Varicella (chickenpox)

Recognizing the importance for physicians to keep up to date on national immunization recommendations, we warmly welcome the new column, Vaccination Snapshots, written by Dr Ben Tan, in which he briefly summarizes the National Advisory Commitee on Immunization's new vaccine recommendations. The first of these is on the varicella vaccine, which is especially timely given that more provinces are moving forward to provide this vaccine. Dr Noni MacDonald, Co-Editor-in-Chief The National Advisory Committee on Immunization's (NACI) February 1, 2004 update on varicella can be retrieved from the Canada Communicable Disease Report online at . The major points are summarized below. NACI recommends the immunization of: All healthy children at 12 months of age; and Any healthy child or adolescent older than 12 months of age who is varicella susceptible. Children and adolescents with previously physician-diagnosed varicella or a clear history of varicella illness from the parent(s), need not be vaccinated (presumed immune). If the history is doubtful, varicella antibody testing of those 13 years of age or older is an option, because these adolescents would require two vaccine doses if they are not immune. In the immunocompetent child who is younger than 13 years of age with no history of varicella, antibody testing is not cost-effective and confirmatory serology is of no benefit. Postexposure immunization is up to 95% effective in preventing disease if it is carried out within three to five days of exposure. This is often impractical, as parents may become aware of the exposure too late or the vaccine may not be readily available. Although postexposure vaccination may be useful in controlling varicella outbreaks, it is best to ensure children are immunized before attending daycare or school. The goal of varicella immunization is to reduce the morbidity and mortality due to complications of the disease. Complications include secondary bacterial skin and soft tissue infections (eg, cellulitis and Group A streptococcal necrotizing fasciitis or toxic shock syndrome, which comprise 50% of the complications), otitis media, pneumonia, osteomyelitis, septic arthritis, hepatitis, thrombocytopenia, cerebellar ataxia (1:4000 cases) and encephalitis (1:5000 cases). Up to one in 200 healthy children with chickenpox require hospitalization, and five to 15 children and adults die of chickenpox complications each year in Canada. Compared with children, adults have a three- to 20-fold higher risk of complications and need for hospitalization. Before the availability of varicella-zoster immune globulin (VZIG) and/or acyclovir therapy, children with impaired immunity had a 30% risk of developing disseminated varicella, which is associated with 7% to 10% mortality. A fetus has a 2% risk of developing congenital varicella syndrome if the mother experiences varicella between the 12th to 20th week of gestation. A newborn has a 20% to 30% mortality risk if he/she catches the infection (without VZIG administration) from a mother who develops varicella during the period between five days before birth and two days after birth. Two refrigerator-stable varicella vaccines containing the Oka virus strain are available in Canada, Varivax-III (Merck Frosst, Canada) and Varilrix (GlaxoSmithKline, Canada). NACI recommends that either vaccine can be used in healthy children and adolescents. For both vaccines, children aged 12 months to 12 years of age require a single dose administered subcutaneously, while adolescents 13 years of age or older require two doses four to six weeks apart. Varicella antibody seroconversion rates of 98% and 99% are seen after one vaccine dose in one- to 12-year-old children and after two doses in adolescents 13 years of age or older, respectively. The varicella vaccine is 70% to 90% effective in preventing varicella disease of any severity, but it is over 95% effective in preventing severe varicella disease. Breakthrough disease may occur in 10% to 20% of vaccinated children after close (household) contact but it is usually mild (in 80%). Immunity is expected to last over 20 years based on evidence from immunized Japanese children with ongoing exposure (boosting) to wild type infection. NACI currently does not recommend any booster doses and will monitor vaccine effectiveness over time. Postvaccination immunity checks in healthy children and adolescents are not recommended. The vaccine antibody studies were performed using specialized test kits, which are not commercially available. The available test kits in most laboratories are sensitive enough to detect antibodies after wild type varicella infection, but are often not able to detect antibody postvaccination (ie, false negative result). Side effects are usually mild postvaccination and can include: Local pain/tenderness and fever in 10% to 20%. A mild, local or more generalized varicella-like rash in 1% to 8% of patients at approximately 10 to 25 days after the first or second (age appropriate) dose. Those who develop the vaccine-related rash are unlikely to infect others. The rate of shingles from the vaccine strain appears to be lower (approximately one-fourth the rate) than after wild type infection. The contraindications and precautions of vaccination include: Anaphylaxis to a previous varicella vaccine dose; Allergy to gelatin (Varivax-III only) or to neomycin (both vaccines); Pregnancy (if exposed, susceptible pregnant women should receive VZIG and NOT varicella vaccine); Untreated tuberculosis; Received blood or blood products within the past three to 11 months (see NACI update for specific wait periods); and Children with immunodeficiency disorders, except those specified below. The manufacturer recommends avoidance of salicylate use for six weeks postvaccination because of the association between varicella and Reye syndrome. Weighing the theoretical risk associated with varicella vaccine against known risk of Reye syndrome following wild varicella, NACI indicates that children with conditions requiring chronic salicylate therapy should be considered for immunization, with subsequent close monitoring. As more children are immunized and chickenpox declines, it has been theorized that shingles occurring in the older (adult) population may increase due to a lack of natural boosting. Studies are currently underway to assess the effect of vaccinating middle-aged adults to boost their immunity against shingles; the results are expected in the next several years. As of August 2004, many Canadian provinces and territories have implemented or announced universal varicella vaccination at 12 months of age with a variety of catch-up programs. Others have varicella vaccine programs for high-risk persons. It is not yet possible to determine the impact of these new vaccination programs in Canada, partly because varicella is under-reported and vaccine coverage estimates are not available. In comparison, varicella immunization coverage has reached 80% nationally in the United States. The disease incidence has dropped by 70% to 85% in three American communities with varicella surveillance. Varicella-related deaths in the United States have also declined by 60% to 80% from the prevaccine era. Although both vaccines have been studied in immunocompromised persons, only Varilrix has been licensed in Canada for some subgroups. NACI recommends that a specialist with expertise in vaccines be consulted before vaccination of these persons because: The number of doses needed (one or two) is controversial. The immune responses may be suboptimal and the duration of immunity is unclear. Specialized antibody and/or lymphocyte stimulation tests may be needed to assess the immune responses over time. This may help determine if VZIG is needed on future exposures to wild type varicella. The subgroups of patients who are potential candidates for immunization include: ○Children and adolescents with isolated immunodeficiency diseases and intact T cell systems (eg, humoral [immunoglobulin] deficiency diseases, neutrophil deficiency disorders, complement deficiency diseases and children who are asplenic). ○Children with acute lymphocytic leukemia who have been in remission for more than 12 months, and whose total lymphocyte count is more than 1.2×109/L. The child must not be receiving radiation therapy and maintenance chemotherapy must be withheld for at least one week before to one week after immunization. ○Children awaiting elective renal and liver transplants. Vaccination should be completed at least four to six weeks before transplantation, making the immunization of persons awaiting urgent organ transplants impractical. ○Children with asymptomatic HIV infection, whose age-specific CD4 percentage is at least 25%. Children having disorders associated with T cell dysfunction should NOT receive varicella vaccine, such as those with advanced HIV infection, severe combined immunodeficiency or post-bone marrow/stem cell transplant in the first one to two years post-transplant (or until they are immunologically reconstituted). Severe and/or prolonged disease caused by the vaccine strain may occur. To protect immunocompromised children, their susceptible healthy household contacts should be immunized with age-appropriate doses of either vaccine.

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