Post-Vaccine Tylenol May Harm Immune Response

Oct. 16, 2009— -- Giving acetaminophen to babies to prevent fever after routine inoculations reduces the immunity that some common vaccines provide, Czech researchers said.

After initial vaccination at 3 to 5 months, infants who received acetaminophen -- commonly known by the brand name Tylenol -- had reduced immune responses to vaccines against pneumococcal disease, Haemophilus influenzae type b (Hib), diphtheria, tetanus, and pertussis (whooping cough), according to Dr. Roman Prymula of the University of Defence in Hradec Kralove, Czech Republic, and colleagues.

After booster doses at 12 to 15 months, children who received acetaminophen to ward off fever still had reduced immune responses to the vaccines against pneumococcal disease, Hib, and tetanus, the investigators reported in the Oct. 17 issue of The Lancet.

"To our knowledge, such an effect of prophylactic [acetaminophen] on post-immunization immune responses has not been documented before," the researchers said.

Although the clinical relevance of the findings isn't clear, they said routine administration of fever-reducing drugs at the time of vaccination "should nevertheless no longer be routinely recommended without careful weighing of the expected benefits and risks."

Although fever after vaccination is not unusual and is generally benign, it can be a concern for parents. As a result, many parents now give their children acetaminophen, particularly after a pertussis shot.

Prymula and his colleagues conducted two randomized controlled trials — one for the initial vaccine dose and one for the booster dose — to explore the effect of preventive acetaminophen on fever and on the children's immune response.

At 10 centers in the Czech Republic, infants were randomized to receive three acetaminophen doses every six to eight hours the day after vaccination (226 patients) or no treatment (233 patients).

For most, the acetaminophen did its primary job. The babies who received it had a significantly lower rate of fever, defined as 100.4°F or higher, after both the initial round of immunizations (42 percent versus 66 percent) and the booster doses (36 percent versus 58 percent).