lthough two widely used vaccines can make children run fevers that in rare cases may cause seizures, the seizures do not lead to lasting problems, researchers are reporting today in the largest study ever done on the side effects of vaccination.

The findings come against a backdrop of recurring fears, in the United States and abroad, that those rare fever-related seizures may be linked to later autism and developmental problems. The fears are unfounded, the study concluded.

"This study is very reassuring," said Dr. Karin Nelson, a child neurologist at the National Institute of Neurological Disorders and Stroke, who was not involved in the research. "It is very large, it confirms the studies that we have done before, and it goes farther in following up children to show there are not additional problems after vaccination."

The study, being published today in The New England Journal of Medicine, looked at medical data for 639,000 children, about 2 percent of all the nation's children under the age of 7. It found that those who got the vaccines and then experienced seizures had no more chance of further seizures or neurological problems later than those who did not have seizures after being vaccinated.

Two vaccines were studied: one given to prevent diphtheria, tetanus and pertussis (or whooping cough), and so called DTP, and the other to prevent measles, mumps and rubella (or German measles), called MMR.

The study was far larger than previous such work, and, in their effort to determine whether seizures caused by high fevers after vaccination, known as febrile seizures, had any long-term effects, the researchers followed a majority of the children past the age of 6. That was typically five years after the first of the vaccinations was given, and three to four years after the second.

"We didn't find any evidence of later problems up to that age, so we think we would have spotted it if something else had occurred," said the lead author, Dr. Robert L. Davis of the University of Washington in Seattle, who worked with the Centers for Disease Control and Prevention in producing the study.

Children in the study were vaccinated from 1991 to 1993. The form of DTP vaccine that they received, which apparently caused rare febrile seizures, is no longer used in the United States. It has been replaced by a form called DTaP, which studies suggest has one-third or fewer of the side effects of the older type.

Previous studies had shown that in rare cases DTP and MMR caused children to have high fevers, as well as febrile seizures that lasted from a few seconds to 15 minutes. But febrile seizures are fairly common in children even without vaccination, occurring in about 4 percent of them by the time they have passed through adolescence.

"The seizures come on when fever is high," Dr. Nelson said, "and it is a terrifying event for the families to have a young child convulse and then look as if they are dead. But terrifying as they are, they are not dangerous in terms of causing problems later."

Of all the children in the study, only 63 had experienced febrile seizures after being vaccinated; 521 had had such seizures as a result of illnesses not related to vaccination.

Seizures not connected to a fever are much more worrisome than febrile seizures, Dr. Nelson and Dr. Davis said. These seizures are sometimes linked to disorders like epilepsy and mental retardation. The study found that between those children who got febrile fevers and those who did not, the rate of nonfebrile seizures was no different.

"This study," Dr. Davis said, "is useful because it gives a clearer picture, and greater confidence that vaccines are not associated with nonfebrile seizures."

Concern and debate over the safety of DTP and MMR have flared in the United States, Britain and elsewhere recently, and periodically during the last 30 years. Indeed, fears about them have caused some parents to avoid vaccinating their children, a decision with sometimes disastrous results. Large outbreaks of disease have followed the drops in vaccination in several countries, and smaller outbreaks of measles and whooping cough have occurred in some American communities.

For example, studies have shown that anti-vaccination arguments by some doctors in Britain three decades ago brought a huge drop in vaccination there, to 30 percent of all children from 80 percent in just a few years. What resulted was an epidemic of 100,000 cases of whooping cough, with at least 34 deaths.

In Japan, a similar drop in vaccination resulted in a 1979 epidemic of 13,000 cases and 41 deaths.

Dr. Robert Chen, an epidemiologist at the Centers for Disease Control and Prevention, said such epidemics were an unintended consequence of "mature vaccination programs" that have nearly eradicated many diseases in developed countries, creating a sense of complacency in doctors and parents who have no personal experience with the diseases the vaccines can prevent.

Many parents today do not realize how serious diseases like measles and whooping cough can be. Around the world, in countries where vaccines are not available, almost 900,000 children a year die of measles.