Local and systemic responses to SARS-CoV-2 infection in children and adults.

It is not fully understood why COVID-19 is typically milder in children1-3. Here, to examine the differences between children and adults in their response to SARS-CoV-2 infection, we analysed paediatric and adult patients with COVID-19 as well as healthy control individuals (total n = 93) using single-cell multi-omic profiling of matched nasal, tracheal, bronchial and blood samples. In the airways of healthy paediatric individuals, we observed cells that were already in an interferon-activated state, which after SARS-CoV-2 infection was further induced especially in airway immune cells. We postulate that higher paediatric innate interferon responses restrict viral replication and disease progression. The systemic response in children was characterized by increases in naive lymphocytes and a depletion of natural killer cells, whereas, in adults, cytotoxic T cells and interferon-stimulated subpopulations were significantly increased. We provide evidence that dendritic cells initiate interferon signalling in early infection, and identify epithelial cell states associated with COVID-19 and age. Our matching nasal and blood data show a strong interferon response in the airways with the induction of systemic interferon-stimulated populations, which were substantially reduced in paediatric patients. Together, we provide several mechanisms that explain the milder clinical syndrome observed in children.

Ride-hailing technology mitigates effects of driver racial discrimination, but effects of residential segregation persist.

We assess racial disparities in the service quality of app-based ride-hailing services, like Uber and Lyft, by simulating their operations in the city of Chicago using empirical data. To generate driver cancellation rate disparities consistent with controlled experiments (up to twice as large for Black riders as for White riders), we estimate that more than 3% of drivers discriminate by race. We find that the capabilities of ride-hailing technology to rapidly rematch after a cancellation and prioritize long-waiting customers heavily mitigates the effects of driver discrimination on rider wait times, reducing average discrimination-induced disparities to less than 1 min-an order of magnitude less than traditional taxis. However, our results suggest that even in the absence of direct driver discrimination, Black riders in Chicago wait about 50% longer, on average, than White riders because of historically informed geographic residential patterns. We estimate that if Black riders in the city had the same wait times as White riders, the collective travel time saved would be worth $4.2 million to $7.0 million per year.

HIV, inflammation, and initiation of methamphetamine use in sexual and gender minorities assigned male at birth.

Methamphetamine use and HIV disproportionately affect sexual and gender minority (SGM) people assigned male at birth. Identifying risk factors for methamphetamine use is crucial to inform preventive interventions. In this cohort study with 1,296 SGM people assigned male at birth, ages 16 to 29, and who resided in Chicago, Poisson regression analyses indicated the prevalence of methamphetamine use increased from 2015 to 2023 [Incidence Rate Ratio (IRR) = 1.07; 95% CI = 1.01 to 1.13; P = 0.02]. This increase was most pronounced among those ages 25 or older at baseline (IRR = 2.20; 95% CI = 1.33 to 3.63; P = 0.002), and 23.9 [Interquartile Range (IQR) = 22.1 to 26.9] was the median age of first-time methamphetamine use. In 826 participants with a prior HIV diagnosis or previous inflammatory measurements, Cox proportional-hazards models examined risk factors for incident, first-time methamphetamine use. Adjusting for other substance use, the rate of incident, first-time methamphetamine use was two-fold greater after HIV diagnosis [adjusted hazard ratio (aHR) = 2.02; 95% CI = 1.27 to 3.23; P = 0.003]. For each SD higher C-reactive protein, the rate of incident, first-time methamphetamine use was 18% greater (aHR = 1.18; 95% CI, 1.05 to 1.34; P = 0.008). HIV seroconversion and inflammation could increase the risk of initiating methamphetamine use in SGM people assigned male at birth.

Health Care Personnel Exposure Risk Assessment and Management During a Mpox Outbreak in Chicago, Illinois, 17 May to 8 July 2022.

This report summarizes risk assessment interviews and follow-up with health care personnel (HCP) after exposure to patients with mpox disease during 17 May to 8 July 2022. HCP-case interactions were assessed using a standard questionnaire to categorize the risk associated with patient encounters. We assessed 150 interactions among 142 HCP and 30 cases. Four (2.7%) interactions were defined as high risk, 5 (3.3%) intermediate, 107 (71.3%) low, and 31 (20.7%) no risk. High and intermediate exposures were offered postexposure prophylaxis; 4 accepted. No documented mpox transmission after exposure was identified. These findings suggest transmission risk in health care settings during routine patient care is low.

Prior Sexually Transmitted Infections and HIV in Patients With Mpox, Chicago, Illinois (June 2022-March 2023).

HIV is associated with severe mpox. Sexually transmitted infections (STIs) could facilitate mpox transmission. We estimated HIV and STI frequency among patients with mpox and compared characteristics associated with mpox severity. Mpox cases during 1 June 2022 to 31 March 2023 were matched to Illinois HIV/AIDS surveillance data. Among 1124 patients with mpox, 489 (44%) had HIV and 786 (70%) had prior or concurrent STI; 307 (39%) had ≥3 STI episodes. More patients with mpox who were living with HIV were hospitalized than those without HIV (10.3% vs 4.1%, P < .001). STI screening visits are opportunities to vaccinate against mpox and provide HIV prophylaxis or treatment.

Comparison of Daily Versus Admission and Discharge Surveillance Cultures for Multidrug-Resistant Organism Detection in an Intensive Care Unit.

BACKGROUND: Admission and discharge screening of patients for asymptomatic gut colonization with multidrug-resistant organisms (MDROs) is a common approach to active surveillance, but its sensitivity for detecting colonization is uncertain. METHODS: Daily rectal or fecal swab samples and associated clinical data were collected over 12 months from patients in one 25-bed medical intensive care unit (ICU) in Chicago, IL and tested for the following MDROs: vancomycin-resistant enterococci; third-generation cephalosporin-resistant Enterobacterales, including extended-spectrum ß-lactamase-producing Enterobacterales; and carbapenem-resistant Enterobacterales. MDRO detection by (1) admission and discharge surveillance cultures or (2) clinical cultures were compared to daily surveillance cultures. Samples underwent 16S rRNA gene sequencing to measure the relative abundance of operational taxonomic units (OTUs) corresponding to each MDRO. RESULTS: Compared with daily surveillance cultures, admission/discharge cultures detected 91% of prevalent MDRO colonization and 63% of MDRO acquisitions among medical ICU patients. Few (7%) MDRO carriers were identified by clinical cultures alone. Higher relative abundance of MDRO-associated OTUs and specific antibiotic exposures were independently associated with higher probability of MDRO detection by culture. CONCLUSIONS: Admission and discharge surveillance cultures underestimated MDRO acquisitions in an ICU. These limitations should be considered when designing sampling strategies for epidemiologic studies that use culture-based surveillance.

Investigation of an Mpox Outbreak Affecting Many Vaccinated Persons in Chicago, Illinois-March 2023-June 2023.

BACKGROUND: After months of few mpox cases, an increase in cases was reported in Chicago during May 2023, predominantly among fully vaccinated (FV) patients. We investigated the outbreak scope, differences between vaccinated and unvaccinated patients, and hypotheses for monkeypox virus (MPXV) infection after vaccination. METHODS: We interviewed patients and reviewed medical records to assess demographic, behavioral, and clinical characteristics; mpox vaccine status; and vaccine administration routes. We evaluated serum antibody levels after infection and compared patient viral genomes with MPXV sequences in available databases. We discussed potential vaccine compromise with partners who manufactured, handled, and administered the vaccine associated with breakthrough infections. RESULTS: During 18 March-27 June 2023, we identified 49 mpox cases; 57% of these mpox patients were FV. FV patients received both JYNNEOS doses subcutaneously (57%), intradermally (7%), or via heterologous administration (36%). FV patients had more median sex partners (3; interquartile range [IQR] = 1-4) versus not fully vaccinated patients (1; IQR = 1-2). Thirty-six of 37 sequenced specimens belonged to lineage B.1.20 of clade IIb MPXV, which did not demonstrate any amino acid changes relative to B.1, the predominant lineage from May 2022. Vaccinated patients demonstrated expected humoral antibody responses; none were hospitalized. No vaccine storage excursions were identified. Approximately 63% of people at risk for mpox in Chicago were FV during this period. CONCLUSIONS: Our investigation indicated that cases were likely due to frequent behaviors associated with mpox transmission, even with relatively high vaccine effectiveness and vaccine coverage. Cases after vaccination might occur in similar populations.

Where you live matters: Area deprivation predicts poor survival and liver transplant waitlisting.

Social determinants of health (SDOH) are important predictors of poor clinical outcomes in chronic diseases, but their associations among the general cirrhosis population and liver transplantation (LT) are limited. We conducted a retrospective, multiinstitutional analysis of adult (≥18-years-old) patients with cirrhosis in metropolitan Chicago to determine the associations of poor neighborhood-level SDOH on decompensation complications, mortality, and LT waitlisting. Area deprivation index and covariates extracted from the American Census Survey were aspects of SDOH that were investigated. Among 15 101 patients with cirrhosis, the mean age was 57.2 years; 6414 (42.5%) were women, 6589 (43.6%) were non-Hispanic White, 3652 (24.2%) were non-Hispanic Black, and 2662 (17.6%) were Hispanic. Each quintile increase in area deprivation was associated with poor outcomes in decompensation (sHR [subdistribution hazard ratio] 1.07; 95% CI 1.05-1.10; P < .001), waitlisting (sHR 0.72; 95% CI 0.67-0.76; P < .001), and all-cause mortality (sHR 1.09; 95% CI 1.06-1.12; P < .001). Domains of SDOH associated with a lower likelihood of waitlisting and survival included low income, low education, poor household conditions, and social support (P < .001). Overall, patients with cirrhosis residing in poor neighborhood-level SDOH had higher decompensation, and mortality, and were less likely to be waitlisted for LT. Further exploration of structural barriers toward LT or optimizing health outcomes is warranted.

When Equality Is Not Equity: The Ethics of Access to Trauma Care: A Surgical Perspective.

OBJECTIVE: The purpose of this surgical perspective is to describe the trauma care needs of the South Side of Chicago and the creation of an adult trauma center at the University of Chicago Medicine and associated hospital-based violence intervention program. BACKGROUND: Traumatic injury is a leading cause of death and disability in the United States. Disparities across the continuum of trauma care exist, which are often rooted in the social determinants of health. Trauma center distribution is critical to timely treatment and should be based on the trauma needs of the area. The previous trauma ecosystem of Chicago was incongruent with the concentration of violent injuries on the south and west sides of the city, leading to a fallacy of distributive justice. METHODS: A descriptive analysis of community partners, trauma program leadership, trauma surgeons, and the violence intervention program director was performed. RESULTS: The UCM trauma center opened in May 2018 and has since been one of the busiest trauma centers in the country, with a 40% penetrating trauma rate. There have been significant reductions in patient transport time on the South Side up to 8.9 minutes ( P <0.001). The violence intervention program employs credible messengers with lived experience representing the community and has engaged over 8000 patients since 2018, developing both community-based and medical-legal partnerships. CONCLUSIONS: The persistent efforts of the community and key stakeholders led to a system change that improved trauma care for the South Side of Chicago.

Assessing Neonatal Intensive Care Unit (NICU) Graduates Across Varied Settings: A Study on the Feasibility of the Baby Moves App.

We assessed the feasibility of obtaining parent-collected General Movement Assessment videos using the Baby Moves app. Among 261 participants from 4 Chicago NICUs, 70% submitted videos. Families living in higher areas of childhood opportunity used the app more than those from areas of lower opportunity.

Perceived Stress, Blood Biomarkers, and Cognitive Functioning in Older Adults.

INTRODUCTION: There is a substantial gap in knowledge regarding how perceived stress may influence the relationship between serum-measured biomarkers for Alzheimer's disease and cognitive decline. METHODS: This study consists of 1118 older adult participants from the Chicago Health and Aging Project (CHAP) (60% Black participants and 63% female participants). Linear mixed effects regression models were conducted to examine the role of perceived stress in the association between three blood biomarkers: total tau (t-tau), glial fibrillary acid protein (GFAP), and neurofilament light chain (NfL) on global cognitive decline. Stratified analysis by stress level was also conducted to evaluate the associations between each blood biomarker and baseline cognitive function and decline. All models adjusted for age, race, sex, education, time, and their interactions with time. RESULTS: The interaction of stress, NfL concentration, and time was statistically significant on global cognition ( ß = -0.064 [SE = 0.028], p = .023) and on episodic memory ( ß = -0.097 [SE = 0.036], p = .007). CONCLUSIONS: Greater stress level worsens the association between high NfL concentration and cognitive decline. Stress management interventions may be helpful to reduce the rate of cognitive decline in individuals with high concentrations of NfL.

Implementing and Enhancing Social and Economic Needs Screening at a Federally Qualified Health Center.

BACKGROUND: Programs to screen for social and economic needs (SENs) are challenging to implement. AIM: To describe implementation of an SEN screening program for patients obtaining care at a federally qualified health center (FQHC). SETTING: Large Chicago-area FQHC where many patients are Hispanic/Latino and insured through Medicaid. PROGRAM DESCRIPTION: In the program's phase 1 (beginning April 2020), a prescreening question asked about patients' interest in receiving community resources; staff then called interested patients. After several refinements (e.g., increased staffing, tailored reductions in screening frequency) to address challenges such as a large screening backlog, program phase 2 began in February 2021. In phase 2, a second prescreening question asked about patients' preferred modality to learn about community resources (text/email versus phone calls). PROGRAM EVALUATION: During phase 1, 8925 of 29,861 patients (30%) expressed interest in community resources. Only 40% of interested patients were successfully contacted and screened. In phase 2, 5781 of 21,737 patients (27%) expressed interest in resources; 84% of interested patients were successfully contacted by either text/email (43%) or phone (41%). DISCUSSION: Under one-third of patients obtaining care at an FQHC expressed interest in community resources for SENs. After program refinements, rates of follow-up with interested patients substantially increased.

Using latent class analysis to investigate enduring effects of intersectional social disadvantage on long-term vocational and financial outcomes in the 20-year prospective Chicago Longitudinal Study.

BACKGROUND: Class and social disadvantage have long been identified as significant factors in the etiology and epidemiology of psychosis. Few studies have explicitly examined the impact of intersecting social disadvantage on long-term employment and financial independence. METHODS: We applied latent class analysis (LCA) to 20-year longitudinal data from participants with affective and non-affective psychosis (n = 256) within the Chicago Longitudinal Research. LCA groups were modeled using multiple indicators of pre-morbid disadvantage (parental social class, educational attainment, race, gender, and work and social functioning prior to psychosis onset). The comparative longitudinal work and financial functioning of LCA groups were then examined. RESULTS: We identified three distinct latent classes: one comprised entirely of White participants, with the highest parental class and highest levels of educational attainment; a second predominantly working-class group, with equal numbers of Black and White participants; and a third with the lowest parental social class, lowest levels of education and a mix of Black and White participants. The latter, our highest social disadvantage group experienced significantly poorer employment and financial outcomes at all time-points, controlling for diagnosis, symptoms, and hospitalizations prior to baseline. Contrary to our hypotheses, on most measures, the two less disadvantaged groups did not significantly differ from each other. CONCLUSIONS: Our analyses add to a growing literature on the impact of multiple forms of social disadvantage on long-term functional trajectories, underscoring the importance of proactive attention to sociostructural disadvantage early in treatment, and the development and evaluation of interventions designed to mitigate ongoing social stratification.

Exploring decision-making performance in young adults with mental health disorders: a comparative study using the Cambridge gambling task.

Decision-making deficits, assessed cognitively, are often associated with mental health symptoms, however, this relationship is not fully understood. This paper explores the relationship between mental health disorders and decision-making, using the Cambridge Gambling Task (CGT). Our study investigated how decision-making varied across 20 different mental health conditions compared to controls in a sample of 572 young adults from the Minneapolis and Chicago metropolitan areas, using a computerized laboratory-based task. Almost all mental health conditions were associated with at least mild (i.e. at least small effect size) impairment in all three studied parameters of the CGT (risk adjustment, quality of decision-making and overall proportion of bet). Notably, binge eating disorder had the largest cognitive impairment and gambling disorder had moderate impairment. Post-traumatic stress disorder (PTSD) was associated with impaired decision-making while obsessive-compulsive disorder (OCD) and depression showed moderate impairment. Additionally, half of the disorders assessed had moderate to large impairment in risk adjustment.These findings suggest that mental health conditions may have a more complex cognitive profile than previously thought, and a better understanding of these impairments may aid in risk assessment and targeted clinical interventions. This study underscores the need for further research to determine the causal pathways between mental health conditions and cognition, as well as to better understand the day-to-day impact of such deficits.

Clinical and diagnostic features of long-COVID patients presenting with neurologic symptoms in Chicago.

Long COVID, a condition characterized by persistent symptoms after COVID-19 infection, is increasingly being recognized worldwide. Neurologic symptoms are frequently reported in survivors of COVID-19, making it crucial to better understand this phenomenon both on a societal scale and for the quality of life of these patients. Between January 1, 2020, and July 31, 2022, Illinois (IL) had a standardized cumulative death rate that ranked it 24th out of the 51 states in the United States (US). However, the US had one of the highest per capita COVID-19 death rates among large, high-income countries. [Bollyky T. et al. 2023] As a result of the increased number of COVID-19 infections, there was a rise in the number of patients experiencing Long COVID. At our neuro-infectious disease clinic in Chicago (IL), we observed an increasing number of patients presenting with cognitive and other neurologic symptoms after contracting COVID-19. Initially, we needed to provide these individuals with a better understanding of their condition and expected outcomes. We were thus motivated to further evaluate this group of patients for any patterns in presentation, neurologic findings, and diagnostic testing that would help us better understand this phenomenon. We aim to contribute to the growing body of research on Long COVID, including its presentation, diagnostic testing results, and outcomes to enlighten the long COVID syndrome. We hypothesize that the neurological symptoms resulting from long COVID persist for over 12 months. We conducted a retrospective analysis of clinical data from 44 patients with long-COVID. Cognitive symptoms were the most common presenting concern. Abnormalities in Montreal Cognitive Assessment, electroencephalogram, serum autoantibody testing, and cerebrospinal fluid were found in minority subsets of our cohort. At 12 months, most patients continue to experience neurologic symptoms, though more than half reported moderate or marked improvement compared to initial presentation. Although most of the patients in this study did not show a consistent occurrence of symptoms suggesting a cohesive underlying etiology, our clinical data demonstrated some features of Long COVID patients in Chicago (IL) that could lead to new research avenues, helping us better understand this syndrome that affects patients worldwide.

Suicide Methods and Trends Across Race/Ethnicity, Age, and Sex Groups in Chicago, Illinois, 2015-2021.

Objectives. To measure differences in suicide rates across race/ethnicity, age, and sex groups in Chicago, Illinois, from 2015 to 2021. Methods. We calculated the incidence rate and annual percentage change in suicides among Asian, Black, Latino/a, and White persons in Chicago. We also analyzed patterns in suicide method across race/ethnicity, age, and sex groups. Results. Suicides increased significantly among Black males (incidence rate ratio [IRR] = 1.10; 95% confidence interval [CI] = 1.01, 1.20), Black females (IRR = 1.18; 95% CI = 1.04, 1.33), and Latino males (IRR = 1.23; 95% CI = 1.11, 1.38) between 2015 and 2021. Suicides decreased overall among White Chicagoans during this period. A significantly greater proportion of Black males than Black females died by suicide using a firearm (55.79% vs 24.05%; P < .001). Similar results were detected for Latino males and females (32.99% vs 9.09%; P = .001) and White males and females (30.10% vs 11.73%; P < .001). Conclusions. Black persons in Chicago were the only group to experience significant increases in suicide among both males and females from 2015 to 2021, although specific methods used varied by race/ethnicity and sex group. (Am J Public Health. 2024;114(3):319-328. https://doi.org/10.2105/AJPH.2023.307511).

Interventions to Mitigate the Impact of COVID-19 Among People Experiencing Sheltered Homelessness: Chicago, Illinois, March 1, 2020-May 11, 2023.

Objectives. To compare the incidence, case-hospitalization rates, and vaccination rates of COVID-19 between people experiencing sheltered homelessness (PESH) and the broader community in Chicago, Illinois, and describe the impact of a whole community approach to disease mitigation during the public health emergency. Methods. Incidence of COVID-19 among PESH was compared with community-wide incidence using case-based surveillance data from March 1, 2020, to May 11, 2023. Seven-day rolling means of COVID-19 incidence were assessed for the overall study period and for each of 6 distinct waves of COVID-19 transmission. Results. A total of 774 009 cases of COVID-19 were detected: 2579 among PESH and 771 430 in the broader community. Incidence and hospitalization rates per 100 000 in PESH were more than 5 times higher (99.84 vs 13.94 and 16.88 vs 2.14) than the community at large in wave 1 (March 1, 2020-October 3, 2020). This difference decreased through wave 3 (March 7, 2021-June 26, 2021), with PESH having a lower incidence rate per 100 000 than the wider community (8.02 vs 13.03). Incidence and hospitalization of PESH rose again to rates higher than the broader community in waves 4 through 6 but never returned to wave 1 levels. Throughout the study period, COVID-19 incidence among PESH was 2.88 times higher than that of the community (70.90 vs 24.65), and hospitalization was 4.56 times higher among PESH (7.51 vs 1.65). Conclusions. Our findings suggest that whole-community approaches can minimize disparities in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission between vulnerable populations and the broader community, and reinforce the benefits of a shared approach that include multiple partners when addressing public health emergencies in special populations. (Am J Public Health. 2024;114(S7):S590-S598. https://doi.org/10.2105/AJPH.2024.307801).

Years of Potential Life Lost and Mean Age of Adults Experiencing Nontraumatic, Out-of-Hospital Cardiac Arrests - Chicago, 2014-2021.

Approximately 1,000 out-of-hospital cardiac arrests (OHCAs) are assessed by emergency medical services in the United States every day, and approximately 90% of patients do not survive, leading to substantial years of potential life lost (YPLL). Chicago emergency medical services data were used to assess changes in mean age and YPLL from nontraumatic OHCA in adults in biennial cycles during 2014-2021. Among 21,070 reported nontraumatic OHCAs during 2014-2021, approximately 60% occurred among men and 57% among non-Hispanic Black or African American (Black) persons. YPLL increased from 52,044 during 2014-2015 to 88,788 during 2020-2021 (p = 0.002) and mean age decreased from 64.7 years during 2014-2015, to 62.7 years during 2020-2021. Decrease in mean age occurred among both men (p<0.001) and women (p = 0.002) and was largest among Black men. Mean age decreased among patients without presumed cardiac etiology from 56.3 to 52.5 years (p<0.001) and among patients with nonshockable rhythm from 65.5 to 62.7 years (p<0.001). Further study is needed to assess whether similar trends are occurring elsewhere, and to understand the mechanisms that underlie these trends in Chicago because these mechanisms could help guide prevention efforts. Increased public awareness of the risk of cardiac arrest and knowledge of how to intervene as a bystander could help decrease associated mortality.

Real-Time Use of a Dynamic Model To Measure the Impact of Public Health Interventions on Measles Outbreak Size and Duration - Chicago, Illinois, 2024.

Measles is a highly infectious, vaccine-preventable disease that can cause severe illness, hospitalization, and death. A measles outbreak associated with a migrant shelter in Chicago occurred during February-April 2024, in which a total of 57 confirmed cases were identified, including 52 among shelter residents, three among staff members, and two among community members with a known link to the shelter. CDC simulated a measles outbreak among shelter residents using a dynamic disease model, updated in real time as additional cases were identified, to produce outbreak forecasts and assess the impact of public health interventions. As of April 8, the model forecasted a median final outbreak size of 58 cases (IQR = 56-60 cases); model fit and prediction range improved as more case data became available. Counterfactual analysis of different intervention scenarios demonstrated the importance of early deployment of public health interventions in Chicago, with a 69% chance of an outbreak of 100 or more cases had there been no mass vaccination or active case-finding compared with only a 1% chance when those interventions were deployed. This analysis highlights the value of using real-time, dynamic models to aid public health response, set expectations about outbreak size and duration, and quantify the impact of interventions. The model shows that prompt mass vaccination and active case-finding likely substantially reduced the chance of a large (100 or more cases) outbreak in Chicago.

Measles Outbreak Associated with a Migrant Shelter - Chicago, Illinois, February-May 2024.

Measles, a highly contagious respiratory virus with the potential to cause severe complications, hospitalization, and death, was declared eliminated from the United States in 2000; however, with ongoing global transmission, infections in the United States still occur. On March 7, 2024, the Chicago Department of Public Health (CDPH) confirmed a case of measles in a male aged 1 year residing in a temporary shelter for migrants in Chicago. Given the congregate nature of the setting, high transmissibility of measles, and low measles vaccination coverage among shelter residents, measles virus had the potential to spread rapidly among approximately 2,100 presumed exposed shelter residents. CDPH immediately instituted outbreak investigation and response activities in collaboration with state and local health departments, health care facilities, city agencies, and shelters. On March 8, CDPH implemented active case-finding and coordinated a mass vaccination campaign at the affected shelter (shelter A), including vaccinating 882 residents and verifying previous vaccination for 784 residents over 3 days. These activities resulted in 93% measles vaccination coverage (defined as receipt of ≥1 recorded measles vaccine dose) by March 11. By May 13, a total of 57 confirmed measles cases associated with residing in or having contact with persons from shelter A had been reported. Most cases (41; 72%) were among persons who did not have documentation of measles vaccination and were considered unvaccinated. In addition, 16 cases of measles occurred among persons who had received ≥1 measles vaccine dose ≥21 days before first known exposure. This outbreak underscores the need to ensure high vaccination coverage among communities residing in congregate settings.