Small Drop in Vaccination Could Triple Number of Measles Cases in United States

Public health experts tout high rates of vaccination in a population for building up “herd immunity” and preventing disease outbreaks, and now, a new study highlights the health and economic consequences of even small decreases in vaccination rates.

Herd immunity occurs when a large percentage of a population is vaccinated, thus, limiting the spread of a disease. In a highly-vaccinated population, the small percentage of those who are unvaccinated are indirectly protected, and for a community to reach herd immunity from most diseases, 80% to 95% of the population typically needs to be vaccinated. The consequences of reductions in immunization rates were highlighted by a recent outbreak of measles in Minnesota that occurred within a Somali community, following a drop in the number of individuals receiving the measles, mumps, and rubella (MMR) vaccine. The measles outbreak was Minnesota’s biggest in more than 2 decades, and was linked to the anti-vaccination movement, which cites unfounded risks of vaccines to promote abstaining from vaccination. Following a smaller measles outbreak in Ontario in 2015, Canadian health officials emphasized that two doses of the MMR vaccine in 95% of a population are needed to achieve herd immunity against measles.

In a new study published in the journal JAMA Pediatrics, researchers found that even a small decline in the MMR vaccination rate can significantly increase the rate of measles infection as well as related costs. For the study, a pair of researchers from Stanford University School of Medicine and Baylor College of Medicine used publicly available data from the Centers for Disease Control and Prevention (CDC) and developed a model to simulate the number of measles cases for different levels of vaccine hesitancy, a term referring to the delay or refusal of vaccines. The researchers found that just a 5% drop in MMR vaccine coverage in the United States would result in 3 times more cases of measles nationally, each year, for children aged 2 to 11 years. The decline in vaccination would also add $2.1 million in public sector costs annually, or $20,000 per additional case; those estimates don’t include unvaccinated infants, adolescents, and adults.

“We focused on measles as a case example of the effects of declining vaccine coverage because it is highly infectious,” said the study’s lead author Nathan Lo, MD, PhD, in a recent press release from Stanford. “It’s likely to be the first infectious disease causing outbreaks if vaccination declines.”

While MMR vaccine coverage for kindergarteners in the United States is close to 95%, and exemptions are low overall, a CDC study published in 2016 noted a slight increase in the national median exemption rate for the 2015-2016 school year over the previous year. While medical exemptions from vaccination are needed for immunocompromised individuals and those with certain health conditions, non-medical and personal belief exemptions are on the rise in several states as parents are opting out of vaccinating their children.

“I think our study is a wake-up call for what we can expect in the coming months and years as vaccine coverage rates continue to decline in the 18 states that now allow non-medical or philosophical belief exemptions,” said senior author Peter Hotez, MD, PhD, in a recent statement.

The authors note that their findings highlight the need to address vaccine hesitancy at the state and national level, and conclude that at a state and national level, policy makers should consider removing personal belief exemptions of childhood vaccination from dialogues.