The refusal of some adults to vaccinate themselves or their children has contributed to a rise in outbreaks of diseases widely considered eradicated in the United States, according to a new study in JAMA.
To understand the role non-vaccinated and under-vaccinated individuals play in spreading viruses, researchers reviewed 18 measles studies published after Jan. 1, 2000, and 32 published reports on pertussis outbreaks since Jan. 1, 1977, that included information on vaccine status.
Both diseases, the researchers note, are considered vaccine-preventable, and measles was officially declared eradicated in the United States in 2000. Yet the researchers identified 1,416 measles cases and 10,609 cases of pertussis in the studies they examined.
Of the 1,416 measles cases, 57 percent of individuals had no history of vaccination and 14 percent "involved individuals with a history of receiving measles-containing vaccine," according to the study.
The researchers also identified a subset of 970 measles cases that included detailed vaccination data. Of those, 574 individuals had not been vaccinated despite being vaccine-eligible, and 405 cited nonmedical reasons for their exemption, such as religious objections.
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In the five largest statewide outbreaks of pertussis, non-vaccination or under-vaccination accounted for 24 to 45 percent of individuals affected in those outbreaks. For pertussis reports that included detailed vaccination information, 59 to 93 percent of cases were among people who intentionally avoided being vaccinated.
But the researchers note that several large pertussis outbreaks occurred in populations with high vaccination rates, which suggests waning immunity against the disease.
Overall, the researchers found that there is a dynamic relationship between the risks to the unvaccinated and the broader population. "The phenomenon of vaccine refusal was associated with an increased risk for measles among people who refuse vaccines and among fully vaccinated individuals," the authors wrote. And despite the role of waning immunity, "vaccine refusal was still associated with an increased risk for pertussis in some populations."
Understanding the risks
Parsing the data further suggests that unvaccinated individuals are among the first people to contract a virus during outbreaks—putting others at risk. "If there are a high number of susceptible or unvaccinated individuals in the community the risk of getting infected—even for vaccinated children—goes up," explains Saad Omer, a pediatrics and epidemiology researcher at Emory University and study co-author.
According to CDC, under ideal conditions, people who receive full courses of vaccination for both pertussis and measles have a 2 and 3 percent chance of contracting the virus upon exposure, respectively.
In a commentary accompanying the study, Matthew Davis, a pediatrician at the University of Michigan, explained that waning vaccine effectiveness and under-vaccination were related issues. Under-vaccination increases the risk of an outbreak starting, and waning vaccine effectiveness can accelerate its spread, he says.
Varun Phadke, a fellow in infectious diseases at Emory University and co-author of the study, says the findings suggest policymakers need to think critically about how to promote vaccination and deal with waning vaccination effectiveness. "States that make it harder not to get vaccinated have lower rates of vaccine-preventable diseases," he notes.
Davis agrees that policymakers should be paying close attention to the issue. "Outbreaks of diseases like measles and pertussis remind us that there are still ways to improve how we use vaccines to safeguard the health of children and adults across the United States," he writes (Rapaport, Reuters, 3/15; Phadke et al., JAMA, 3/15; Healy, "Science Now," Los Angeles Times, 3/15; Reinberg, HealthDay/U.S. News & World Report, 3/15).
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