CDC report: Persistent measles importations challenge U.S. elimination efforts

A recent Centers for Disease Control and Protection (CDC) report published in the Morbidity and Mortality Weekly Report (MMWR) assessed the status of measles elimination in the United States (U.S.) and reported on the recent increase in cases by analyzing surveillance data from January 2020 to March 2024.

Study: Measles — United States, January 1, 2020–March 28, 2024. Image Credit: sulit.photos/Shutterstock.com

Background 

Measles, a highly contagious viral illness, was considered eliminated in the U.S. in 2000 due to high coverage with the measles, mumps, and rubella (MMR) vaccine.

Despite this, the elimination status faced challenges in 2019 following significant outbreaks in under-vaccinated communities in New York, contributing to a notable rise in cases.

Further research is needed to address gaps in vaccine coverage and enhance surveillance systems to prevent future outbreaks and sustain measles elimination.

About the study 

Confirmed measles cases are reported to the Centers for Disease Control and Prevention (CDC) by state health departments through the National Notifiable Disease Surveillance System and directly via email or telephone to the National Center for Immunization and Respiratory Diseases.

The Council of State and Territorial Epidemiologists classifies measles cases. Cases are deemed import-associated if they originate internationally, are epidemiologically linked to an imported case, or show viral genetic evidence of an imported measles genotype.

Conversely, cases without an epidemiologic or virologic connection to an imported source are categorized as having an unknown origin.

For analytical purposes, unique sequences are identified as those varying by at least one nucleotide in the N-450 sequence, following the World Health Organization’s (WHO)’s recommendations for describing sequence variants.

Patients who were unvaccinated yet eligible for vaccination were identified based on the criteria set by the Advisory Committee on Immunization Practices.

A surveillance system is considered effective if at least 80% of cases meet three criteria: classification as import-associated, comprehensive reporting on key surveillance indicators, and confirmation through laboratory testing.

Additionally, measles cases are categorized into chains of transmission based on known epidemiologic links. These include isolated cases, two-case chains involving two linked cases, and outbreaks consisting of three or more linked cases.

The analysis of two-case chains and outbreaks also involves assessing the potential for missed cases by examining the time intervals between the onset of measles rashes, with intervals exceeding one maximum incubation period suggesting possible unreported cases. 

Study results 

From January 1, 2020, to March 28, 2024, the CDC was notified of 338 confirmed measles cases across 30 jurisdictions. A noticeable portion of these cases, particularly 12 of 13 reported before the COVID-19 mitigation measures began in March 2020, marked the early stages of this period.

In the following years, 2021 and 2022 witnessed 170 reported cases, 78% of which were linked to distinct outbreaks. Notably, 47 of the 49 cases in 2021 were among Afghan evacuees at U.S. military bases during Operation Allies Welcome, and 86 of the 121 cases in 2022 stemmed from an outbreak in Central Ohio.

By 2023, 48% of the 58 cases reported were associated with four outbreaks. As of late March 2024, 97 cases had been reported for the year, signifying a significant increase over previous first-quarter averages.

The median age of the affected individuals was three years, ranging from newborns to 64 years old, with 58% of the cases occurring in the 16 months to 19 years age group.

A significant majority, 91% of patients, were either unvaccinated or had an unknown vaccination status, and of these, 84% were eligible for vaccination.

Hospitalization data showed that 46% of the patients were hospitalized, primarily children under five years old, with 92% being unvaccinated or of unknown vaccination status. There were no reported deaths due to measles.

Regarding the origin of the cases, 96% were linked to importation. A detailed breakdown reveals that the majority of the 326 import-related cases involved U.S. residents eligible yet unvaccinated or of unknown vaccination status.

The most frequent origins of these imported cases were from the Eastern Mediterranean and African WHO regions. However, the first quarter of 2024 saw increased European and Southeast Asian cases.

Surveillance quality was notably high, with nearly all cases (98%) including comprehensive data on key indicators. Timeliness of reporting showed that 58% of cases were reported to health departments on or before the day of rash onset.

Laboratory confirmations were achieved in 93% of cases, with a large majority verified through real-time reverse transcription-polymerase chain reaction (rRT-PCR) testing and successful genotyping of most specimens, identifying predominant measles genotypes B3 and D8.

Transmission patterns were categorized into 92 chains, with the majority being isolated cases. However, 20 chains qualified as outbreaks involving three or more cases, and the typical outbreak lasted 20 days, highlighting the rapid transmission potential of measles.

Notably, none of the two-case chains or larger outbreaks experienced a gap exceeding the maximum incubation period.