Worries that cramming multiple vaccinations into the first months of life slows brain development have no basis in fact, researchers said.
There was no evidence of neurodevelopmental delays or deficits associated with on-time vaccination in an intensively studied cohort of more than 1,000 children, according to Michael J. Smith, MD, and Charles R. Woods, MD, of the University of Louisville, in the June issue of Pediatrics.
"These data may reassure parents who are concerned that children receive too many vaccines too soon," wrote Smith and Woods.
Although the link between certain childhood vaccines and autism has now been thoroughly debunked, the sheer number of vaccines that infants are supposed to receive is "another area of parental angst," the researchers noted.
"Although the number of parents who completely refuse vaccines remains low, many families are requesting alternative immunization schedules that space out and delay receipt of the recommended childhood vaccines," they wrote.
To determine whether these concerns may be valid, Smith and Woods analyzed data from the Vaccine Safety DataLink study, in which 1,047 children between the ages of 7 and 10 underwent 42 in-depth neuropsychological tests.
The study, which began in 1993, was originally conceived to examine whether thimerosal-containing vaccines were associated with later development of autism.
Children were enrolled from 1993 to 1997. At that time, the recommended vaccination schedule included two doses of hepatitis B vaccine, three for diphtheria-tetanus-pertussis, three for Haemophilus influenzae B, and two for polio during the first year.
Just under half the cohort (491) received the entire series in a timely way, defined as receiving each dose within 30 days of the recommended age.
There were some significant differences between the children who received timely vaccination and those who did not. Those with on-time vaccinations were slightly younger at the time of neuropsychological assessment (mean 9.2 versus 9.4 years), their mothers were better educated (56.8% with college degrees versus 46.8%), and they came from higher-income households (448% of poverty level versus 380%), all with P values of 0.001 or less.
Not one of the 42 tests showed better outcomes for those who did not have timely vaccine administration.
On some of the tests, timely vaccination was actually associated with improved performance compared with the untimely group, but only two of these associations remained significant in multivariate analysis, which included 16 potential confounders: speeded naming in the Developmental Neuropsychological Assessment test of speech and language, and the performance IQ subscale in the Wechsler Intelligence Scale for Children.
Smith and Woods also conducted a secondary analysis involving 310 children in the cohort whose vaccinations came closest to the recommended schedule and 112 with the least timely vaccinations.
The "most timely" group had received a mean of 11.2 doses through seven months of age, compared with a mean of 4.2 in the "least timely" group.
Multivariate analysis of these subgroups showed no differences in outcomes on any of the 42 tests.
The unadjusted univariate results indicated better performance in the most timely group on 15 tests; children with the least timely immunizations didn't outperform the most timely group on any measure.
"The lack of any statistically significant results that favored delayed receipt of vaccines in the first year of life sends a clear public health message that should be comforting to many parents with vaccine safety concerns: Children can receive their immunizations on time and expect to have the same neurodevelopmental outcomes as children with any other pattern of vaccine receipt," Smith and Woods concluded.
Limitations to the study included lack of a significant number of children receiving no vaccinations, and the enrollment period in the mid-1990s when the recommended vaccination schedule was different from today's.
Also, children in the cohort may not be fully representative of the population at large, the researchers noted.
No external funding for the analysis was reported. The Vaccine Safety DataLink was supported by the CDC.
Smith and Woods are or have been unfunded subinvestigators for cross-coverage purposes on vaccine clinical trials for which their colleagues receive funding from Wyeth, Sanofi Pasteur, GlaxoSmithKline, MedImmune, and Novartis.
Woods has received honoraria for speaking engagements from Merck, Sanofi Pasteur, Pfizer, and MedImmune and has received research funding from Wyeth and Sanofi Pasteur.
- Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, RN, BC-ADM, CDE, Nurse Planner