Parents often give their infants acetaminophen (Tylenol) after immunizations and booster shots. As it turns out, this may do more harm than good. The medication may actually weaken infants’ immune systems, according to research from the Czech Republic.

Acetaminophen is sometimes given to infants and small children following shots to prevent fever, a common reaction to vaccinations. This study examined the effects of acetaminophen on immune response after initial and booster vaccinations.

Two hundred twenty-six children from 10 different medical centers were randomly selected to receive three doses every six to eight hours for 24 hours after immunizations. Just 42 percent of these children experienced fevers (100.4 degrees F or above), compared to 66 percent of a control group of 233.

When it was time for booster shots, parents were asked to repeat the process. There was a lower incidence of fever in both groups—36 percent for those who took acetaminophen and 58 percent for those who did not.

Children who received acetaminophen , however, had fewer antibodies, indicating more susceptibility to disease. In the studied children, pneumonia, hepatitis B, whooping cough, polio, diphtheria and tetanus antibodies were fewer following vaccinations and boosters with acetaminophen use.

Antibodies are proteins in the immune system (immunoglobulins) that bind to harmful pathogens (viruses) to keep them away from healthy white blood cells. Without these antibodies, the body’s immune system is more susceptible to disease.

When vaccines fail to protect against diseases, the health of the entire community is at stake because viruses are more easily spread.

“This point has implications, especially for Haemophilus influenzae, for which higher and sustained antibody concentrations are needed (to reduce transmission),” says Dr. Robert T Chen of the Centers for Disease Control in Atlanta, “…and for pertussis, the bacterial vaccine-preventable disease that is least controlled.”

Although they’re not entirely sure why children taking acetaminophen had fewer antibodies, doctors plan to explore the issue further. They stress the importance of this information, and advise against unnecessary acetaminophen use.

“(A)dministration of antipyretic drugs at the time of vaccination should nevertheless no longer be routinely recommended without careful weighing of the expected benefits and risks,” wrote lead author Dr. Roman Pryula of the University of Defense in Hradec Kralove, Czech Republic.

The study appears in this month’s online issue of The Lancet.