A measles outbreak spanning Texas, New Mexico and Oklahoma reached at least 453 cases Friday and might be linked to cases in other states and across borders.
Texas has reported 400 outbreak-associated cases, New Mexico reported 44, and Oklahoma reported nine – seven confirmed and two probable – as of Friday.
Friday’s updates are 74 cases higher than the last update Tuesday, but experts say these numbers are a severe undercount because many cases are going unreported.
The World Health Organization said in an update Thursday that the US outbreak is an “unusual event with potential significant public health impact” and had spread to Mexico.
“Additionally, cases linked to the outbreak in the State of Texas, United States, have been reported in Mexico. Measles is a highly contagious, airborne viral disease that can lead to severe complications and death,” the agency said.
“Viruses don’t need a passport,” said Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center. People can become infected and then travel and spread illness among heavily unvaccinated communities which are essentially pockets of susceptible people, he said.
Twenty-three measles cases have been reported in Kansas, and the state health department said they could also be linked to the outbreak.
“The confirmed cases in Kansas have a possible link to the outbreaks in Texas and New Mexico. While genetic sequencing of the first Kansas case reported is consistent with an epidemiological link to the Texas and New Mexico outbreaks, the source of exposure is still unknown,” Jill Bronaugh, communications director for the Kansas Department of Health and Environment, said in an email Wednesday.
According to the state, children account for 21 of the Kansas cases, and 20 of the cases are in people who are unvaccinated. The Kansas measles cases are concentrated in the southwestern part of the state and have spread to six counties.
A resident of a Bucks County, Pennsylvania, near Philadelphia, also tested positive for measles after traveling to Texas, the state health department said Thursday. It is unclear exactly what part of Texas the person visited and whether there is a direct link to the outbreak. The person was a vaccinated adult and had a “relatively mild infection.”
Although breakthrough cases can happen in those who are vaccinated, they are uncommon, and the person will shed less virus, meaning they are less likely to pass it on to others, said Dr. Paul Offit, director of the Vaccine Education Center at the Philadelphia Children’s Hospital. Their infection will also result in more mild symptoms with a lower risk of complications.
If you’ve gotten the vaccine, “it would be very unlikely you get severely infected. And that was true in the Bucks County case. It was a mild case,” he said.
Experts attribute these rising measles cases to undervaccination, especially in specific locations and communities.
One dose of the measles-mumps-rubella (MMR) vaccine is 93% effective against measles, and two doses are 97% effective. Experts say this is the best way to prevent infection with the measles virus.
“There’s often a source of measles. There are all these little sparks that often happen in this country, and when immunization rates were high and community immunity was high, you wouldn’t see that spark being lit to become fire,” Offit said. “But now, those little sparks are becoming lit to become a fire, because there’s a critical percentage of people who aren’t vaccinated.”
Some states have reported individual cases related to international travel.
“Even single cases evoke intense public health responses. And of course, that is an important commitment of public health resources wherever that occurs,” Schaffner said.
The multistate outbreak has put 43 people in the hospital. Children under age 4 account for 139 of the cases, and 177 are in young people 5 to 17. One death has been reported in Texas, and a death in New Mexico is under investigation. Cases have been identified in 17 Texas counties and two New Mexico counties.
Although most of the cases are in people who were not vaccinated or who had unknown vaccination status, six cases have been reported in people who said they have received at least one dose: two in Texas and four in New Mexico.
Given the contagious nature of the illness, officials expect more spread. Even though New Mexico reported only two additional cases this week, the state health department says this may continue to grow.
“We’re also realistic that there’s enough of the population who’s unvaccinated that we may see cases continuing for quite some time,” said Dr. Chad Smelser, deputy state epidemiologist for the Infectious Disease Epidemiology Bureau of the New Mexico Department of Health.
One of these states is Ohio, which reported an outbreak of 10 cases this week. None of the affected individuals was vaccinated, and state officials say there is no known link to the Texas outbreak.