Chickenpox Vaccine Less Effective After First Year

The effectiveness of the vaccine against chickenpox, or varicella, decreases significantly after the first year, new research indicates. Moreover, the vaccine is less effective in the first year in children younger than 15 months old than in older children. However, cases of breakthrough infection are usually mild.

The effectiveness of the vaccine against chickenpox, or varicella, decreases significantly after the first year, new research indicates. Moreover, the vaccine is less effective in the first year in children younger than 15 months old than in older children. However, cases of breakthrough infection are usually mild. These findings are based on a study of 339 children diagnosed with chickenpox and 669 children without the disease drawn from group practices in southern Connecticut. The report is published in the Journal of the American Medical Association. Dr. Eugene Shapiro, from Yale School of Medicine in New Haven, Connecticut, and colleagues found that although the overall vaccine effectiveness was 87 percent, this figure did not hold steady throughout the postvaccination period. In the first year after vaccination, effectiveness was 97 percent, whereas in subsequent years it was only 84 percent. When the vaccine was given at 15 months of age or older, the effectiveness in the first year was 99 percent. In contrast, giving the vaccine to younger children resulted in a first year effectiveness of just 73 percent. After the first year, however, the age at vaccination did not seem to influence effectiveness. Most of the chickenpox cases that occurred were mild, the authors note, and the vaccine’s overall effectiveness against moderate or severe cases was 98 percent. “This study indicates that at least through the first 8 years after vaccination, the overall effectiveness…varicella vaccine remains good, although breakthrough varicella is not rare,” the researchers state. “It is important to monitor closely the incidence of varicella and the effectiveness of the vaccine over time to determine if a booster dose is needed to improve its effectiveness,” they add. (SOURCE: Journal of the American Medical Association, Reuters Health, Feb, 2004)