No Evidence of Safety Concerns
Upon reviewing stakeholder concerns and scientific
literature regarding the entire childhood
immunization schedule, the IOM committee finds
no evidence that the schedule is unsafe. The committee’s
review did not reveal an evidence base
suggesting that the U.S. childhood immunization
schedule is linked to autoimmune diseases,
asthma, hypersensitivity, seizures, child developmental
disorders, learning or developmental disorders,
or attention deficit or disruptive disorders.
Existing mechanisms to detect safety signals —
including three major surveillance systems
of FDA-approved products maintained by the
CDC and a supplemental vaccine safety monitoring
initiative by the FDA—provide further confidence
that the current childhood immunization
schedule is safe.
Despite the reassuring available evidence, the
committee calls for continued study of the immunization
schedule using existing data systems.
Answering research questions of the most
importance to stakeholders could be done through a variety of methods. The committee does not
endorse conducting a new randomized controlled
clinical trial that would compare the health outcomes
of unvaccinated children with their fully
immunized peers. Although this is the strongest
study design type, ethical concerns prohibit this
study, as unvaccinated individuals and communities
intentionally would be left vulnerable to
morbidity and mortality. While stakeholder concerns
should be one, but not the only, element that
drives continued searches for scientific evidence,
the committee writes that these concerns alone,
absent epidemiological or biological plausibility
of potential safety problems, do not warrant further
A new observational study, a complex undertaking
that also would require a considerable
investment, would be less likely than a randomized
controlled clinical trial to conclusively reveal
differences in health outcomes between children
who are fully immunized and unimmunized children.
Fewer than 1 percent of Americans refuse all
immunizations. Enrolling sufficient numbers of
unvaccinated children and matching them with
vaccinated children of the same age, gender, ethnicity,
and geographic location — a necessary step
to rule out chance findings — would be prohibitively
difficult and time-consuming.
The IOM committee finds analysis using existing
databases to be the most feasible approach to
studying the safety of the childhood immunization
schedule. It concludes that the Vaccine Safety
Datalink (VSD), a collaborative effort between
the CDC and nine managed care organizations that monitors potentially rare and serious side
effects after vaccines are marketed, is the best
available system for studying the U.S. immunization
schedule. VSD data represent more than 9
million children and adults—roughly 3 percent of
the U.S. population—and include medical details,
such as the diagnoses and procedures associated
with outpatient, inpatient, and urgent care visits.
For this reason, the committee concludes that
the VSD is currently the best available system for
studying the childhood immunization schedule.
The committee notes one potential limitation
of the VSD: children who are immunized with
alternative vaccination schedules may differ in
meaningful ways from children who adhere to
the schedule, and these differences could make
it difficult to tease out health differences that
are attributable to the immunization schedule.
In order to bridge such data gaps, the VSD system
could be modified to enable new analyses of
important questions, participants could be asked
additional questions, and medical records could
be reviewed. The federal government also should
continue to build on this component of its robust
vaccine safety net by enhancing the quality of
VSD’s demographic information and including
more diversity in its study populations.
Since the late 1970s, IOM committees have conducted
more than 60 studies of vaccine safety,
attesting to society’s sustained interest in safely vaccinating populations from preventable disease.
This committee’s report is unique in that it is the
first to attempt to examine the entire childhood
immunization schedule as it exists today.
In this most comprehensive examination of the
immunization schedule to date, the IOM committee
uncovered no evidence of major safety concerns
associated with adherence to the childhood immunization
schedule, which should help to reassure a
diverse group of stakeholders. Indeed, rather than
exposing children to harm, following the complete
childhood immunization schedule is strongly associated
with reducing vaccine-preventable diseases.
As scientific advances continue and new vaccines
are developed, the childhood immunization
schedule may grow even more complex. Looking
to the future, the IOM supports HHS’s efforts to
ensure that stakeholders are more fully involved
in addressing benefits and concerns regarding the
safety of the childhood immunization schedule.