When it infects a cell, the vaccine  a type that attacks rather than prevents disease  can produce thousands of copies of itself, which then spread to nearby cancer cells to kill them.

The combined vaccine against measles, mumps and chickenpox comes with a small risk of fever-related seizures in toddlers, that's not true in older children, according to a US study.

Researchers, whose report was published in Pediatrics, found no evidence that a second dose of the measles-mumps-rubella-varicella (MMRV) vaccine, given between the ages of four and six, causes fever-related seizures in those older children.

The MMRV vaccine has been available in the United States since 2005. It combines the traditional MMR vaccine with the anti-chickenpox shot so young children don't have to undergo as many shots.

After its release, the vaccine was found to carry a small risk of fever-related seizures in one- to two-year-olds, the age at which the first dose is given. Some parents now opt for two separate shots, which seems to cut the odds of a seizure.

Though the seizures are “very scary” for parents, “they are not dangerous, and they do not lead to later epilepsy or seizure disorders,” said lead researcher Nicola Klein, co-director of the Kaiser Permanente Vaccine Study Centre in Oakland, California.

The study is based on medical records for nearly 87,000 four- to six-year-olds who received the MMRV shot between 2006 and 2008. Another 67,000-plus received the MMR and varicella vaccines separately, on the same day, between 2000 and 2008.

One child had a fever-related seizure seven to 10 days after the MMRV vaccine, the time frame in which one- to two-year-olds appear to be at risk. No seizures were recorded in children who had the MMR and varicella shots separately.

Fever-related seizures are fairly common in children, according to the National Institutes of Health. About one in every 25 children will have at least one, although they most often affect toddlers.

So it's not surprising that the MMRV shot has been linked to seizures in toddlers but not older children, Klein's team said - even though the risk is quite small for toddlers as well.

“It is more common for a child to have a febrile seizure caused by a cold than by an immunisation,” Klein said.

In a 2010 study, Klein's team found that toddlers who had the combined MMRV vaccine had twice the rate of fever-related seizures than children who got the shots separately.

But even so, that translated to one additional seizure for every 2,300 doses of the MMRV shot than for the separate shots.

The vaccine is not the only one that's associated with fever-related seizures in very young children. A recent study found a small risk among babies getting the combined vaccine against diphtheria, tetanus, whooping cough, polio and Haemophilus influenzae type 2.

Klein and some of her co-researchers on the study have received past research funding from vaccine makers. - Reuters