Measles Outbreaks Can Occur in Intense Exposure Settings Despite High Immunity Levels

In populations with high presumptive immunity to measles, the infection can still spread when there is intense contact between patients or high levels of exposure. This was demonstrated in a measles outbreak in a United States Immigrations and Customs Enforcement (ICE) Facility during May and June 2016 which affected not only detainees who may not have had immunity, but also ICE staff who did have presumptive immunity.

“This was the first documented measles outbreak in an ICE facility in the United States,” observed EIS field officer Heather Venkat, DVM, MPH, in her oral presentation on the topic at the 2017 Annual EIS Conference in Atlanta, Georgia, on April 24, 2017. “Measles was declared eliminated in the United States in 2000, but sporadic outbreaks occur due to importation of the virus,” she continued. Because vaccination records are often not available for detainees, the facility performed testing to distinguish between primary and secondary immune response among the infected population. Infection was confirmed through laboratory testing, or through nasal swabs, daily symptom and temperature monitoring, face-to-face interviews, and online interviews with facility staff.

“Primary immune responses indicated that a patient had not been vaccinated or that they had a low immune response to the vaccination,” Dr. Venkat explained, adding that secondary immune response meant that either a previous vaccination attempt had failed or that the patient had experienced a previous measles infection.

In total, 31 patients contracted measles in the facility: 22 detainees and nine staff members.

“Of the 31 confirmed cases, 26 were lab-confirmed and five were confirmed by epidemiologic linkage,” she said. One unit in the facility, unit A, experienced significantly higher “attack rates” than the rest of the units (B-F), but attack rates were not significantly different among detainees and staff.

After testing, the measles genotype was revealed to be endemic to India, Nepal, and Bangladesh; detainees from these areas were housed in unit A. “The outbreak may have started from an area housing a detainee from one of those countries and spread,” Dr. Venkat said, although she emphasized that they were unable to confirm this hypothesis.

After the outbreak was identified, the facility implemented control measures that included isolating detainees and staff who had been exposed to the infection and vaccinating 1424 of the 1425 detainees. The facility also vaccinated or verified immunity for 445 of 510 facility staff and quarantined affected housing units. “Staff participation in the first vaccination campaign was low,” Dr. Venkat stated, noting that a second campaign conducted on July 15, 2016 and July 19, 2016 resulted in a total of 87% of the staff being considered to have “evidence of immunity.”

The research team noted several limitations in the study, including that they were unable to interview all detainees who may have been exposed to measles and that there was a likelihood of underreporting due to detainees hiding symptoms. “We were notified of the first case on May 25, 2016,” said Dr. Venkat. The first rash onset occurred on May 6, 2016 and the facility reported later that it had “heard reports of rash illness on May 10, 2016 on units A and F.”

“There are ongoing discussions regarding different policies regarding vaccinations [in ICE facilities]. At least at this facility, they are vaccinating detainees for measles, mumps, and rubella,” Dr. Venkat concluded. “In terms of similar outbreaks, I do not think that they have had measles outbreaks in these types of facilities before. It is on their radar now.”