Routine childhood varicella vaccination was implemented in the United States in 1995. From 1970 to 1994, the age at which most deaths from varicella occurred shifted from younger than 20 years to older than 20 years, but mortality rates remained highest in children younger than one year. After universal vaccination began, the incidence of the disease dropped 71 to 84 percent in five years. Nguyen and colleagues studied national death records from 1990 to 2001 to determine whether the decrease in incidence was also evident in mortality rates.
The researchers used the 1990 to 2001 national death records to identify mortality trends before and after vaccine implementation. They examined records of all varicella-related deaths, whether varicella was listed as the underlying cause or as a contributing cause. They also looked for seasonal trends and preexisting conditions that would increase the chances of severe infection or sequelae (e.g., pneumonia, central nervous system sequelae, hemorrhage, secondary infection). Patients in whom vaccination would be contraindicated (e.g., those with cancer or immune deficiency) were classified as high risk.
The absolute number of deaths between 1990 and 2001 was 1,465. From 1990 to 1994, there was an average of 145 deaths per year; after 1999, this number declined to an average of 66, marking a statistically significant drop in the death rate. Death rates in which varicella was the underlying cause dropped in all age groups younger than 50 years. Death rates in which varicella was a contributing cause also declined, though not as steeply. The greatest age-specific reduction in mortality rates was in children one to four years of age, both where varicella was the underlying cause and where it was a contributing factor. There were no significant racial or ethnic differences. High-risk conditions were present in 18 percent of deaths caused by varicella from 1990 to 1994, and 14 percent from 1999 to 2001. Varicella sequelae were common, and although the absolute number declined during the study in the age groups younger than 50 years, the percentage of deaths that involved complications in these age groups increased.
By 2001, 76 percent of children 19 to 35 months of age had been vaccinated. This age group experienced the most significant decline in varicella-related death, though mortality also decreased in infants, older children, adolescents, and adults younger than 50 years. The authors attribute this to a combination of vaccination and the effect of “herd immunity.” Overall, the authors conclude that routine varicella immunization is associated with a significant decline in varicella-related deaths. These findings support the success of the U.S. varicella vaccination program.