Initial report retrieved via the scientific literature on 14 July 2011. Two unidentified children (ages unknown) who had received a 5th dose of DTaP (trade names, manufacturer names, lot numbers, routes and sites unknown) in the arm on unspecified dates and were subsequently hospitalized on unspecified dates because of local reactions, both with a diagnosis of cellulitis of the upper arm. Both patients were discharged from the hospital after 2 days. Diagnostic tests and/or laboratory data performed were not provided. Outcomes were not provided. Objective of study: To assess whether the risk of medically attended local reactions after the fifth dose of the DTaP vaccine varied according to the injection site (arm versus thigh)." DTaP formulations were not identified, so possible variations in risk of reactions according to formulations could not be assessed. Method: Retrospective cohort study involving children age 4-6 years of age in the Vaccine Safety Datalink population that received DTaP between 2002 through 2006. Medically attended local reactions were confirmed by medical record review. Limitations included: unidentified DTaP vaccine formulations, and the children were not randomly assigned to received an arm or thigh injection. Providers may have elected to vaccinate in the arm or thigh on the basis of either patient characteristics, such as age, BMI, or deltoid muscle mass, or other factors such as local standards and practices. Results: 1,017 of the 2,333,616 children who were in the study had confirmed medically attended local reaction to the 5th dose of DTaP vaccine. Characteristics of age of the study population according to the site of injection were as follows: 68.2% (arm injection) and 46.9% (thigh injection) were 4 years of age. 30.0% (arm injection) and 50.9% (thigh injection) were 5 years of age. 1.8% (arm injection) and 2.3% (thigh injection) were 6 years of age. 51.1% (arm injection) and 50.6% (thigh injection) were Male. Two of the 1,017 patients experienced cellulitis of the upper arm (mentioned above), and the rest of the patients experienced the following reactions (which were not matched to any particular patient): erythema, swelling, erythema and/or swelling (of diameters reported, the range was from maximum diameter of < 5cm, 5-9 cm, 10-14 cm, and > to or equal to 15 cm), warmth, tenderness, induration, pain, itching, history of fever, rash, proximal lymphadenopathy (1.3% [arm injection], 6.9% [thigh injection]), and ulceration (1.0% of arm injection group only). The extent of the local reactions included: entire limb, extended to or past elbow or knee, or none of these. There were 13.8% 9 who experienced difficulty walking (thigh vaccinations only), and some were diagnosed with cellulitis (22.5% of arm injection group, and 23.3% of thigh injection group). Timing of the symptoms onsets ranged from day of vaccination to 4 days after vaccination (some were not specified). The clinical encounter setting included outpatient, urgent care, emergency department and 2 inpatient (identified above). The rates of risk of local reaction was 78% higher in those who had received the vaccination in the arm (47.4/10,000 vaccinations) compared with the thigh (32.1/10,000 vaccinations). Conclusion: The findings suggested that the thigh location may be considered over the arm location as an acceptable site of injection for the 5th dose of DTaP vaccination, which was noted to be currently different from recommendations by ACIP (Advisory Committee on Immunization Practices) and the Academy of Pediatrics who recommend the deltoid muscle as the preferred site in children age 1 years and older for intramuscular vaccinations. Documents held by sender: None.