The order in which babies receive their scheduled vaccinations really can make a difference to how much pain they feel when they get the shots, a Canadian study has found.

Researchers from Toronto's Hospital for Sick Children found that babies who are first given the DPTaP-Hib combo vaccine (which protects against diphtheria, polio, tetanus, pertussis and Hemophilus influenza type b) followed by PCV, the pneumococcal conjugate vaccine, seem to experience less pain than those who get their shots in the opposite order.

It's common for babies to receive more than one shot at a single doctor's visit. But many pediatricians have complained that sometimes parents refuse to have a child vaccinated in because of the pain caused by multiple vaccines. Therefore, reducing that pain could increase immunization rates and prevent a resurgence of infectious diseases, the authors of this study say.

"Because some vaccines cause more pain than others, the order in which they are given may affect the overall pain experience," the authors write in the May issue of Archives of Pediatrics & Adolescent Medicine, a theme issue on vaccines.

Dr. Moshe Ipp at SickKids led a team that studied 120 healthy infants aged two to six months who were getting routine immunizations at a local pediatric clinic in 2006 or 2007.

Sixty infants received the PCV before the DPTaP-Hib vaccine, while 60 received the DPTaP-Hib vaccine first. All of the babies received the shots in their thighs while they were being held by one of their parents.

The procedure was videotaped and pain was assessed on a scale that considered the infant's facial expression, crying and body movements after vaccination. Parents were also asked to rate their children's pain levels on a scale of zero to 10.

The researchers found that difference in pain expression was about 25 per cent less among the babies given DPTaP-Hib first, compared to those given the PCV.

"Infants given the less painful DPTaP-Hib vaccine first followed by the more painful PCV experienced less pain overall when compared with those given the vaccines in the reverse order," the authors write, noting that "pain increased from the first to the second injection, regardless of the order of vaccine injection."

The authors speculate that giving the more painful injection first may activate the pain processing centres in the brain, which leads to a more intense pain signal in response to any shots given afterward.

They note that simply varying the order of vaccine administration is a simple, effective, and cost-freeway to reduce the baby's pain and improve the immunization experience for parents and doctors.

"On the basis of these results, we recommend that the order of vaccines be the DPTaP-Hib vaccine followed by the PCV," they conclude.

The researchers aren't sure why different shots hurt more than others, but they speculate that it may be related to the pH levels, or acidity, of the different vaccines.

They add that vaccine manufacturers should also work on making vaccine formulations that are less painful.