Varicella (Chickenpox)

Varicella, also known as chickenpox, is a very common and highly infectious childhood disease that is found worldwide. Symptoms appear 10 to 21 days after infection and last about 2 weeks. The defining symptom is a characteristic blister-like rash, which can cause severe irritation. Most children have a relatively mild illness, but severe illness may occur in adults and people with depressed immunity because of existing illness or because of a treatment that they are receiving (e.g. chemotherapy). The National Advisory Committee on Immunization (NACI) recommends immunization against varicella.

About Varicella

Agent of disease

Varicella is a generalized viral disease caused by varicella-zoster virus (VZV), a DNA virus of the herpesvirus family.

Symptoms

The first noticeable symptom is the onset of a slight fever, which is usually followed by some mild constitutional symptoms, such as a headache, runny nose and a general feeling of malaise.

The defining symptom of varicella is the eruption of skin lesions on all areas of the body, including on the scalp and on the mucous membranes of the mouth and upper respiratory tract. These fluid-filled lesions or vesicles occur in "crops", so that several stages of old and new lesions will be present at the same time. Varicella varies in severity from very mild, with just a few spots, to severe, with fever and a widespread rash. The more severe form is seen more often in adults.

Following the initial varicella illness, varicella-zoster virus establishes latency in the sensory nerve ganglia, which may be reactivated later in life as herpes zoster (also known as shingles).

Period of communicability

It may take 10 to 21 days for symptoms to appear after infection has occurred. A person is most contagious from 1 to 2 days before to shortly after the onset of rash. Contagiousness persists until the skin lesions crust over.

How it is transmitted

Varicella is solely a human disease and is one of the most readily infectious illnesses. The virus can be spread by direct contact with fluid in the lesions or through the airborne spread from the respiratory tract. The attack rate among susceptible contacts in household settings is estimated at 65%-87%.

Worldwide Distribution

Varicella is very common worldwide and in densely populated metropolitan communities. In the pre-vacccine era, most cases of varicella occurred in children, with 50% of children becoming infected by the age of 5 years and 90% by the age of 12 years. With the implementation of universal childhood immunization programs, a greater number of cases are occurring in adolescents and adults, although the overall incidence in this age group has been greatly reduced. Some data suggest that people from the tropics are less likely to acquire immunity in childhood and therefore have higher rates of susceptibility as adults.

Prevention and control

Varicella can be prevented by immunization. The National Advisory Committee on Immunization (NACI) recommends that healthy children 12 months to 12 years of age should receive two doses of varicella-containing vaccine (univalent varicella or MMRV) for primary immunization. For further information about varicella vaccine and its use in adolescents, adults and special populations, please refer to the most recent version of the Canadian Immunization Guide.

Cases should be reported to the local health authority. In addition, isolation can mitigate the spread of illness:

• People who have or suspect that they have the disease should avoid public places for at least 5 days after the first crop of lesions appears. If it is necessary to see a physician, call first. There are numerous people to whom varicella can be life-threatening, including newborns, people with depressed immunity and individuals with serious illnesses like cancer, any of whom could be visiting the physician's office.
• Keep infected children away from school or public places until the last lesions have scabbed over. Wash or disinfect articles that may have been soiled by the vesicle fluid or by any discharge from the nose or throat.

Contacts, especially children, must have their immunization status verified. If immunization status is incomplete and no contraindications are identified, necessary doses of vaccine should be given.

Varicella Epidemiology in Canada

In the pre-vaccine era, 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 is difficult because varicella infections are significantly under-reported, less than 10% of the expected cases being reported through the Canadian Notifiable Disease Surveillance System (CNDSS) annually.

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 Immunization Monitoring Program, ACTive (IMPACT) for the periods 1990 to 1996 and 1999 to 2009. These data indicate that the majority of hospitalizations occur in previously healthy children. For the most recent period, 1999 to 2009, a total of 2,297 pediatric varicella related hospitalizations were reported from 12 sites across Canada, averaging 208 hospitalizations annually for children age up to 16. Among these cases, children at pre-school ages were affected mostly and accounted for 14% (age < 1) and 66% (age 1-6) of the total hospitalizations, respectively. Since the public funded vaccine programs began in 2004 in Canada, the annual hospitalizations of varicella dropped from 288 (1999 to 2004) to 114 (2005-2009).

The complications of varicella 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.

Congenital varicella syndrome is rare when infection occurs before the 13th or after the 20th 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.

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). In the pre-vaccine era in the United States, adults accounted for only 5% of cases but 55% of the approximately 100 varicella deaths each year. In Canada, 70% of the 59 varicella -related deaths in the pre-vaccine years (1987 to 1997) occurred in those over 15 years of age. Since 2000, a total of 11 pediatric deaths due to varicella were reported by IMPACT with a range of 0-3 deaths per year.

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.

Varicella Surveillance in Canada

In Canada, limited surveillance information on varicella is available through Canadian Notifiable Disease Surveillance System (CNDSS) and Immunization Monitoring Program, ACTive (IMPACT).

Varicella Data

The following two tables contain recent data on the number of reported cases and incidence of rubella in Canada from 2005 to 2008. For further surveillance data, please see the Notifiable Diseases On-Line webpage as well as the Publications section below.