More than one quarter of two-year-olds in the US may not be in compliance with the recommended vaccine schedule, government researchers reported.
In a study of more than 17 000 US children between the ages of 19 and 35 months, researchers at the Centres for Disease Control and Prevention (CDC) found that 28 percent had either missed a dose of a recommended vaccine altogether, or had gotten doses at the wrong age or at wrong intervals.
For childhood vaccines to be most effective, they must be given at particular ages and in specific intervals. For example, children should get their first MMR shot against measles, mumps and rubella no earlier than 12 months of age, but before they are 15 months old; a second dose should be given when they are between four and six years old.
The CDC researchers found that of children who were not compliant with the recommended vaccination schedule, two thirds had missed a vaccine dose entirely.
In other cases, children received a dose too early, lead author Dr. Elizabeth T. Luman and her CDC colleagues report in the American Journal of Preventive Medicine.
For example, 14 percent of the more than 17 000 toddlers in the study had gotten their third hepatitis B vaccine dose before the recommended age of six months old.
When a child gets such an "invalid" vaccine dose, the official advice is that the shot be repeated at the proper time, according to Luman.
Past studies on childhood vaccination rates have focused on whether children had gotten all of their vaccination doses. The new findings show that when other factors are taken into account - such as whether children got their doses at the right time - vaccine compliance is even lower than previously thought.
"We were surprised that accounting for other recommendations affected results so much," Luman said in a statement.
It's not clear what impact these cases of non-compliance may have had on children's health. The recent, highly publicised measles outbreaks in places like San Diego and Wisconsin affected people who had not received the MMR at all, according to the CDC.
Luman acknowledged that in the real world, parents sometimes find it hard to bring their children to the doctor at the scheduled vaccination times. And doctors will sometimes administer a shot earlier than recommended rather than tell parents to come back for another appointment.
"I have two kids, and from a parent's point of view, it can be hard, logistically, to come in when scheduled," she said. "But I know how important timely vaccination is for the health of my children and my community."