One of the more frequent claims of antivaccine activists often comes in
the form of a question, usually something like, "If your child is
vaccinated, why are you worried about my children? They don't pose any
danger to you." Of course, the premise behind that question is, ironically,
one that conflicts with the belief that vaccines are ineffective: that
vaccines are so effective that there's no reason for the parents of a
vaccinated child to be concerned if that child comes in contact with another
child with a vaccine-preventable disease.
Another claim is that it isn't the
unvaccinated who are causing outbreaks, but the vaccinated.
People point out that most of the infected
in an outbreak are vaccinated, which is, of course, not uncommonly true.
However, this neglects how small the number
of unvaccinated children usually are relative to the vaccinated.
People unfamiliar with mathematical probability
don't realize that raw numbers mean little. What really needs to be
examined is the relative risk of infection of the unvaccinated compared to
the vaccinated during an outbreak, and, depending on how effective the
vaccine is, that relative risk is usually rather high. For instance, for
pertussis, being unvaccinated is associated with a
23-fold increased risk of infection.
The fact is, not vaccinating children not only
endangers them, but everyone's children.
this week yet another study was published that finds yet the same thing
again. However, given misunderstanding and the
misinformation,it's always good to see a new
review in a high impact journal like the
Journal of the American Medical Association (JAMA) confirming
just that. This time, it's a systematic review of the evidence for measles
and pertussis by Phadke et al entitled
Association Between Vaccine Refusal and Vaccine-Preventable Diseases in the
United States: A Review of Measles and Pertussis. The senior author was
Saad B. Omer,
MBBS, MPH, PhD at Emory University.
on studies that examined risk of disease in the unvaccinated and vaccinated
since measles was declared eliminated in the
United States 16 years ago and since pertussis reached its lowest point of
its incidence (after 1977). They also looked at
vaccine delay and exemptions, including medical and non-medical (i.e.,
philosophic or religious) exemptions in order to determine how vaccine
refusal affects risk of disease in both the unvaccinated and the vaccinated.
As a result, they were able to estimate, for example, that over half of the
victims of US measles outbreaks are unvaccinated, often intentionally.
In their search, the authors identified 18 published measles studies (9
annual summaries and 9 outbreak reports). These studies described 1,416
measles cases ranging in age from 2 weeks to 84 years of age, with 178 of
them younger than 12 months. Of these cases, a total of 199 cases (14%) were
people with a history of being vaccinated against measles, while more than
half of the total measles victims 804 (nearly 57%) had no history of measles
vaccination. There were 970 measles cases with detailed vaccination data, of
which 574 were unvaccinated, and, of these, 405 (71%) had nonmedical
exemptions, making up 42% of the total number of cases). One particularly
pertinent observation is how the unvaccinated predominate among cases early
in the outbreak:
The outbreaks evaluated in the cumulative epidemic curve included
cases that occurred up to 5 generations of spread after the index case,
with the latest related case occurring 12 weeks after identification of
the index case. When viewed by week of outbreak, unvaccinated
individuals constituted a larger fraction of the total measles cases per
week in the earliest weeks of an outbreak (eg, earlier generations).
So basically, most of the measles cases were in the unvaccinated.
majority of the unvaccinated were old enough to receive the vaccine, and
there was no medical reason for them not to be vaccinated. Their parents had
refused the vaccine for nonmedical reasons. This demonstrates that refusing
vaccinations causes harm, and existing studies allowed the authors to estimate how much
refusing vaccinations increases the risk of harm in the whole population.
Reviewing the relative risk of measles in unvaccinated children, the
authors found studies demonstrating that the unvaccinated were anywhere from
22 to 35 times more likely to contract the measles during an outbreak.
Worse, higher rates of vaccine exemption in a community were associated with
greater measles incidence in that community, among both the exempt and
nonexempt population. Curious, I went back to look up the
article cited by
Phadke et al, which used mathematical modeling to estimate
risks. Depending on assumptions of the model about the degree of mixing between
exempted and nonexempted, "an increase or decrease in the number of
exempt[ed] would affect the incidence of measles in nonexempt populations.
If the number of exempt[ed] doubled, the incidence of measles infection in
nonexempt individuals would increase by 5.5%, 18.6%, and 30.8%,
respectively, for intergroup mixing ratios of 20%, 40%, and 60%."
This confirms that an
increased proportion of unvaccinated children does degrade herd immunity and
does increase the risk of disease in the vaccinated. Remember, no vaccine is
100% effective. The Mumps-Measles-Rubella (MMR)
vaccine is very effective against measles, over 90%, but not
100%. Yes, the vaccinated can still be infected; it's just that they're much
less likely to be.
As far as pertussis goes, the numbers for the
unvaccinated aren't good either. The authors identified 32 reports of nonoverlapping pertussis
outbreaks covering 10,609 cases among individuals ranging in age from 10
days to 87 years. The five largest statewide pertussis outbreaks
included substantial numbers of victims who had
been vaccinated or who had been inadequately vaccinated (i.e., missed doses). Part of the
that complicates the pertussis picture is, of course, the problem of waning
immunity, but it's clear with pertussis as well that being unvaccinated
carries with it a substantially increased risk of developing the disease:
Three studies evaluated the individual risk of pertussis associated
with vaccine refusal—1 retrospective cohort study used Colorado
pertussis surveillance and immunization data from 1987-1998 and
determined that those with exemptions were 5.9 times more likely to
acquire pertussis compared with fully vaccinated individuals. A
different case-control study analyzed pertussis cases from 1996-2007
within a large managed care organization and computed a nearly 20-fold
increased risk of pertussis among individuals with exemptions—11% of the
pertussis cases in that cohort were attributed to vaccine refusal.
Another case-control study used pooled longitudinal data (2004-2010)
from 8 Vaccine Safety Datalink sites and determined that even
undervaccinated individuals had an increased risk of pertussis, with the
risk being proportional to the number of missed doses of DTaP.
Overall, the authors concluded that vaccine refusal is associated with an
increased risk of vaccine among both the unvaccinated and vaccinated.
Although waning immunity to pertussis is an issue in pertussis
outbreaks (as I've discussed before), vaccine
refusal still contributes significantly to the risk of infection in some populations.
The authors observed:
This review has broad implications for vaccine practice and policy.
For instance, fundamental to the strength and legitimacy of
justifications to override parental decisions to refuse a vaccine for
their child is a clear demonstration that the risks and harms to the
child of remaining unimmunized are substantial. Similarly, central to
any justification to restrict individual freedom by mandating vaccines
to prevent harm to others is an understanding of the nature and
magnitude of these risks and harms. However, the risks of vaccine
refusal remain imperfectly defined, and the association between vaccine
refusal and vaccine-preventable diseases may be both population- and
disease-specific. Vaccine refusal–specific strategies to optimize
vaccine uptake could include state or school-level enforcement of
vaccine mandates, or increasing the difficulty with which vaccine
exemptions can be obtained.
It is sometimes said that parents' freedom and right
to raise their children as they see fit shouldn't be limited by vaccine
mandates because their unvaccinated children are harming no one. While this
often gets a sympathetic hearing, the evidence for pertussis
and particularly for measles, at least, demonstrates that it is clearly
not true. Refusing vaccinations for pertussis and measles puts both
vaccinated and unvaccinated children at risk.
As is often the case with major articles like this, there was an
accompanying editorial, in this case by Matthew Davis at the University
of Michigan. Davis first notes that, in the case of pertussis, waning
immunity and vaccine refusal are different challenges, but they are related.
The reason is that nonmedical exemptions for childhood vaccination decrease
overall community immunity and thus increase the risk of infection for
children with waning immunity or, in the case of the children of vaccine
refusers, no immunity at all. Outbreaks then occur, and
are sometimes used to try to 'prove' that the
benefits of vaccination are being oversold and therefore not important.
Davis also notes:
An important priority is to ensure high reliability in US vaccination
efforts. Current US vaccination efforts are not optimally effective, as
measured by outbreaks of vaccine-preventable diseases and vaccination
coverage rates that fail to reach target levels. Currently, no single
entity is accountable for monitoring and coordinating the multiple
stakeholders with interests in maximizing vaccination rates. These
multiple stakeholders include parents, physician practices, private
insurance, public health institutions, community pharmacies, and
government agencies. Given the public health importance of effective
vaccination, a more reliable system is needed.
The airline and nuclear power industries have established a culture
that values consistent and standardized practices to promote highly
reliable performance. In the United States, efforts to achieve complete
vaccination rates in the population do not follow the standards
established by these industries. By standardizing procedures and
continuously evaluating the effectiveness of new initiatives to increase
vaccination rates, it may be possible to reduce exemptions and waning
immunity and achieve more complete vaccination of children and adults.
Exactly. Davis almost drolly notes at the end that "without a centralized
infrastructure focused on the goal of maximizing community immunity,
high-reliability vaccine coverage remains challenging in the United States."
That's an understatement. The infrastructure in this country for tracking
vaccination rates could use considerable improvement. It's a patchwork of
state systems, some of which do a good job, some of which do not. In some
states school-level vaccination rates are reported; in others not.
Unfortunately, because it is states that are responsible for setting vaccine
requirements, this is not a situation likely to be improved much any time
Still, the message of this review article needs to be repeated
over and over again. Despite claims to the
contrary, the evidence is that those who refuse
vaccinations are likely to cause harm to themselves and their entire
community - including the vaccinated.
Note: The opinions expressed here should not be
construed as representing the opinions of institutions with which the author
is professionally affiliated, nor any other person or entity. Medical
commentary is not to be construed in any way as medical advice.