Most childhood vaccines are given according to the recommended schedule, but some children may get the vaccine at a much earlier or a much later age. There are many potential reasons for this, including a high risk of exposure due to a current disease outbreak or because family members have the disease, or due to missed well care visits, shortages of the vaccine, parental or physician concerns about the recommended vaccine schedule, misunderstanding of the recommended schedule, or medical errors, etc. As an example, while the first dose of MMR is recommended at age 12 to 15 months, in one health plan, 22 percent of children were recorded to have received it later and 0.7 percent to have received it before their first birthday, with 0.3 percent receiving it before 6 months of age. Nationwide, even half a percent adds up to a fairly large number, and it is important to evaluate the safety of the vaccine for those children, so that a contraindication warning can be issued if there is a major safety problem.
After the 2004 recommendation to give influenza vaccines to 6- to 23-month-old children, Hambidge et al. (2006) used data from VSD to conduct an influenza vaccine safety study specific to this age group, looking at a wide variety of potential adverse events.
Health plan data capture all vaccinations at whatever age they occurred, so such data are useful not only for evaluating the safety of vaccines in special age groups but also for characterizing the real-world age distribution of vaccinated children.
With its national coverage, VAERS data can also be used to monitor vaccine safety in specific age groups. While no denominator data are directly available, a large number of adverse event reports in children outside the recommended age range could be the first indication that an age-specific problem exists.
If a change in the recommended age that a vaccine should be given is anticipated, a randomized trial may be warranted. For that to occur, there needs to be some uncertainty as to whether the currently recommended time is safe and some evidence, based on, for example, observational data, that an alternative age is safer. If the question is simply whether the vaccine should be contraindicated for certain age groups or whether the vaccine