One dose of chickenpox vaccine may be insufficient to prevent school outbreaks of chickenpox, according to a report in the medical journal Pediatrics.
Outbreaks of chickenpox continue to be reported, even in highly vaccinated populations, the authors explain.
In Arkansas, a chickenpox vaccination requirement for entry into kindergarten was introduced in 2000, so by September 2003 children in kindergarten through third grade were protected. Nonetheless, a large number of cases of chickenpox still occurred in an elementary school in 2003.
Dr. Sandra L. Snow from the Arkansas Department of Health, Little Rock, and colleagues investigated the outbreak.
Among the 545 children attending the school, 96 percent who had never had chickenpox were vaccinated, the team found, including 14 children who received two doses of the vaccine.
Forty-three of 48 students (90 percent) who developed chickenpox had been vaccinated, the findings indicate, and the highest attack rate occurred in a first grade classroom where all of the students had been vaccinated.
Most of the vaccinated patients had mild disease, the researchers note, with only six percent appearing sick and only an average of two days of school being missed.
The overall vaccine effectiveness was 82 percent for chickenpox of any severity and 97 percent for moderate or severe cases, the report indicates.
None of the previously reported risk factors for chickenpox appeared to have a significant role in any cases that developed in this study.
"The effectiveness of one dose of varicella vaccine is not adequate to provide sufficient ... immunity levels to prevent outbreaks in school settings where exposure can be intense," the investigators report.
Although it is currently recommended that a second dose of chickenpox vaccine be given during an outbreak, a routine two-dose recommendation would be more effective in preventing a outbreak compared with one dose of the vaccine, Snow and colleagues conclude.