Scientists in Denmark said Sunday they had found genetic clues to explain why a small number of children have febrile seizures -- brief convulsions -- after receiving the measles, mumps and rubella (MMR) vaccine.
They stressed there was no need to scrap the MMR vaccine -- caught in a health scare in 1998 that watchdogs later declared groundless -- and described its use as a "great achievement" in saving lives.
Reporting in the journal Nature Genetics, the team found that febrile seizures occurred in roughly one in every 1,000 children who were given the MMR vaccine.
Two genetic variants came to light that pointed to a higher risk of a febrile seizure in the second week following MMR vaccination, they said.
They lie on genes that play an important role in how the immune system reacts to viral intruders.
Febrile seizures are the term for when a child develops a high temperature, loses consciousness and spasms.
The episode usually lasts for a minute or two, but apart from causing alarm for parents and the need for a checkup afterwards, is typically not dangerous.
A known but rare side effect of vaccination or viral infection, febrile seizures are different from epileptic seizures, which occur without fever.
In addition, four other variants were found that link to febrile seizures in general, and have no connection with the MMR vaccine.
These four lie on genes that help to govern ion channels, an essential communications link between nerve cells.
Children who had the highest tally of the four variants were almost four times likelier to have febrile seizures than counterparts with the least number.
The six variants are unlikely to account for more than just a small proportion of the genetic causes for seizures, the researchers said.
Further work should tease out other genetic culprits, hopefully leading to a diagnostic test to show which children could be at greater risk of a febrile seizure after a jab.
The study was unable to say whether children who had seizures did so as a result of getting a triple immunisation, as opposed to getting single shots to protect against the three diseases. The triple vaccine is the only available in Denmark.
In addition, the gene trawl was only carried out in Denmark, and it is unknown whether the same risk exists, or exists to the same degree, in children elsewhere.