Community spread of COVID-19 in New York City revealed by contact tracing

In a recent study posted to the Research Square* preprint server, researchers assessed the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in New York City (NYC) communities by contact tracing.

Study: Contact tracing reveals community transmission of COVID-19 in New York City. Image Credit: WESTOCK PRODUCTIONS


A detailed understanding of SARS-CoV-2 transmission in community-based settings is essential to formulate tailored public health strategies and policies for particular geographical areas and communities with persistent and spatial SARS-CoV-2 transmission. Analyzing community-level SARS-CoV-2 transmission has been challenging since community testing, and surveillance data are lacking. Additionally, the impact of preventive measures on SARS-CoV-2 community transmission in urban regions needs further evaluation.

About the study

In the present study, researchers assessed SARS-CoV-2 transmission in NYC communities by contact tracing.

Contact tracing data were obtained from case investigations and SARS-CoV-2 antigen and molecular test reports for 644,029 coronavirus disease 2019 (COVID-19) cases and their close contacts in NYC during the second wave of the COVID-19 pandemic to quantify the spatial pattern of community-level SARS-CoV-2 transmission. The study was conducted between 1 October 2020 and 10 May 2021.

Cases reported to the NYC department of health and mental hygiene (DOHMH) and traced by the NYC test & trace corps were analyzed for characterizing the operational performance of contact tracing and understanding SARS-CoV-2 within- and cross-ZIP code SARS-CoV-2 transmission. Self-reported chains of SARS-CoV-2 transmission were reconstructed by linking contact tracing records to the laboratory-confirmed COVID-19 cases and mapped across modified ZIP code tabulation areas (MODZCTAs).

For symptomatic COVID-19 cases, the team estimated the infection date based on the SARS-CoV-2 incubation period mentioned in a previous study. For asymptomatic COVID-19 cases, the date of sample collection was used to estimate the date of infection by applying Bayesian inference to findings of viral load modeling.

Further, the impact of public health interventions on community transmission of SARS-CoV-2 was evaluated. Data on 4,642 events of non-household SARS-CoV-2 transmission were obtained from mobile phones with documented weekly counts of persons at points of interest (POIs) such as grocery stores, restaurants, bars, and gyms in geographical areas with different ZIP codes used. Conditional autoregressive (CAR) modeling was performed with Poisson generalized linear mixed models (GLMM) to assess the spatial transmission of SARS-CoV-2 in NYC communities.


Samples were obtained, findings were reported to DOHMH within two days, and almost all index cases (97%) were summoned by contact tracers within two days of sending reports to DOHMH. Over 68% of close contacts were summoned the day reports were sent to the test & trace contact tracing team.

Over 66% of contacts underwent SARS-CoV-2 testing within seven days of receiving the notification. For symptomatic SARS-CoV-2 infections, 13% and 87% of individuals underwent testing prior to and post the onset of COVID-19 symptoms, respectively.

Most of the index cases were 20 to 49 years old, with frequent social interactions among similarly aged individuals and greater inter-generation interactions in households. SARS-CoV-2 transmission chains were heterogeneous and comprised 947,042 persons belonging to 242,486 disconnected clusters. Clusters of SARS-CoV-2 exposure were diverse and ranged from single spreader exposure networks to those with several spreaders.

More than 50% of clusters belonged to Brooklyn and Queens and within the clusters, 59% (n=1,195 individuals) of traced index cases comprised within-ZIP code contacts and 42% (n=817 individuals) cross-ZIP contacts. In addition, 58,474 clusters of potential SARS-CoV-2 transmission were identified, comprising two individuals on average, representing 20% (n=135,478 individuals) of reported COVID-19 cases.

The largest identified cluster of SARS-CoV-2 transmission comprised 12 cases with a maximum of seven secondary COVID-19 cases per index case. Cross-ZIP code transmission events mainly occurred within a radius of 10 km; however, transmission ≤40 km was observed. Out of 72,191 events of SARS-CoV-2 transmission in known residential areas, 11% (n=7,826) of cross-ZIP codes were found to be tightly interlinked by SARS-CoV-2 exposure and dissemination.

The within-ZIP code and cross-ZIP code SARS-CoV-2 transmission reduced by 28% and 15%, respectively, among 12.5% of individuals who received COVID-19 vaccinations recently. Greater coverage of vaccination and fewer POI visitors were associated with lower within-ZIP code and cross-ZIP code SARS-CoV-2 transmission.


Overall, the study findings highlighted the spatial pattern of SARS-CoV-2 transmission in NYC communities and potential measures for curtailing SARS-CoV-2 transmissions such as vaccination and reduced POI visits in high-prevalence communities.

*Important notice

Research Square publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
  • Pei, S. et al. (2022) "Contact tracing reveals community transmission of COVID-19 in New York City". Research Square. doi: 10.21203/

Posted in: Medical Science News | Medical Research News | Disease/Infection News

Tags: Antigen, Coronavirus, Coronavirus Disease COVID-19, covid-19, Hygiene, Laboratory, Pandemic, Public Health, Research, Respiratory, SARS, SARS-CoV-2, Severe Acute Respiratory, Severe Acute Respiratory Syndrome, Syndrome

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Pooja Toshniwal Paharia

Dr. based clinical-radiological diagnosis and management of oral lesions and conditions and associated maxillofacial disorders.

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