Vaccination Shot to Arm Causes Amplified Local Reaction

The researchers assessed if the occurrence of local reactions to intramuscular (IM) administered vaccines vary by injection site (arm versus thigh) in children 1-6 years of age.

The results showed that the study cohort received a total 6 million IM vaccines during the study period. For inactivated influenza and hepatitis A vaccines, local reactions were relatively uncommon, and there was no difference in risk of these events with arm versus thigh injections. But for the DTaP vaccine, 72,795 shots were given in the thigh, with 184 reactions, compared with 125 reactions to the 18,715 arm shots, a relative risk (RR) of 1.88. For toddlers 35 months and younger, the RR was also 1.88, but with wider confidence intervals; for children ages 3 to 6, the RR was 1.41, although the difference was not statistically significant.

A secondary analysis examined combinations of two vaccines, as long as the shots were given in the same muscle – both in the thigh or both in the arm. The analysis of two-vaccine combinations found only one statistically significant difference by injection site: DTaP and hepatitis A vaccines, when injected into the arm together, were twice as likely to lead to a medically attended reaction, with a RR of 2.13.

“Injection in the thigh is associated with a significantly lower risk of a medically attended local reaction to a DTaP vaccination among children 12 to 35 months of age, supporting current recommendations to administer IM vaccinations in the thigh for children younger than 3 years of age,” concluded lead author Lisa Jackson, MD, of the Group Health Research Institute, and colleagues.

DTaP immunization is a combination vaccine that protects against three bacterial illnesses: diphtheria, tetanus, and pertussis (whooping cough). DTaP may cause mild side effects, which usually only last a few days, including fever, crankiness, soreness at the injection site, vomiting, and decreased appetite. A warm, damp cloth or a heating pad may help to reduce soreness. Frequently moving or using the arm or leg that has received the injection is recommended and often reduces the soreness.

Related Links:
Group Health Research Institute
Kaiser Permanente Southern California
US Centers for Disease Control and Prevention