Even though one of the most prominent studies that raised the issue of vaccine safety—Andrew Wakefield's link between the measles, mumps, and rubella (MMR) vaccine and autism—has been thoroughly discredited, the fear of vaccines has proven remarkably malleable. In addition to MMR, fears focused on a mercury-based preservative called thimerosal. Both of those have been cleared by subsequent research, but that hasn't stopped people from questioning vaccine safety. Instead, a pattern emerged: some people doubted the exculpatory research, as others continued to blame the same cause (vaccines) while casting about for a new mechanism.

One of the more recent targets of the latter group has been the recommended vaccine schedule, which calls for a significant number of injections within the first year of age. A number of people have argued that this schedule causes a response that ultimately produces neurological damage. This idea has now taken a hit from a study published in the journal Pediatrics. The study found little correlation between vaccination schedules and performance on a battery of tests of mental performance that were administered seven years later—the few associations that did turn up indicated that the prevailing vaccination schedule actually improves mental performance.

To perform the new analysis, some researchers took advantage of a cohort of children that had been organized to check on the safety of thimerosal. Seven years after vaccination, the 1,047 kids had been given a large battery of tests that measured a variety of mental processes, from hyperactivity scores to metacognition.

It turns out that not all of these children had been given the full recommended suite of vaccines during their first seven months. Nearly half of the children fell into the "timely receipt" category, while about 20 percent never received the full suite of vaccinations; the remainder got them all, but on a delayed schedule. This provided the opportunity to compare test performance among these groups.

Factors such as parental education and income were included in the analysis to control for known influences on test performance. (Parents who kept to the vaccination schedule tended to have higher incomes and education levels, which tend to correlate with many of these test scores.) The researchers performed a binary comparison between the top and bottom 20 percent when it came to vaccination scheduling, along with a multivariate analysis.

The binary comparison indicated that, if anything, adhering to the recommended vaccination schedule improved the children's cognitive development, as those kids outscored the least timely group on 12 of the 42 tests, whereas the converse didn't occur. Most of these differences vanished in the multivariate analysis, however, which suggests they weren't very robust. Still, it's anything but support for the contention that the vaccination schedule can be harmful.

The authors caution that there could be some other demographic factor that they haven't accounted for that influences the test results. Still, if vaccines were doing anything significant here, they argue that it should have been apparent. They also note that the recommended vaccination schedule has changed since the time of the study, but it has resulted in a reduced exposure to antigens during the first year of life.

There's little question that the paper was intended to put fears about vaccine safety to rest. "These data may reassure parents who are concerned that children receive too many vaccines too soon," the authors conclude, while noting that there are some very clear medical reasons to get the vaccinations early on, such as for diseases that have higher fatality rates for younger infants.

Still, I'd place the emphasis on "may" in that sentence. The antivaccine community hasn't completely given up the mercury and MMR claims despite copious evidence, and has shown a remarkable ability to invent new reasons to question the safety of vaccines as each of their prior justifications is debunked. Still, there's almost certain to be a population that is committed to vaccinations but is unsure about the recommended schedule, and this study may go a long way towards convincing them.

Pediatrics, 2010. DOI: 10.1542/peds.2009-2489 (About DOIs).