Chickenpox vaccine may need boosters

An outbreak of chickenpox among a group of children in New Hampshire, USA shows that the virus that causes chickenpox can be highly infectious even among those who have been vaccinated, according to a recent report.In seven previous studies of the effectiveness of the varicella vaccine conducted since it was licensed, the effectiveness was 71 to 100 percent against disease of any severity and 95 to 100 percent against moderate and severe disease. The researchers from the Disease Control and Prevention in Atlanta, investigated an outbreak of chickenpox, which is caused by the Varicella virus in a population of children with a high proportion of vaccination who were attending a day-care centre in a small community in New Hampshire. A total of 88 parents returned a questionnaire that aimed to gauge prior chickenpox illness and vaccination among the children. 25 children came down with chickenpox between December 2000 and January 2001. The index case occurred in a healthy child who had been vaccinated three years previously and who infected more than 50 percent of his classmates who had no history of chicken pox. At the time of the outbreak, roughly 73% of children old enough for chickenpox vaccine had received it. The effectiveness of the vaccine was found to be 44% against disease of any severity and 86% against moderate or severe disease. Children who had been vaccinated 3 years or more before the outbreak were at greater risk of vaccination failure than those who had been vaccinated more recently. Immunity against chickenpox weakens as time passes after vaccination. However, the reasons for the poor performance of the vaccine are not apparent. The findings in this investigation raise concern that the current vaccination strategy may not protect all children adequately. Vaccination remains the most effective strategy for protecting children and adults against illness and death due to chicken pox. The investigators point out that the illness is much less of a threat today than it was before the era of chickenpox vaccination. Current guidelines call for one dose of chickenpox vaccine for children between the ages of 1 and 12 years and two doses of vaccine for people over 13. It was suggested that a second dose of chickenpox vaccine should decrease the number of children who have vaccine failure and might also prevent waning immunity, if it does indeed currently occur. The time for exploring the possibility of routinely administering two doses of varicella vaccine to children seems to have arrived.