The study looked at 459 infants randomly assigned to receive three prophylactic paracetamol doses every six to eight hours, or no paracetamol, in the first 24 hours after a routine childhood jab for pneumococcal disease, tetanus or polio.
After the initial vaccinations, 42 per cent of infants in the paracetamol group had temperatures above 38 degsC compared with 66 per cent of infants in the placebo group.
But infants given paracetamol had lower antibody geometric mean concentrations, a measure of immune response, than infants in the placebo group.
The researchers, led by Professor Robert Prymula from the faculty of Military Health Sciences, University of Defence, Hradec Kralove, Czech Republic, said: 'To our knowledge, such an effect of prophylactic paracetamol on post-immunisation immune responses has not been documented before.
'The interference could result from the prevention of inflammation.'
The researchers said paracetamol may cut immune responses by interfering with the early phase of post-vaccination immune reactions that require interaction between dendritic cells, T-cells and B-cells.
They suggested paracetamol should no longer be routinely recommended without careful weighing of benefits and risks.
Oxford GP Dr Anthony Harnden, a child infections expert, said: 'Infants should not routinely be given paracetamol following immunisation - they should be given paracetamol only if they develop a high fever or prolonged irritability.'
But Dr George Kassianos, RCGP immunisation spokesman, said the study involved vaccines not used in the UK.
'Post-vaccination fever should be managed with cooling the room, tepid sponging and dressing the child lightly. Paracetamol has a place if the child develops a temperature above 38 degsC.'
