Vaccine-Preventable Diseases

Varicella

Varicella-zoster virus (VZV) is a DNA virus of the herpesvirus family. VZV causes a primary illness (varicella or chickenpox) and establishes latency in the sensory nerve ganglia, which may be reactivated later as herpes zoster (shingles). VZV is spread by the airborne route as well as by direct contact with the virus shed from skin lesions. The incubation period is from 10 to 21 days, usually in the range of 14 to 16 days. Infectiousness begins 1 to 2 days before onset of the rash and lasts until the last lesion has crusted. The attack rate among susceptible contacts in household settings is estimated at 65%-87%.

Epidemiology

Varicella is mainly a disease of childhood, developing in 50% of children by the age of 5 years and 90% by the age of 12 years. People from the tropics are less likely to acquire immunity in childhood and therefore have higher rates of susceptibility as adults.

Varicella has been considered to be a benign disease in otherwise healthy children aged up to 12 years. However, this group accounts for 80% to 85% of varicella-associated physician visits, 85% to 90% of hospitalizations and nearly 50% of fatal cases. The complications of chickenpox include secondary bacterial skin and soft tissue infections, otitis media, bacteremia, pneumonia, osteomyelitis, septic arthritis, endocarditis, necrotizing fasciitis, toxic shock-like syndrome, hepatitis, thrombocytopenia, cerebellar ataxia, stroke and encephalitis. Varicella increases the risk of severe invasive group A streptococcal infection in previously healthy children by 40- to 60-fold. Complications are more common in adolescents, adults and immunocompromised people, who have higher rates of pneumonia, encephalitis and death.

Varicella case fatality rates are highest among adults (30 deaths/100,000 cases), followed by infants under 1 year of age (7 deaths/100,000 cases) and then those aged 1 to 19 years (1-1.5 deaths/100,000 cases). Since 2000, a total of six pediatric deaths due to varicella were reported by the the Immunization Monitoring Program ACTive (IMPACT) system, with a range of 0-3 deaths per year. In the pre-vaccine era in the United States, adults accounted for only 5% of cases but 55% of the approximately 100 chickenpox deaths each year. In Canada, 70% of the 59 chickenpox-related deaths in the pre-vaccine years (1987 to 1997) occurred in those over 15 years of age.

Congenital varicella syndrome is rare when infection occurs before the 13^th or after the 20^th week of gestation. The risk is approximately 2% when infection occurs at 13-19 weeks of gestation. Congenital infection results in a wide clinical spectrum, which may include low birth weight, ophthalmic abnormalities, skin scarring, limb atrophy, cerebral atrophy and a variety of other anomalies. Almost one-third of affected infants die by early in the second year of life. Maternal varicella occurring in the 5 days before to 2 days after birth is associated with severe neonatal varicella in 17% to 30% of infants, with high case fatality for the newborn.

Before varicella vaccine became available, approximately 350,000 varicella cases were estimated to occur each year in Canada. However, assessing the effect of varicella immunization programs on the incidence of varicella and zoster disease is difficult because varicella infections are significantly under-reported, less than 10% of the expected cases being reported through the national Notifiable Diseases Reporting System (NDRS) annually. Furthermore, zoster is not a nationally notifiable disease. Given that the risk of having at least one reactivation to herpes zoster is 15% to 20%, there are likely a significant number of zoster cases occurring each year in Canada. Post-herpetic neuralgia lasting longer than 6 months is more frequent at older ages, occurring in 35% of those aged ≥ 50 years.

A review of data from the Canadian Institute for Health Information for 1994 to 2000 showed that over 1,550 varicella hospitalizations occur annually for all age groups. Information on pediatric hospitalized cases and deaths are available from the IMPACT system for the periods 1990 to 1996 and 1999 to 2004. These data indicate that the majority of hospitalizations occur in previously healthy children. For the most recent period, 1999 to 2004, a total of 2,058 pediatric hospitalizations due to varicella or herpes zoster were reported from 12 sites across Canada, averaging 343 hospitalizations annually. Of these cases, just over half were males, and the most affected age groups were children 1 to 4 years old (accounting for 45% of hospitalizations) and those 5-9 years old (30% of hospitalizations).

The total medical and societal costs of varicella in Canada were estimated in a multicentre study to be $122.4 million yearly or $353.00 per individual case. Eighty-one percent of this amount went toward personal expenses and productivity costs, 9% toward the cost of ambulatory medical care and 10% toward hospital-based medical care.

Benefits from varicella immunization have been seen in the United States after varicella vaccine was licensed in 1995. From 1995 to 2005, the United States recommended that children 12-18 months of age receive a single dose of varicella vaccine, with catch-up vaccination of older, susceptible children and adults. Varicella disease incidence in children 19-35 months old declined by 70%-85% in three U.S. communities that had achieved vaccine coverage levels of 75%-85%. Varicella-related hospitalizations in the United States decreased from 2.3-5 per 100,000 population in the pre-vaccine era (1993-1995) to 0.3-1.3 per 100,000 population in 2001-02. Ambulatory care visits for varicella also declined, by 59%. In 2000, the number of varicella-related deaths in the United States had declined by 78% in the < 20 year age group and by 63% in the 20-49 year age group, as compared with the pre-vaccine years, 1990-94.

Source: Canadian Immunization Guide, 7^th edition, 2006

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Links to more information

Guidelines and Recommendations

• For information about Varicella vaccine, see the Canadian Immunization Guide, 7^th edition 2006

Information Sheets

• Chickenpox Vaccine