COVID-19 Case Investigation and Contact Tracing in the US, 2020.

Importance: Contact tracing is a multistep process to limit SARS-CoV-2 transmission. Gaps in the process result in missed opportunities to prevent COVID-19. Objective: To quantify proportions of cases and their contacts reached by public health authorities and the amount of time needed to reach them and to compare the risk of a positive COVID-19 test result between contacts and the general public during 4-week assessment periods. Design, Setting, and Participants: This cross-sectional study took place at 13 health departments and 1 Indian Health Service Unit in 11 states and 1 tribal nation. Participants included all individuals with laboratory-confirmed COVID-19 and their named contacts. Local COVID-19 surveillance data were used to determine the numbers of persons reported to have laboratory-confirmed COVID-19 who were interviewed and named contacts between June and October 2020. Main Outcomes and Measures: For contacts, the numbers who were identified, notified of their exposure, and agreed to monitoring were calculated. The median time from index case specimen collection to contact notification was calculated, as were numbers of named contacts subsequently notified of their exposure and monitored. The prevalence of a positive SARS-CoV-2 test among named and tested contacts was compared with that jurisdiction's general population during the same 4 weeks. Results: The total number of cases reported was 74 185. Of these, 43 931 (59%) were interviewed, and 24 705 (33%) named any contacts. Among the 74 839 named contacts, 53 314 (71%) were notified of their exposure, and 34 345 (46%) agreed to monitoring. A mean of 0.7 contacts were reached by telephone by public health authorities, and only 0.5 contacts per case were monitored. In general, health departments reporting large case counts during the assessment (≥5000) conducted smaller proportions of case interviews and contact notifications. In 9 locations, the median time from specimen collection to contact notification was 6 days or less. In 6 of 8 locations with population comparison data, positive test prevalence was higher among named contacts than the general population. Conclusions and Relevance: In this cross-sectional study of US local COVID-19 surveillance data, testing named contacts was a high-yield activity for case finding. However, this assessment suggests that contact tracing had suboptimal impact on SARS-CoV-2 transmission, largely because 2 of 3 cases were either not reached for interview or named no contacts when interviewed. These findings are relevant to decisions regarding the allocation of public health resources among the various prevention strategies and for the prioritization of case investigations and contact tracing efforts.

An outbreak of Escherichia coli O157:H7 infections among visitors to a dairy farm.

BACKGROUND: Outbreaks of Escherichia coli O157:H7 infections have involved direct transmission from animals and their environment to humans. We describe an outbreak among visitors to a Pennsylvania dairy and petting farm that provides public access to animals. METHODS: We conducted both a case-control study among visitors to a farm to identify risk factors for infection and a household survey to determine the rates of diarrheal illness among these visitors. We performed an extensive environmental study to identify sources of E. coli O157:H7 on the farm. RESULTS: Fifty-one patients with confirmed or suspected E. coli O157:H7 infection were enrolled in the case-control study. The median age of the patients was four years, and the hemolytic-uremic syndrome developed in eight. Contact with calves and their environment was associated with an increased risk of infection, whereas hand washing was protective. The household survey indicated that visitors to the farm during the outbreak had higher than expected rates of diarrhea. Environmental studies showed that 28 of the 216 cattle on the farm (13 percent) were colonized with E. coli O157:H7 that had the same distinct pattern on pulsed-field gel electrophoresis that was found in isolates from the patients. This organism was also recovered from surfaces that were accessible to the public. CONCLUSIONS: In a large outbreak of E. coli O157:H7 infections among visitors to a dairy farm, predominantly children, high rates of carriage of E. coli O157:H7 among calves and young cattle most likely resulted in contamination of both the animals' hides and the environment.

Reevaluation of epidemiological criteria for identifying outbreaks of acute gastroenteritis due to norovirus: United States, 1998-2000.

BACKGROUND: Noroviruses are believed to be the most common etiologic agent of foodborne outbreaks of gastroenteritis, yet diagnostic tests for these agents are not readily available in the United States. In the absence of assays to detect norovirus, several clinical and epidemiologic profiles--the criteria of Kaplan et al. (vomiting in >50% of patients, mean incubation period of 24-48 h, mean duration of illness of 12-60 h, and no bacterial pathogen) and the ratios of fever to vomiting and diarrhea to vomiting--have been used to distinguish foodborne outbreaks of gastroenteritis caused by noroviruses from those caused by bacteria. METHODS: To examine how well clinical and epidemiological profiles discriminate between foodborne outbreaks of gastroenteritis due to noroviruses and those due to bacteria and to estimate the proportion of reported outbreaks that might be attributable to noroviruses, we reviewed subsets of the 4050 outbreaks reported from 1998 to 2000. RESULTS: The set of criteria of Kaplan et al. is highly specific (99%) and moderately sensitive (68%) in discriminating confirmed outbreaks due to bacteria from those due to norovirus and was the most useful diagnostic aid evaluated. Each individual component of the criteria, the fever-to-vomiting ratio, and the diarrhea-to-vomiting ratio were more sensitive, yet less specific, and therefore less useful, than the criteria of Kaplan et al. We estimated that, at a minimum, 28% of all the foodborne outbreaks reported to the Centers for Disease Control and Prevention may be attributed to norovirus on the basis of these criteria. CONCLUSION: Until norovirus diagnostic tests become widely available, the criteria of Kaplan et al. remain the most useful and discriminating diagnostic aid to identify foodborne outbreaks of gastroenteritis due to noroviruses.