In our country, we think the doctor should have all the medical records, said Dr. James McElligott, a pediatrician at the Medical University of South Carolina who worked on the study. I like the idea of putting the ownership back in Mom's hands and empowering her a little bit.

When parents kept a so-called shot card, their child's odds of being up-to-date on vaccinations rose by more than half.

Experts agree that kids aren't getting the vaccines they need, from those for measles, mumps, and rubella to those for polio and the flu. Tapping into national vaccination data, McElligott and his colleague Dr. Paul Darden found that about 81 percent of 2-year-olds were considered up-to-date according to national guidelines.

But no one has figured out the best way to meet national goals. One potential solution is using shot cards.

In their study, McElligott and Darden, who is now at the University of Oklahoma, found that about 40 percent of the toddlers had a shot card, and 84 percent of these had up-to-date vaccinations. By contrast, only 79 percent of the children without a card had all their shots.

The timing of vaccinations is important because toddlers' immune systems have not yet matured enough to fight off many diseases, said Dr. Robert M. Jacobson, a professor of pediatrics at the Mayo Medical School in Rochester, Minnesota, who was not involved in the study.

In principle, experts would like for at least 95 percent of children to have up-to-date-vaccinations, Jacobson added. But in the real world, numbers fall well short of that. In some poor communities, it's about 50 percent.

The new study found that shot cards were particularly effective when mothers had little education or had many children, and when a child had multiple health-care providers.

McElligott said the findings strengthened the case for holding on to your child's vaccination records.

It turns out that not only does it make a big difference, but it seems to work in the people who need it the most, he said.

Pediatric societies already recommend using the shot card as a way to ensure that children get vaccinated. But some states have been more hesitant to adopt the card than others. In Indiana, for example, the researchers found that less than one in five kids had it, while in Kansas, more than half did.

With an ever-expanding list of shots, it may be difficult for parents to keep track of which vaccines their kids already have and which ones they still need.

You need a vaccination record in part to remind yourself and in part to share with providers when you move, said Jacobson.

From the study, however, it is impossible to determine whether the card itself led to more vaccinations. It could be that the kinds of parents who are organized enough to keep their own records are the kinds of parents who remember to take their kids to the doctor regularly-or vice versa.

Nonetheless, Jacobson urges parents and providers to use the card. The fact is that it doesn't have a downside and it's cheap, he said.