Our findings provide reassuring evidence that the vaccine is not associated with acute seizure events and is safe for routine immunization in early childhood, they write in the journal Pediatrics.

An earlier version of the vaccine -- which also protects against diphtheria and tetanus -- had stoked concern, because it tripled the risk of fever-related seizures in infants. While most such seizures are harmless, they are frightening to watch for parents and could cause babies to choke on food.

The new study is the largest so far to look at seizure risk with the current vaccine, known as DTaP, which has been recommended in the U.S. since 1997. It includes data from more than 430,000 infants who were vaccinated between 1997 and 2006.

The researchers followed the toddlers until just before they turned two; most were given four vaccine shots, starting at two months of age. About 5,200 of the babies had seizures at some point, but only 112 occurred within four days of the shot.

The seizure rate during these four days was 1,208 per 100,000 infants followed over a year, compared to a baseline rate of 1,083. Statistically speaking, that difference could easily have been due to chance.

And when accounting for other factors linked to seizures, such as age and the measles-mumps-rubella vaccine (MMR), infants actually had fewer seizures in the days after their shot than at other times. Again, however, that difference could have been due to chance.

It seems that public concerns regarding vaccine safety in general has grown significantly in recent years, said James M. Baggs of the CDC, who led the new research.

Our message to parents is that this study finds very reassuring results that DTaP vaccination is not related to seizure events after vaccination, he added in an e-mail to Reuters Health.

The findings agree with earlier results from Canada, said vaccine expert Dr. David Scheifele, of the University of British Columbia in Vancouver, who was not involved in the research.

We have continued to monitor seizures after vaccination, he told Reuters Health in an e-mail. Canadian infants have enjoyed a minimal risk of febrile seizures since the switch was made to newer vaccines in 1997-8.

The few cases that still occur, he said, are more likely due to other infections than vaccination.

Medicine