Vaccines are one of the greatest breakthroughs in medical history, and continue to play a key role in preventing and eradicating many diseases. Unfortunately, a recent rise in opposition to vaccines has led to outbreaks of diseases that were once on the verge of eradication in the United States, such as measles. Those who are unvaccinated – whether out of misguided and discredited fears over vaccine safety, or for a legitimate medical reason such as a compromised immune system — rely on a phenomenon known as herd immunity. Herd immunity exists when a high proportion of a population is vaccinated, preventing disease from spreading and providing a measure of protection for those that who are unvaccinated. However, when immunization rates decline, the effectiveness of herd immunity falls, leading to an increase in disease incidence.
Herd immunity effectiveness is determined by the infectiousness of the underlying disease, the effectiveness of the vaccine itself, and overall immunization rates within a community. When anti-vaccine proponents opt to keep their children unimmunized, they lessen herd immunity and increase the risk of disease for others. A decline in herd immunity can be especially dangerous for children with compromised immune systems and others who for medical reasons are unable to receive some vaccines.
Herd immunity also helps protect vaccinated individuals, since no vaccine is 100 percent effective. That means that even if you vaccinate your own children, they can still be harmed when others in their school or community decline vaccines for non-medical reasons.
Science Blogs has a nice summary of a recent Stanford University study which examined the consequences could be if the rate for the measles, mumps, and rubella (MMR) vaccine was to fall nationwide. The study found the following:
We found that a 5% decline in MMR vaccine coverage in US children would result in a 3-fold increase in national measles cases in this age group, for a total of 150 cases and an additional $2.1 million in economic costs to the public sector (Figure). With declining vaccination coverage, the size of outbreaks increased. Increased MMR vaccine coverage in children, through elimination of children with nonmedical exemptions or other mechanisms, increased national MMR coverage to 95% prevalence (state variation, 91%-98%).We predicted that this strategy would reduce annual cases of measles by 20% (from 48 to 38 cases) and was an effective strategy to mitigate annual measles cases and costs.
This modeling analysis likely predicts a conservative estimate for a rise in measles cases associated with declining immunization status because of foundational model assumptions and since we limited our analysis to US children (age 2-11 years). The model is designed to simulate outbreaks in highly immunized population and nonendemic settings. However, large reductions in MMR vaccine coverage could allow for measles to become endemic again, which is not accounted for in this model, and would likely result in thousands of annual measles cases.
Owing to constraints on data for immunization status in the adult population and social mixing structures, we limited our analysis to children ages 2-11 years who contribute approximately 30% of the annual measles cases. However, the number of cases of measles would be much larger when accounting for other age groups, such as infants, adolescents, and adults.
This increased number would be, in part, driven by infants younger than 12 months who are not yet eligible to receive measles vaccines, as illustrated in the sensitivity analysis modeling an expanded age group (0-11 years). Finally, the upper limit of the prediction interval in the base case analysis demonstrated substantial nonlinearity, suggesting the possibility for large outbreaks due to random chance with small reductions in vaccine coverage.
Read the full blog coverage here.