BODY WATCH : Making Shots Timely for Younger Siblings : Immunizations: A study finds that second kids are less likely than firstborns to be up to date on vaccinations. Why? Experienced parents may worry less about disease.

Second children are less likely to receive timely immunizations than first children, according to a recent study, which also found that younger siblings of first children who are immunized late are particularly at risk for delayed vaccinations.

The study, published in the Archives of Pediatric and Adolescent Medicine, was conducted by researchers at the University of Rochester School of Medicine in New York.

Pediatricians Stanley J. Schaffer and Peter G. Szilagyi examined the immunization records of 892 children from 446 families born from 1983 to 1991 who were regular patients at the Pediatric Continuity Clinic of Strong Memorial Hospital. The clinic treats children of the poor; 75% of patients are covered by Medicaid.

Schaffer and Szilagyi examined the immunization histories of first and second children, considering the timing of DTP injections to protect against diphtheria, tetanus and pertussis; oral polio vaccine, and the vaccine that protects against measles, mumps and rubella.

They found that second children were significantly less likely than firstborns to be up to date on their immunizations in the first year of life. In families where first children were immunized later than recommended by the American Academy of Pediatrics, the delay was more pronounced.

Such delays, the authors speculate, may be because of a decrease in parental worry about disease or an increase in the logistics of getting multiple children to medical appointments.

“Inexperienced first-time parents may seek added reassurance about the health of their children,” they wrote, “and may be especially vigilant about having their children receive preventive health services.”

Experienced parents, they say, may be less apprehensive and “thus less concerned that they receive timely immunizations.”

To improve compliance with immunizations, the authors suggest flagging the charts of patients whose older siblings were delayed and furnishing parents with help such as transportation assistance in keeping appointments.