Baby boys who have been circumcized still a standard practice in the US, even among Christians are more likely to feel the pain of vaccination.
Researchers believe that circumcision conditions baby boys to pain, and the conditioning may last for several months.
The Canadian study throws new light on the nature of pain and, on a more prosaic level, confirms that circumcision is a painful procedure, which surgeons in the US had often denied.
The study team found that pain responses fell among babies aged between four and six months if they were first given a local anesthetic before having a triple vaccine.
In all, 42 baby boys were studied; half were given a local anesthetic cream, EMLA, and half given a placebo one hour before injection. They were then all vaccinated in the thigh with the DPT vaccine, and 18 boys also had the Hib vaccine in the opposite thigh a few minutes later.
Of the 42, 30 had been circumcized, and they all showed greater pain, although many were helped by the cream.
The researchers are now calling for a properly conducted trial to decide if circumcision indeed causes long-term pain responses.