Evaluation of the impact of an HMO’s varicella vaccination program on incidence of varicella. Mullooly JP, Maher JE, Drew L, Schuler R, and Hu W. Vaccine 2004;22(11-12): 1480-1485
Did the number of new cases of chickenpox (varicella) decrease after the introduction of a chickenpox vaccination program in an HMO?
This study examined the number of new cases (or incidence) of chickenpox in children 0 to 18 years of age during the time period 1976–1999. Varicella vaccination began to be introduced in 1995 at the Kaiser Permanente Northwest (KPNW) HMO in Portland, Oregon.
Researchers looked at how many children had been immunized in different age groups and compared that number to the number of chickenpox cases in those same age groups for 1996 through 1999. They also contrasted this to the period before vaccine was available (1976-1991).
What was the impact of the chickenpox vaccination program? To answer that question, the researchers sought if any reduction in chickenpox cases (in the different age groups) could be related to the increasing number of those that were immunized. If the HMO received fewer chickenpox cases as more children were immunized, it could be shown that the vaccination program was effective.
In the 15 years (1976-1991) studied to determine the occurrence of chickenpox, chickenpox cases varied by season. During 1996 through 1999, the number immunized increased beginning with young children and increased as more and more young children received vaccine plus those who had vaccine previously became older. There were fewer cases of chickenpox as the number of immunized children in each age group increased.
The researchers estimated that for 2-year olds chickenpox decreased by 68.3%. The decrease in the 2-year olds was greater than the other age groups because more of them had been vaccinated (73%). Overall, the number of new chickenpox cases for all children decreased by an estimated 34.2% during the 1996–1999 after the vaccination program was implemented.
The chickenpox vaccination program significantly reduced the incidence of chickenpox among all children attending the clinic. As more children were immunized the rates of chickenpox decreased. As these children became older, the rates of chickenpox were reduced in the older children as well. In the oldest group of children, 10-18 years of age, the impact of vaccine had not yet been observed.
This study confirms the effectiveness of the chickenpox vaccine and shows the increasing impact of the vaccine as more children are immunized. Immunization of the older children at the same time as the younger children might have reduced the number of new chickenpox cases among the older children more rapidly.