Modified Measles: Surveillance Challenges When Clinical Presentation Is Non-Classic

BACKGROUND: A modified form of measles can occur in people with some degree of immunity to the virus, including certain previously vaccinated persons, and those with passive immunity following receipt of immunoglobulin ( IG) after exposure to a measles case. The incubation period may be as long as 21 days and symptoms tend to be milder and last a shorter period of time than those of a normal measles presentation. Surveillance for modified measles is challenging because cases occur infrequently and clinical manifestations vary. The illness may not have the classic features of prodrome, Koplik spots, or even rash so recognizing the syndrome can be difficult for healthcare providers and public health professionals alike. Results of laboratory testing for these cases can be difficult to interpret and additional testing at a national reference laboratory is usually required. Potential for transmission of virus from a modified measles case to others is thought to be low, but data is scarce. Our objective is to describe the clinical, epidemiologic and laboratory findings for cases of measles and results of contact investigations for these individuals.

METHODS: We present case scenarios for three modified measles investigations in Washington (WA) State.  For each, we present risk factors, incubation period, clinical presentation, laboratory findings, settings with potential for transmission of measles virus to others, control measures used, and outcomes.

RESULTS: Two lab-confirmed cases of modified measles occurred in children previously immunized with one dose of measles-containing vaccine. One case had an unknown exposure.  One suspected case in a 41-year old adult teacher with unknown immunization history and potential exposure to measles at an international airport was ruled out by further testing at CDC.  Investigation and confirmation of each of these three cases required intensive use of public health resources at the local, state and federal level. Measures used to control transmission varied. No transmission of measles from these modified cases to others was documented.

CONCLUSIONS:  Modified measles is an infrequent but ongoing vaccine-preventable disease surveillance issue. It is likely that many modified cases are never identified at all, or a negative result on a test for immunoglobulin M antibody may lead to a modified case being considered “ruled out”. It is important to promote increased awareness in the public health and healthcare communities about variable clinical manifestations of modified measles, along with appropriate uses of confirmatory laboratory tests, and factors to consider when implementing disease control measures.