The researchers examined whether children were sufficiently protected at age 6, when they were vaccinated as part of a 1999-2000 national campaign. Those children are now entering adolescence, a more high-risk age.

Dr. Matthew Snape and colleagues studied the antibody levels in blood samples from 999 adolescents ages 11 to 20 who were vaccinated against meningitis C during the campaign.

The study, published in the British Medical Journal, found that children who were 10 or older when vaccinated maintained protective levels of antibodies for longer. However, more than 20 percent of those ages 11 to 13 were lacking full protection.

A booster dose of vaccine may be needed to sustain protection against meningitis C among teenagers who were not fully protected, the researchers said.