Disease Risk – Varicella

 

Risk to a child from Varicella if not vaccinated until after the age of 5:

 

Incidence Rates:  Varicella vaccine was licensed in the United States in 1995. In the prevaccine era, varicella was endemic in the United States, and virtually all persons acquired varicella by adulthood. As a result, the number of cases occurring annually was estimated to approximate the birth cohort, or approximately 4 million per year.[1]  In 2008, varicella incidence in the United States was 29 cases per 100,000 population.[2]

Case Fatality Rate: In the 1 – 4 age group, deaths with Varicella as an underlying or contributing cause decreased from 18.4 per year in 1990 – 1994 to 2.0 in 1999 – 2001 [3].  Based on census data [4], the annual mortality rate in the 1-4 age group was 0.011 per 10,000 population (1 in 874,000) in 1990 – 1994 and 0.001 per 10,000 (1 in 7, 726,000) in 1999 – 2001.

Rate of Long Term Sequelae or Injury: The proportion of hospitalizations with complications ranged from 51% – 54% from 1993 – 1995, and from 39% – 54% from 1996 – 2001 [3].  Permanent sequelae from varicella is rare, and numbers were not found.  For the purposes of this analysis, it is assumed that 2% of varicella hospitalizations with complications result in permanent injury for children ages 1-4.  Based on census data [4], the annual permanent-injury rate in the 1-4 age group was 0.84 per 10,000 population (1 in 119,000) in 1993 – 1995 and 0.012 per 10,000 (1 in 813,000) in 2001.

Incremental Risk in Population with Low Rates of Vaccination (if a child does not vaccinate by age 5): Varicella risk to children less than one year old excluded from the incremental risk analysis since they are not eligible for vaccination under the current guidelines.  The incremental cumulative risk of varicella permanent-injury to a child not vaccinated by age five was calculated to be 0.13 per 10,000 or 1 in 74,000.  The cumulative risk of death was calculated to be 0.046 per 10,000 or 1 in 218,000.

Incremental Risk in Highly Vaccinated Population (if a child does not vaccinate by age 5): This analysis utilizes the vaccination coverage rate in 1999 – 2001 [5], and assumes that all varicella permanent-injuries and deaths in 2001 occurred to unvaccinated children. Varicella risk to children less than one year old excluded from the incremental risk analysis since they are not eligible for vaccination under the current guidelines.  The incremental cumulative risk of varicella permanent-injury to a child not vaccinated by age five was calculated to be 0.019 per 10,000 or 1 in 408,000.  The cumulative risk of death was calculated to be 0.0052 per 10,000 or 1 in 1,931,000.

 

References:

[1] Centers for Disease Control and Prevention: Epidemiology and Prevention of Vaccine-Preventable Disease. Atkinson W, Wolfe S, Hamborsky J, McIntyre L. eds. 11th edition. Washington D.C.: Public Health Foundation, 2009.

[2] http://cdc.confex.com/cdc/nic2010/webprogram/Paper22595.html

[3] Centers for Disease Control and Prevention. Nguyen, H., Jumaan, A., Seward, J. Decline in Mortality Due to Varicella after Implementation of Varicella Vaccination in the United States.  NEJM 2005;352: 450-8

[4] United States Census Bureau. The 2010 Statistical Abstract; Table 78: Live Births, Deaths, Marriages and Divorces: 1960 to 2007.

[5] Davis MM, Patel MS, Gebremariam A. Decline in varicella-related hospitalizations and expenditures for children and adults after introduction of varicella vaccine in the United States. Pediatrics 2004;114:786–92.