The order in which vaccine injections are given affects infants’ pain response, new research shows.

During routine childhood immunization, infants given an injection of DPTaP-Hib vaccine before pneumococcal vaccine (PCV) experience less pain than those given the vaccines in the opposite order.

The finding suggests that when two vaccines are to be given, the less painful one - in this case DPTaP-Hib - should be given before the more painful one -in this case PCV.

“To our knowledge, the effect of varying the order in which vaccines of different degrees of painfulness are given has not been previously examined,” Dr. Moshe Ipp, from The Hospital for Sick Children, Toronto, and co-researchers note in the Archives of Pediatrics and Adolescent Medicine.

Their trial included 120 healthy infants, 2 to 6 months of age, who were seen at a pediatric clinic in Toronto. The infants’ pain was assessed using a standard behavioral pain scale and by parents’ ratings.

Pain after both vaccine injections was significantly lower in the group of infants who were given DPTaP-Hib first and then PCV. This was seen with both the pain scale and the parents’ assessment.

As to why the order of vaccinations affects the pain response, the investigators suggest that when the more painful injection is given first, it “focuses the infant’s attention on the procedure and activates central and peripheral mechanisms of pain processing that together result in amplification of the pain signal during subsequent injections administered immediately thereafter.”

SOURCE: Archives of Pediatrics and Adolescent Medicine, May 2009.