MONDAY, July 24, 2017 (HealthDay News) -- With a small percentage of U.S. parents not vaccinating their children for "non-medical" reasons, a new study warns that even a few such families can trigger a big jump in local measles cases.
Based on data collected by the U.S. Centers for Disease Control and Prevention, the analysis projects that just a 5 percent drop in MMR (measles, mumps and rubella) vaccine coverage would triple the number of American kids aged 2 to 11 who would catch the highly contagious virus.
"The meaning is that even small declines in vaccine coverage in children owing to vaccine hesitancy may have substantial public health and economic consequences that will be larger when considering unvaccinated infants, adolescents and adults," explained study author Nathan Lo.
Lo is an M.D./Ph.D. candidate in the division of epidemiology at Stanford University's School of Medicine. He conducted the research with senior study author Dr. Peter Hotez, dean of Baylor's National School of Tropical Medicine, in Houston.
The study authors pointed out that -- barring a medical (or other) exemption -- all 50 states require that all children get the MMR vaccine, alongside a full complement of childhood vaccines, before enrolling in day care or elementary school.
The rationale is that measles is both a deadly and easily transmitted illness. Even without face-to-face contact, the virus is capable of remaining airborne and infectious in any given space for upwards of two hours.
"Measles is really the canary in the coal mine," noted Dr. Paul Offit, director of the Vaccine Education Center at the Children's Hospital of Philadelphia. "It's that contagious. Much more so than influenza or chickenpox or polio."
Measles "is the first virus to come back when you start seeing fraying in what we call the phenomenon of 'herd immunity,' which is when enough of the population is immunized to keep the rest of the population safe," Offit explained.
In the case of measles, the researchers said that epidemiologists believe that 90 percent to 95 percent of all residents need to be vaccinated to prevent an outbreak.
Public health efforts to hit that goal have been largely successful, following the introduction of the measles vaccine in 1963. U.S. cases have generally been limited to between several dozen to several hundred each year, most attributed to infected Americans returning home after exposure while traveling abroad.
But that safety threshold has increasingly come under pressure, as some parents avail themselves of a "personal belief" vaccine exemption (available in 18 states) or a "religious exemption" (available in 47 states). Neither exemption needs to be grounded in scientific reasoning.
Several recent outbreaks have already raised concern, including one that struck 383 members of the Amish community in Ohio in 2014; another that originated at Disneyland in California in 2015; and one just this year that affected Somali immigrants (many of whom are unvaccinated) living in Minnesota.
To get a handle on the growing risk, the researchers analyzed CDC data concerning children aged 2 to 11 in 257 counties.
In the end, the investigators found "that small declines in vaccine coverage can really reduce the 'herd immunity' effect."
The findings were published online July 24 in the journal JAMA Pediatrics.
Offit said the finding is not surprising. "The way the herd effect works is that everyone is obviously best off being vaccinated and living in a highly vaccinated community," he noted.
"But the next best situation is being unvaccinated in a highly vaccinated community, which is better than being vaccinated in a highly unvaccinated community. Because you're much more likely to be exposed if those around you aren't protected," Offit explained.
"So 'anti-vacciners' -- many of whom believe they can Google the word vaccine and know just as much as any expert -- are a real concern," Offit said.
"We already have school mandates. And they work well. But we also have exemptions," he said. "So, in a perfect world, we just have to educate people to better understand the risk. Unfortunately for some, the real educator will be the virus. When outbreaks grow more common, and more people die, a lot more people will understand."
There's more on the measles vaccine at the U.S. Centers for Disease Control and Prevention.