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Vaccine refusal endangers everyone, not just the unvaccinated

Science Blogs: Respectful Insolence
17 March, 2016

Reproduced with permission.

Dr. David M. Gorski*

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. Just this week yet another study was published that finds yet the same thing again. However, given misunderstanding and the circulation of 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.

They concentrated 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.  Moreover, the 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 problem 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 these outbreaks 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 soon.

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.


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