZV is spread by direct contact with the virus shed from skin lesions or in oral secretions as well as by the airborne route. The incubation period is from 10 to 21 days, usually in the range of 14 to 16 days. Infectiousness begins 1 or 2 days before the rash appears and lasts until the last lesion has crusted.
11 Indication:
Lifetime risk of developing Varicella is 95%. 15 to 20% will develop shingles. 90% of cases occur in children up to age 12. The vaccine is not mandatory. Suitable candidates are children from 12 to 18 months of age, as part of routine immunization, preferably at the same time as MMR. (Note, if given at the same time as MMR, a separate needle and syringe should be used and a different injection site.. Older children, non-pregnant women of childbearing age , health care workers and those in correctional institutions may also be vaccinated if they have not already had the chicken pox.
111 Prevention
Vaccination.
Part B Vaccine
Three varicella vaccines are licensed in Canada. These are live vaccines, Varivax®, Varivax II® (Merck Frosst Canada & Company) and Varilrix® (GlaxoSmithKline). Each is a preparation of lyophilized, live, attenuated varicella virus designated the Oka strain, which was developed in Japan in the early 1970s. Only Varivax II® is currently marketed in Canada.
1 Composition and efficacy:
Clinical studies of healthy children have shown detectable varicella antibodies in
over 99% from 1 to 4 years after receipt of one dose and in over 96% up to 7 to 10 years after immunization, a level comparable with that of children with a history of natural infection. Over 97% of healthy adolescents and adults who received two doses 4 to 8 weeks apart demonstrated detectable antibody up to 3 years later. In pre-licensure placebo-controlled trials of the vaccine in healthy children aged 1 to 14 years, the observed protective efficacy was 100% in the first year and 96% in the second year after immunization. No placebo-controlled trials have been done in adolescents and adults. In actual use, it is estimated that the vaccine will offer 70% to 90% protection against varicella of any severity and 95% protection against severe varicella for at least 7 to 10 years after immunization, the observation period reported to date. Varicella in vaccine recipients is associated with a significantly reduced number of lesions (fewer than 50, as compared with several hundred among unvaccinated people) and no or little fever. Illness in those who experience vaccine failure does not appear to increase in severity with time since immunization. People who are immunocompromised because of disease or treatment may not achieve as high a level of protection with immunization as healthy people.
11 Contraindications:
The vaccine should not be given to individuals receiving high doses of systemic steroids, or immunocompromised individuals, except acute lymphoblastic leukemia. Vaccinees who develop a rash should avoid contact with immunocompromised individuals for the duration of the rash.
Before immunization
A reliable history of varicella disease is adequate evidence of immunity, and there is little value in administering the vaccine to such people. A history of varicella disease should therefore be obtained before immunization.
For those aged >= 13 years with an unknown history of prior varicella infection,
serologic testing before immunization may be helpful in determining the need for
immunization. In routine adolescent immunization programs, it may be more
efficient to offer vaccine to those without a history of chickenpox than to screen prior to immunizing.
111 Precaution (pregnancy, nursing mothers, special conditions):
Pregnant women should not be immunized with varicella vaccine because the effects on fetal development are unknown. Women should postpone pregnancy for 1 month after immunization with the two-dose series. To assist in evaluation of outcomes of immunization during or before pregnancy, incidents of inadvertent immunization during pregnancy, pregnancy occurring within 3 months after immunization should be reported to
Merck Frosst Canada Inc., Medical Services (tel: 1 800 684 6686).
Breast-feeding is not a contraindication to varicella immunization of the mother or
child. Varicella vaccine may be given to people in households with a newborn.
1V Drug interaction
Adverse events have not been reported in association with the use of salicylates
after varicella immunization. Because of the association of varicella and Reye
syndrome in children given salicylate therapy, the manufacturer recommends
avoidance of salicylate use for 6 weeks after varicella immunization. Despite this,
children with rheumatoid arthritis or other conditions requiring chronic salicylate
therapy are at higher risk of Reye syndrome following wild varicella and should be considered for immunization, with close subsequent monitoring. Physicians need to weigh the theoretical risks associated with varicella vaccine against the known risks of wild varicella in children taking long-term salicylates.
V Adverse reactions:
Reactions are usually mild and include injection site reactions, a non-injection site rash and low-grade fever.
Part C Administration
1 Dose (amount and schedule, booster)
Schedule and Dosage
Children aged 12 months to 12 years: A single dose given after the 1st birthday.
Adolescents aged >= 13 years and adults: Two doses given at least 4 weeks (28
days) apart. There is no need to re-start the schedule if administration of the second dose has been delayed.
The dose is 0.5 mL, containing at least 1350 plaque-forming units (PFU) of VZV for Varivax II® and not less than 1995 PFU for Varilrix®.
Simultaneous Administration with Other Vaccines
Varicella vaccine may be administered at the same time as but at a separate injection site from MMR vaccines routinely given at 12 months, DPT-containing
vaccines given at 18 months and school entry, or adult tetanus-diphtheria vaccine given in adolescence and adulthood. When not given at the same time as other live virus vaccines, administration of the vaccines should be separated by a 4-week interval.
Varicella vaccine should not be given concurrently with immune globulin (IG),
including varicella zoster immune globulin (VZIG). For administration of varicella
vaccine after IG, blood or plasma transfusions, follow the guidelines for
measles containing vaccine in the section on Recent Administration of Human
Immune Globulin Products (Part 1 of the Guide).
Booster Doses and Re-immunization
The need for booster doses at this time is unknown. Follow-up evaluation of children immunized during pre-licensure clinical trials in the US reveals
protection for at least 11 years, and studies in Japan indicate protection for at least 20 years. Follow-up of clinical trial subjects is under way to determine the need for additional doses. It is unknown whether protection may be less durable when the incidence of natural varicella declines as a result of vaccine use and allows fewer opportunities for "natural boosting". As with measles and rubella vaccines (other live virus vaccines), protection may persist throughout life in people who have a primary response to immunization.
11 Route
The lyophilized varicella vaccine should be reconstituted with the diluent provided for this purpose immediately before administration and given subcutaneously.
111 Post immunization care
Post-immunization serologic testing for immunity is not recommended because of the high level of immunity conferred by the vaccine. As well, currently available
commercial laboratory tests are not sufficiently sensitive to detect vaccine-induced antibodies, unlike the highly sensitive but commercially unavailable gpELISA, which was used in clinical trials for testing immunogenicity
References: Canadian Immunization Guide Edition 2002