People who do not have MMR or DTaP vaccines run a higher risk of contracting several diseases.
Immunization can cause minor side effects, but the Centers for Disease Control and Prevention (CDC) urge people to seek immunization rather than risk developing the disease.
Overall, the US has a high level of routine vaccination coverage. In 2013, the rate of vaccination with at least one dose of the measles, mumps and rubella (MMR) vaccine was 91.9%, and with four or more doses of diphtheria, tetanus and acellular pertussis (DTaP), the figure was 83.1%.
Routine immunization means that deaths from measles in the US are now rare. Measles was officially declared eradicated in the US from January 1, 2000, and the lowest number of pertussis cases was recorded at the end of 1976.
Growing trend of not vaccinating
However, more recent outbreaks of both diseases have occurred, while the last two decades have seen a growing number of requests for non-medical exemptions, for moral or religious reasons.
An outbreak of measles affected 111 people in 2014, with cases reported in seven states plus Canada and Mexico, originating in Disneyland in Anaheim, CA. Around half of those who contracted the disease had refused vaccination, mostly for non-medical reasons.
Health service employees and officials have been calling on the American public to take the growing trend of refusing or delaying vaccination seriously.
In a meta-analysis of 18 studies, Saad B. Omer, PhD, of Emory University in Atlanta, GA, and colleagues have been looking at links between outbreaks of measles and pertussis and the tendency to delay vaccination or not vaccinate at all, whether as a result of refusal or through being exempt.
The team examined reports of measles outbreaks since January 2000 and of pertussis since January 1977, as well as studies evaluating the risk of disease among individuals who are not vaccinated.
High representation of unvaccinated people in outbreaks
The studies covered 1,416 cases of measles, affecting people aged from 2 weeks to 84 years, with 178 cases involving infants under 12 months. Detailed vaccination data was available for 970 cases.
Among those who caught measles, 57% had not been vaccinated. Of the 970 whose data were available, 574 were unvaccinated, although they were eligible for vaccination, and 71% of these had exemptions for religious or philosophical rather than medical reasons. This represented 42% of the total number of those who caught measles.
Since January 1977, 32 pertussis outbreaks have been recorded. Vaccination status was available for 10,609 individuals, aged from 10 days to 87 years. There was detailed vaccination data for 12 outbreaks.
In the five biggest epidemics, 24-45% of people were either unvaccinated or incompletely vaccinated. In eight of the 12 outbreaks with vaccination data, 59-93% of those who were unvaccinated had refused vaccination.
However, a number of outbreaks occurred in populations that were highly vaccinated, suggesting that immunity itself is on the decline.
The authors note that the precise risks of vaccine refusal remain unclear, and they may vary according to individual populations and diseases.
"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."
The researchers say the results have implications for vaccine practice and policy, and they call for a better understanding of the association between vaccine refusal and the occurrence of measles and pertussis.
Need for a centralized system of immunization
In an accompanying editorial, Dr. Matthew M. Davis, of the University of Michigan in Ann Arbor, points out that the US population wants safe, effective, long-lasting and timely immunization, and in the case of emerging diseases, such as Zika or Ebola, they would hope that the system for immunization would function seamlessly.
However, he says: "Without a centralized infrastructure focused on the goal of maximizing community immunity, high-reliability vaccine coverage remains challenging in the US."
He suggests that, as measles and pertussis present challenges, they also offer a chance to develop and assess ways of improving the effectiveness of vaccination coverage, of preventing declines in immunity and of ensuring that diseases that could be prevented by vaccination are kept to a minimum.
Last year, Medical News Today reported on research suggesting that falling levels of immunization in the US increase the risk of an epidemic.