An outbreak of measles in an undervaccinated community

Abstract

Measles is readily spread to susceptible individuals, but is no longer endemic in the United States. In March 2011, measles was confirmed in a Minnesota child without travel abroad. This was the first identified case-patient of an outbreak. An investigation was initiated to determine the source, prevent transmission, and examine measles-mumps-rubella (MMR) vaccine coverage in the affected community. Investigation and response included case-patient follow-up, post-exposure prophylaxis, voluntary isolation and quarantine, and early MMR vaccine for non-immune shelter residents >6 months and <12 months of age. Vaccine coverage was assessed by using immunization information system records. Outreach to the affected community included education and support from public health, health care, and community and spiritual leaders. Twenty-one measles cases were identified. The median age was 12 months (range, 4 months to 51 years) and 14 (67%) were hospitalized (range of stay, 2-7 days). The source was a 30-month-old US-born child of Somali descent infected while visiting Kenya. Measles spread in several settings, and over 3000 individuals were exposed. Sixteen case-patients were unvaccinated; 9 of the 16 were age-eligible: 7 of the 9 had safety concerns and 6 were of Somali descent. MMR vaccine coverage among Somali children declined significantly from 2004 through 2010 starting at 91.1% in 2004 and reaching 54.0% in 2010 (χ(2) for linear trend 553.79; P < .001). This was the largest measles outbreak in Minnesota in 20 years, and aggressive response likely prevented additional transmission. Measles outbreaks can occur if undervaccinated subpopulations exist. Misunderstandings about vaccine safety must be effectively addressed.

Keywords: immunization coverage assessment; immunization information system; measles; measles outbreak; measles transmission; measles-mumps-rubella vaccine; post-exposure prophylaxis; undervaccinated subpopulation; vaccine hesitancy; vaccine-preventable diseases; voluntary isolation and quarantine.

FIGURE 1

Comparison of 24-month-old children born in Minnesota of Somali descent and non-Somali descent; MMR versus varicella vaccinations, Hennepin County, Minnesota. MMR vaccine coverage among Somali children declined significantly from 2004 through 2010, starting at 91.1% in 2004 and reaching 54.0% in 2010 (P < 0.001). Varicella vaccine also declined significantly from 90.2% to 75.1 % (P < 0.001).

FIGURE 2

Confirmed measles cases by exposure site and rash onset date, February 15 – April 24, 2011.