NEW YORK (Reuters Health) Aug 12 - During an outbreak of varicella in an elementary school in Arkansas, students who were vaccinated once or twice still came down with the illness, though their symptoms were mild.
Furthermore, vaccine effectiveness was no better among those who received two inoculations prior to the outbreak than those who received only one, researchers report in the Pediatric Infectious Disease Journal for August.
"As our understanding of the effectiveness of one dose of varicella has developed (with) postlicensure studies, additional studies will be needed to assess the impact of the routine two-dose recommendation by the ACIP (Advisory Committee on Immunization Practices)," lead author Dr. Philip L. Gould and his associates write.
In June 2006, the ACIP recommended a second dose of varicella vaccine for children at age of school-entry and a catch-up second dose for all ages. According to the authors, an outbreak of chickenpox at the school in Arkansas was the first chance to investigate the effectiveness of this second dose. They note that the outbreak was the largest reported in recent years in the US.
Children were considered vaccinated if their first or second dose was received at least 42 days before rash onset. Among children with no history of varicella, vaccination coverage was 97% for at least one dose and 39% for the second dose. In addition, the report indicates, 15% had a positive history for varicella.
There were 84 cases of varicella between September 1 and December 18. Twenty-five cases had received two doses, 53 had received one dose, and 6 were unvaccinated with a history of the disease. Overall disease severity was mild, the authors note.
Dr. Gould, from the Centers for Disease Control and Prevention in Atlanta, and his team estimated vaccine effectiveness as 85.4% for one dose and 89.1% for two doses, not a significant difference. The vaccine was 100% effective against severe disease.
"It is possible," they contend, "that higher two-dose coverage at the school would have prevented the second outbreak from occurring, or that high coverage with two doses of varicella vaccine will reduce the number of outbreaks in the population."
Pediatr Infect Dis J 2009;28:678-681.