This study was designed as a randomized controlled trial comparing simultaneous and sequential administration of intramuscular vaccines in infants.

The study was carried out in the pediatric outpatient vaccination clinic of a private hospital in Istanbul, Türkiye, between 1 September and 15 November 2021.

Infants were eligible if they were younger than 12 months, required two routine intramuscular vaccines during the same visit, and were accompanied by a parent or legal guardian able to provide informed consent.

Infants were excluded if they had received analgesics within six hours prior to vaccination, received vaccines other than hepatitis B, pentavalent combination vaccine (DTaP-IPV/Hib), or pneumococcal conjugate vaccine, or had known neurological, developmental, or motor impairments.

Infants were randomly assigned in a 1:1 ratio to either the simultaneous injection group or the sequential injection group using computer-generated randomization (www.randomizer.org). Allocation was completed prior to the vaccination procedure.

A total of 150 infants were assessed for eligibility; 40 did not meet inclusion criteria. The final sample included 110 infants (55 per group). No participants were lost to follow-up.

Inclusion Criteria:

• The following criteria were determined: the child was ordered to receive two of the routine childhood IM vaccines (hepatitis B, quintuple combination vaccine, pneumococcal vaccine), the parents were willing to participate in the study, the child had not received painkillers in the last 6 hours, and the child did not have mental and motor retardation.

Exclusion Criteria:

• It was determined that the child should be 1 year of age or older, the vaccines other than the quintuple mixed vaccine, Pneumococcal vaccine and Hepatitis B vaccine should have been ordered by the physician.

Institutional policy does not allow

All assessments were performed by the same experienced pediatric nurse who was not part of the research team and was unaware of the study hypotheses.