Why small groups of vaccine refusers can make large groups sick


A patient gets a flu shot in Seattle in January. Small groups of vaccine refusers can hinder the effectiveness of the vaccination as a whole. (Ted S. Warren / AP)

Saad B Omer holds the William H. Foege Chair in Global Health and Professor of Global Health, Epidemiology and Pediatrics at Emory University.

Infectious diseases such as chickenpox and measles – once a rite of passage for American children – have been made uncommon because of vaccines. However, in recent years, an increasing number of parents are refusing vaccines, leading to epidemics.

The overall vaccination rate in the United States remains fortunately despite this worrying trend. For example, according to the Centers for Disease Control and Prevention, more than 90% of American children aged 19 to 35 months are properly vaccinated against measles and chicken pox. Why, then, do we continue to see epidemics of preventable diseases in the United States?

One of the reasons is the epidemiological phenomenon of clustering of susceptible individuals, which occurs when a group of unvaccinated individuals in a specific area becomes large enough to make protection against rates of protection less effective. globally high vaccination.

The current outbreak of chicken pox at a Waldorf School in Asheville, NB, is the most recent example of this problem. The epidemic has been the largest outbreak of chickenpox in North Carolina for more than two decades. Waldorf schools focus on "independent and inclusive" education and academic rigor, and alternative educational institutions such as Waldorf have much higher vaccine refusal rates than the Waldorf schools. public schools and other private schools. In a study of California schools, we found that Waldorf schools had a 19-fold higher rate of refusal of vaccination than public schools.

Alternative schools, especially Waldorf schools, have also been associated with epidemics outside the United States. Several recent epidemics of measles and other preventable diseases in the United States have begun among geographical clusters of vaccine refusers. Epidemics in groups of vaccine refusers mean that public health authorities and physicians need to focus on these areas of vulnerability, not just on increasing overall immunization rates.

Vaccines protect communities not only by directly protecting those vaccinated against viruses and bacteria, but also by reducing the risk of contracting these infections. Since people who are vaccinated have a considerably lower risk of contracting and thus transmitting the infection, there is simply less chance for anyone to contract it. When enough people are vaccinated, the risk of an epidemic decreases. when the vaccination rate is high enough, the risk of the epidemic is virtually eliminated. This phenomenon is called collective immunity (or community immunity) and the vaccination rate at which the risk of an epidemic is eliminated is called the collective immunity threshold. This threshold depends on the infectivity of a virus or a bacterium and therefore varies depending on the disease.

The immunity heard is important for several reasons. First of all, not everyone can be vaccinated: children with certain medical conditions, such as certain types of immune deficiency, and those who are undergoing cancer chemotherapy can not receive most vaccines, for example. These vulnerable people, especially children, need collective immunity to protect themselves from diseases that vaccines can prevent. And since few vaccines are 100% effective, some vaccinated children remain unprotected individually and also depend on herd immunity. People suffering from chronic diseases such as asthma or diabetes are particularly vulnerable to the serious consequences associated with vaccine-preventable diseases. For example, diabetics are six times more likely to be hospitalized for influenza-related reasons than non-diabetics and three times more likely to die as a result of influenza infection. Similarly, people with asthma have a higher risk of whooping cough.

Clusters of refusal to vaccinate exceed the herd immunity threshold in specific schools or neighborhoods, which means that outbreaks can occur even though vaccination rates are globally high.

Equally importantly, in an analysis of measles outbreaks in the United States since 2000, we found that vaccine refusers are disproportionately represented in the early stages of outbreaks. People who are not deliberately vaccinated can provide the critical mass of sensitive individuals that can help trigger outbreaks that vaccination would otherwise have prevented.

Although we do not fully understand why vaccine refusal groups are developing, the evidence suggests an aggregation based on shared values, demographics, and socioeconomic status. As our research has shown, school districts have different immunization mandates.

One thing is clear: these clusters exist in both left and right communities. For example, Vashon Island, Washington, home to many wealthy "ex-hippies", has had notoriously high vaccine refusal rates. On the other hand, some of the highest rates of vaccine refusal in the state of Washington concern the rural county of Ferry, which voted for Donald Trump at a margin of about 30 percentage points .

Most public health and policy interventions do not explicitly focus on the grouping of vaccine refusers. But some potential approaches can target this problem. For example, state health services could ensure that all schools apply vaccination requirements correctly. Community education and communication efforts could target communities with a high rate of refusal of vaccination. In all interventions to increase the acceptance of vaccines, health care providers should be the focus of attention, as research shows that they are the source of information on the most vaccines. reliable, even among those who refuse them. Evidence-based state-level interventions can also reduce local clustering. For example, Washington State recently enacted a law requiring that the board of health care providers be exempted from school-level immunization requirements. Not only has this law resulted in more than 40% reduction of state-level vaccine exemption rates, but there has been a substantial reduction in the pooling of vaccine refusals. At the individual level, parents should demand that states and schools share vaccination refusal rates in their schools and communities.

The grouping of vaccine refusers increases the likelihood of epidemics beyond what one would expect from globally high vaccination rates. As the rate of vaccine refusal increases, public health authorities, physicians and parents need to pay close attention to clustering as well as the overall rate of vaccine refusal. Otherwise, we will likely continue to see more and larger epidemics.

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