Acetaminophen, which is the active ingredient in Tylenol, weakens the immune system towards vaccines in infants, as per a new study.

Infants often get a mild fever after getting vaccines. Some pediatricians use acetaminophen in a routine to prevent vaccine-related fever.

An international research team led by Roman Prymula, MD, of the University of Defense, Czech Republic, found that its was not such a brilliant idea.

The study found that over-the-counter pain killers hamper the vaccine-induced immune responses.

The effect of other fever-reducing drugs is not clear as yet but the researchers warn doctors and parents to try to avoid using acetaminophen or other fever-reducing drugs to resist vaccine- related fever.

Robert T. Chen, MD, chief of vaccine safety for the CDC's National Immunization Program said, "A fever is likely a critical part of the immune response to any infection or vaccination, so dampening fever after immunization is probably not a good idea for most kids".

He adds that the issue is never about the child having fever but whether the child is sick. Chen says that if the child is happy after the vaccine then it is the best situation but if he looks sickly then he should be immediately taken to a doctor.