Although the MMR vaccine has been linked to a heightened risk of a rare blood disorder, other childhood vaccines do not appear to be, researchers reported Monday.
The disorder is called immune thrombocytopenic purpura, or ITP, and it arises when the immune system destroys blood cells called platelets. That limits the blood's ability to clot, which can cause bleeding under the skin, bruising and nosebleeds—or, in rare cases, serious problems like bleeding in the brain.
Doctors have long known that the MMR vaccine against measles, mumps and rubella can cause ITP—usually triggering a mild, temporary case. It's estimated that ITP arises once for every 40,000 MMR shots.
But it had not been clear whether any other childhood vaccines might cause ITP.
"It's such a rare disease to start with, and then it would only rarely be triggered by a vaccine," said Dr. Sean T. O'Leary of Children's Hospital Colorado in Aurora, the lead researcher on the new study.
That means scientists need data on a very large number of children to weed out a possible connection to any one vaccine.
So for their study, O'Leary and his colleagues looked at medical records for 1.8 million U.S. infants and children who received routine vaccines between 2000 and 2009.
Over that time, 197 kids were diagnosed with ITP.
The researchers compared a child's risk of developing ITP within 42 days of receiving a vaccine compared with other times. And they found no evidence that any vaccine other than MMR was linked to an increased risk of ITP in young children.
The picture was different, however, with older kids, the researchers report in the journal Pediatrics.
Among 7- to 17-year-olds, hepatitis A vaccination was tied to an increased ITP risk. And in 11- to 17-year-olds, the chickenpox vaccine and the vaccine against tetanus, diphtheria and acellular pertussis (Tdap) were both linked to the blood disorder.
However, that was based on just one or two cases of ITP tied to each vaccine.
"So it's by no means conclusive that there are associations," O'Leary stressed.
ITP in children usually goes away on its own and may not require treatment.
In general, most cases affect children between the ages of 1 and 3. And O'Leary said his team can be "fairly comfortable" in saying that for that age group, there is no association between ITP and vaccines other than MMR.
There is still a question with older kids and teenagers. But ITP is so rare at those ages, O'Leary said it will be very challenging to study the potential links further.
On the other hand, since ITP is so rare, it should reassure parents.
"I don't think this is something parents should be worried about," O'Leary said.
Even with the MMR vaccine, experts stress that parents should not be deterred by the risk of ITP. In fact, children are more likely to develop ITP if they become infected with the viruses that the MMR prevents.
A child's risk of ITP after measles, for instance, is about 10 times higher than it is with the MMR, O'Leary pointed out.
Researchers aren't sure why MMR, in particular, would carry a risk of ITP. But it may be because the viruses MMR prevents are more closely associated with ITP than other viruses are.
The MMR vaccine contains weakened (rather than "killed") versions of the viruses; it's thought that in some children, the immune system's response to the vaccine viruses leads to an abnormal attack on platelets—possibly in kids who have a genetic susceptibility.
O'Leary noted that some parents worry that childhood vaccines have been little studied.
"But this study serves as an example that we really are looking at the safety of vaccines," he said.
"They are being continually monitored for significant side effects," O'Leary added, "and this (ITP) is one of the few known significant events associated with vaccination."