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Background Measles elimination (interruption of endemic measles virus transmission) in the United States was declared in 2000; however, the number of cases and outbreaks have increased in recent years. We characterized the epidemiology of measles outbreaks and measles transmission patterns after elimination to identify potential gaps in the US measles control program. Methods We analyzed national measles notification data from 1 January 2001 to 31 December 2019. We defined measles infection clusters as single cases (isolated cases not linked to additional cases), 2-case clusters, or outbreaks with ≥3 linked cases. We calculated the effective reproduction number (R) to assess changes in transmissibility and reviewed molecular epidemiology data. Results During 2001–2019, a total of 3873 measles cases, including 747 international importations, were reported in the United States; 29% of importations were associated with outbreaks. Among 871 clusters, 69% were single cases and 72% had no spread. Larger and longer clusters were reported since 2013, including 7 outbreaks with >50 cases lasting >2 months, 5 of which occurred in known underimmunized, close-knit communities. No measles lineage circulated in a single transmission chain for >12 months. Higher estimates of R were noted in recent years, although R remained below the epidemic threshold of 1. Conclusions Current epidemiology continues to support the interruption of endemic measles virus transmission in the United States. However, larger and longer outbreaks in recent postelimination years and emerging trends of increased transmission in underimmunized communities emphasize the need for targeted approaches to close existing immunity gaps and maintain measles elimination.

Between 2001 and 2014, 78 reported measles cases resulted from transmission in US healthcare facilities, and 29 healthcare personnel were infected from occupational exposure, 1 of whom transmitted measles to a patient. The economic impact of preventing and controlling measles transmission in healthcare facilities was $19 000-$114 286 per case.

Background Measles, a vaccine-preventable viral illness that can cause serious complications, was declared eliminated from the United States in 2000 because of a successful measles vaccination program. Recent years have seen an increase in the number of measles cases and outbreaks. We summarized measles epidemiology in the United States during 2017–2019. Methods We reviewed US national surveillance data on confirmed measles cases reported to the Centers for Disease Control and Prevention during January 1, 2017–April 26, 2019. We describe the demographic characteristics, vaccination status, and disease epidemiology of measles cases. Results During 2017–April 2019, 1,196 measles cases were reported in 37 US States and Washington DC, including 146 (12%) importations from 37 countries; 108 (74%) of importations were US residents returning from travel abroad, of which 60 (56%) were unvaccinated and 31 (29%) had unknown vaccinations status. Among 1,148 cases who were US-residents, the highest incidence of measles was among infants and children aged 6–11 and 12–15 months (112 cases [19 cases/million person-years] and 106 cases [27 cases/million person-years], respectively). Among US-resident cases, 846 (74%) were unvaccinated and 163 (14%) had unknown vaccination status; 777 (68%) were considered to have preventable measles (i.e., were eligible for vaccination but unvaccinated). Among the 1,196 cases, 85 were single cases, and the remaining 1,111 represented 19 two-case chains and 34 outbreaks of 3 or more cases linked epidemiologically; the median outbreak size and duration was 6 cases (range, 3 to 452 cases) and 19.5 days (range, 5 to 205 days). A total of 934 (78%) of the 1196 cases and 13 (38%) of the 34 outbreaks occurred in under-immunized close-knit communities; eight outbreaks are ongoing. Conclusion Outbreaks of measles in the United States result from recurring measles introductions and subsequent measles spread, especially in under-immunized close-knit communities. To sustain measles elimination, it will be necessary to maintain timely routine high coverage with MMR vaccine, improve implementation of pretravel recommendations to minimize importations, and close immunity gaps in communities of US residents who remain unvaccinated. Disclosures All Authors: No reported Disclosures.

Background. Measles affected entire birth cohorts in the prevaccine era but was declared eliminated in the United States in 2000 because of a successful measles vaccination program.Methods. We reviewed US surveillance data on confirmed measles cases reported to the Centers for Disease Control and Prevention and data on national measles-mumps-rubella (MMR) vaccination coverage during postelimination years 2001–2008.Results. During 2001–2008, a total of 557 confirmed cases of measles (annual median no. of cases, 56) and 38 outbreaks (annual median no. of outbreaks, 4) were reported in the United States; 232 (42%) of the cases were imported from 44 countries, including European countries. Among case-patients who were US residents, the highest incidences of measles were among infants 6–11 months of age and children 12–15 months of age (3.5 and 2.6 cases/1 million person-years, respectively). From 2001 through 2008, national 1-dose MMR vaccine coverage among children 19–35 months of age ranged from 91% to 93%. From 2001 through 2008, a total of 285 US-resident case-patients (65%) were considered to have preventable measles (ie, the patients were eligible for vaccination but unvaccinated). During 2004–2008, a total of 68% of vaccine-eligible US-resident case-patients claimed exemptions for personal beliefs.Conclusions. The United States maintained measles elimination from 2001 through 2008 because of sustained high vaccination coverage. Challenges to maintaining elimination include large outbreaks of measles in highly traveled developed countries, frequent international travel, and clusters of US residents who remain unvaccinated because of personal belief exemptions.

In 1969, the United States established its national rubella vaccination program. With the success of the program, 32 years later, reports of rubella reached record low numbers. To assess the achievement of elimination of rubella and congenital rubella syndrome (CRS) in the United States, 7 epidemiological criteria were used. Rubella cases reported to the National Notifiable Diseases Surveillance System from 1998 through 2004 and CRS cases reported to the National Congenital Rubella Syndrome Registry from 1998 through 2004 were analyzed. During 1998-2000, the median number of reported rubella cases was 272, whereas, during 2001-2004, the median number reported was 13. The incidence of rubella decreased significantly, from 0.1/100,000 population in 1998 to 0.005/100,000 population in 2004. Since 2001, 5 infants with CRS have been reported--3 were born in 2001, 1 was born in 2003, and 1 was born in 2004. The epidemiological evidence strongly supports the claim that rubella is no longer endemic in the United States. To prevent future rubella outbreaks and CRS cases, current strategies must be maintained.

To determine trends in international importations of measles, data from the National Notifiable Diseases Surveillance System were analyzed. Of the 2632 measles cases reported between 1993 and 2001, 449 cases (17%) were internationally imported. An additional 186 cases (7%) resulted from spread of measles virus from these imported cases, and 388 cases (15%) had virological evidence of importation. The number of imported cases averaged 50 per year (range, 26-79 cases). The proportion of cases imported increased from an average of 14% in 1993-1996 to an average of 35% in 1997-2001. Imported measles cases were acquired in 63 countries, with 6 countries (Japan, Germany, China, the Philippines, Italy, and the United Kingdom) accounting for 44% of all imported cases. Further reduction of measles in the United States requires international cooperation and improved global surveillance and control of measles.

The status of measles elimination is best summarized by evaluation of the effective reproduction number R; maintaining R1 is necessary and sufficient to achieve elimination. Previously described methods for estimating R from the sizes and durations of chains of measles transmission and the proportion of cases imported were applied to the measles data reported for the United States in 1997-1999. These comprised 338 cases, forming 165 chains of transmission, of which 43 had1 case. One hundred seven cases were classified as importations. All 3 methods suggested that R was in the range 0.6-0.7. Results were not sensitive to the minimum size and duration of outbreak considered (so long as single-case chains were excluded) or to exclusion of chains without a known imported source. These results demonstrate that susceptibility to measles was beneath the epidemic threshold and that endemic transmission was eliminated.

As part of a strategy to eliminate measles, 7 indicators were adopted in the United States in 1996 to ensure the quality of measles surveillance. This report summarizes the US experience with these indicators during 1996-1998. The indicators are compiled from data reported to the Centers for Disease Control and Prevention (CDC) during routine surveillance supplemented with information collected directly from states. Measles case investigations are generally thorough, and sufficient information is collected to control and monitor disease. A high proportion of measles cases are imported from other countries, suggesting that investigations are complete. For some states, the lag from disease onset to reporting is long, and the number of health department investigations of measleslike illnesses is low. Most of these investigations include laboratory testing of clinical specimens. Collection of measles virus specimens from cases for genetic analysis needs improvement. The CDC and health departments need to continue efforts directed at health care professionals to ensure the recognition, proper diagnostic workup, and reporting of measles.

Evidence that endemic measles has been eliminated in the United States rests on the performance of the surveillance system. Information from national surveillance data allows us to evaluate the adequacy of national surveillance to detect the circulation of endemic measles. Sources of data include measles report dates, international importation status, and the size of chains of measles transmission. The proportion of chains of measles transmission that can be epidemiologically linked to international importations is high (62%), as would be expected if measles is no longer circulating; the number of imported cases, although lower than estimated expected values, is within a reasonable range of expectation. National surveillance detects even small outbreaks, so larger outbreaks that are the marker for endemic transmission would almost certainly be detected. Few unreported cases of measles are detected when health departments conduct careful investigations in response to reports of an index case. Surveillance appears to be adequate to support the contention that measles is no longer endemic in the United States.

To evaluate the extent of measles virus circulation and populations at risk in the United States, we reviewed measles outbreaks during 1993-2001. A total of 120 measles outbreaks, constituting 1804 outbreak-related cases, were reported during this period. The maximum outbreak size decreased from 233 cases in 1993-1995 to 119 cases in 1996-1998 and 15 cases in 1999-2001. The maximum outbreak duration decreased from 127 days in 1993-1995 to 65 days in 1999-2001. The majority of outbreaks resulted from documented spread from an internationally imported case (42%) or had a strain of measles virus not endemic in the United States (12%). Outbreaks in which adults were the predominant age group affected accounted for 35% of all outbreaks, compared with 29% of outbreaks predominantly affecting preschool children, 30% predominantly affecting school-aged children and adolescents, and 6% with no predominant age group. The extremely limited size and duration of measles outbreaks indicates very high population immunity to measles and suggests that measles is no longer endemic in the United States.

The elimination of endemic measles from the United States has been a national goal since the introduction of measles vaccine, and measles surveillance has been crucial to guide the elimination efforts. The United States surveillance system is geared towards detection of measles virus transmission, rapid discovery of measles outbreaks to facilitate outbreak control, and identification of risk factors for measles. The surveillance system is a passive reporting system that, when activated by a reported case of suspected measles, triggers a search for additional cases around the reported case. Cases are typically reported by health care providers or from schools and day care centers. The sensitivity of the system is increased through reporting and investigation of all suspected measles cases by means of an inclusive case definition (generalized maculopapular rash and fever), and the specificity is increased through laboratory testing for measles of all suspected cases.

This report describes measles virus surveillance in the United States for 1989-2001. During the resurgence of measles in the United States between 1989 and 1992, only viruses of genotype D3 were isolated. In contrast, virological surveillance conducted after the resurgence period showed that at least 12 different genotypes were associated with the greatly reduced number of measles cases. Eight different genotypes were identified for 27 chains of transmission in which the source of infection was unknown. The diversity of measles virus genotypes observed in the United States between 1994 and 2001 reflected multiple imported sources of virus and indicated that no genotype of measles is endemic in the United States. Therefore, the data obtained from virological surveillance are consistent with the conclusions made by disease surveillance and epidemiological investigations that measles is no longer an endemic disease in the United States.

Genetic analysis was conducted on 28 wild type measles viruses isolated from outbreaks or cases in the United States during 1995-1996. These viruses were members of at least 6 distinct genetic groups. However, none of these viruses was related to the group 2 viruses that were associated with the resurgence of measles in the United States between 1989 and 1992 except for a single importation from the Philippines. The sequence data support and extend previous findings showing that transmission of group 2 viruses within the United States was interrupted after 1993. The data also suggest that all measles cases that occurred in the United States in 1995-1996 were the result of importation of virus, even in instances when the source was unknown. Molecular epidemiologic studies can provide a means to measure the success of measles control programs by helping to identify the transmission pathways of the virus.

Background Measles is a highly contagious vaccine-preventable disease that can lead to serious complications. Although measles was declared eliminated from the United States in 2000, measles cases and outbreaks continue to occur each year as a result of importations of the disease from countries in which it remains endemic. We describe the epidemiology of importations of measles virus into the US during the post-elimination era. Methods Measles is nationally notifiable in the USA and local and state health departments report confirmed measles cases to the Centers for Disease Control and Prevention (CDC). A case is considered internationally imported if at least some of the exposure period (7-21 days before rash onset) occurred outside of the USA, rash onset occurred within 21 days of entering the USA, and there was no known exposure to measles in the US during that time. We describe the demographic characteristics, source regions, and vaccination status of measles importations during 2001–2015. Results From 2001 to 2015, 2,012 measles cases were reported to CDC; 535 (27%) were imported. A median of 28 importations occurred each year (range: 18–80). The median age of imported cases was 18 years (range

Background Measles, a vaccine-preventable disease that can cause severe complications, was declared eliminated from the United States in 2000. The last published summary of US measles epidemiology was during 2001-2008. We summarized US measles epidemiology during 2009-2014. Methods We compared demographic, vaccination, and virologic data on confirmed measles cases reported to the Centers for Disease Control and Prevention during January 1, 2009-December 31, 2014 and January 1, 2001-December 31, 2008. Results During 2009-2014, 1264 confirmed measles cases were reported in the United States, including 275 importations from 58 countries and 66 outbreaks. The annual median number of cases and outbreaks during this period was 130 (range, 55-667 cases) and 10 (range, 4-23 outbreaks), respectively, compared with an annual median of 56 cases (P = .08) and 4 outbreaks during 2001-2008 (P = .04). Among US-resident case-patients during 2009-2014, children aged 12-15 months had the highest measles incidence (65 cases; 8.3 cases/million person-years), and infants aged 6-11 months had the second highest incidence (86 cases; 7.3 cases/million person-years). During 2009-2014, 865 (74%) of 1173 US-resident case-patients were unvaccinated and 188 (16%) had unknown vaccination status; of 917 vaccine-eligible US-resident case-patients, 600 (65%) were reported as having philosophical or religious objections to vaccination. Conclusions Although the United States has maintained measles elimination since 2000, measles outbreaks continue to occur globally, resulting in imported cases and potential spread. The annual median number of cases and outbreaks more than doubled during 2009-2014 compared with the earlier postelimination years. To maintain elimination, it will be necessary to maintain high 2-dose vaccination coverage, continue case-based surveillance, and monitor the patterns and rates of vaccine exemption.