Nearly 9 million U.S. kids -- or about one in eight infants through 17-year-olds -- are at risk of contracting measles due to gaps in vaccination rates, according to research presented today at IDWeek, an infectious disease conference being held in San Diego.
This is the first estimate to look at the overall number of measles-susceptible children in the U.S. It includes not only unvaccinated children, but also accounts for delayed immunization, which would leave kids vulnerable until their first dose of the measles, mumps and rubella (MMR) vaccine.
Measles is one of the most contagious of the vaccine-preventable diseases and can lead to complications like pneumonia, hospitalization and occasionally death.
Some children are unprotected due to their inability to get the vaccine, either because they are too young or for medical reasons. But some parents choose not to vaccinate their kids or delay vaccination for personal reasons.
According to the Centers for Disease Control and Prevention, measles was declared eliminated in the U.S. in 2000. But last year, there were a record 668 cases of measles reported, and 2015 started off with a multi-state outbreak that originated in Disneyland and sickened at least 117 people.
Measles is currently not widespread in the U.S. thanks to herd immunity -- meaning the majority of people across the country have been vaccinated. This ensures the number of people vulnerable to infection is small and helps protect those who can't be vaccinated by preventing their exposure to the virus in the community.
For the study, researchers from Emory University analyzed data from the National Immunization Survey-Teen and found that the current percentage of children immune to measles is very close to the range of 92 to 94 percent. But below this threshold, measles outbreaks could become more common and more severe.
"Although we eliminated continuous measles transmission in the United States about 15 years ago thanks to the effectiveness of the MMR vaccine and robust vaccination rates, these study results show that we can't get complacent," Robert Bednarczyk, lead author of the study and assistant professor in the Hubert Department of Global Health, Rollins School of Public Health at Emory University, said in a statement.
"While we currently have overall immunity in the population that should prevent sustained measles transmission, if the virus is introduced, there is the potential for large outbreaks. This is because there are clusters of unvaccinated children in some communities, which could allow a large outbreak to occur with spread to similar communities."
The analysis also found that nearly one in four children aged three or younger are at risk for measles, and that nearly five percent of 17-year-olds had not received any doses of the vaccine.
The researchers estimate that if the percentage of vaccinated children drops to just 98 percent of current levels, more than 14 percent of children -- about one in seven -- would be susceptible to measles.
In a press conference this morning, Bednarczyk urged parents and primary care doctors to vaccinate children on time in order to protect their kids and help maintain current immunity levels. Children should receive two doses of the MMR vaccine, the first at 12 to 15 months and the second at four to six years old.
"We know some parents have concerns about vaccines and may want to avoid or delay vaccination, or follow an alternative schedule than the one recommended because they're concerned about the safety of the vaccine," he said. "In fact, the vaccine is very safe, while not vaccinating is highly risky, leaving their children -- and others -- vulnerable to a serious illness that can cause a large number of complications. Currently, these children are protected because of the high vaccine coverage of the population, but that will change if we begin having more outbreaks and the percentage of children vaccinated declines."