Case growth analysis to inform local response to COVID-19 epidemic in a diverse U.S community

Abstract

Early detection of new outbreak waves is critical for effective and sustained response to the COVID-19 pandemic. We conducted a growth rate analysis using local community and inpatient records from seven hospital systems to characterize distinct phases in SARS-CoV-2 outbreak waves in the Greater Houston area. We determined the transition times from rapid spread of infection in the community to surge in the number of inpatients in local hospitals. We identified 193,237 residents who tested positive for SARS-CoV-2 via molecular testing from April 8, 2020 to June 30, 2021, and 30,031 residents admitted within local healthcare institutions with a positive SARS-CoV-2 test, including emergency cases. We detected two distinct COVID-19 waves: May 12, 2020-September 6, 2020 and September 27, 2020-May 15, 2021; each encompassed four growth phases: lagging, exponential/rapid growth, deceleration, and stationary/linear. Our findings showed that, during early stages of the pandemic, the surge in the number of daily cases in the community preceded that of inpatients admitted to local hospitals by 12-36 days. Rapid decline in hospitalized cases was an early indicator of transition to deceleration in the community. Our real-time analysis informed local pandemic response in one of the largest U.S. metropolitan areas, providing an operationalized framework to support robust real-world surveillance for outbreak preparedness.

Figure 1

Distribution of cumulative SARS-CoV-2 infections across ZIP Code Tabulation Areas in Harris County, Texas, from April 8, 2020 to June 30, 2021. All SARS-CoV-2 infections were summed across ZIP Codes and then across US Census ZIP Code Tabulation Areas (ZCTAs) based on the individuals’ address information. Red circles indicate the location of Texas Medical Center (TMC) facilities. Geospatial distribution patterns vary from light to dark blue according to increasing quintiles of cumulative infections during the analysis period. 255 records were excluded due to missing ZIP Code information. Figure was generated using ArcMap (ArcGIS version 10.8.2).

Figure 2

SARS-CoV-2 growth rate, acceleration and stage among Harris County residents (left) and a subgroup of Harris County residents admitted to Texas Medical Center(TMC) hospitals with a positive SARS-CoV-2 test (right) from April 8, 2020 to June 30, 2021.

Figure 3

Transition periods to SARS-CoV-2 outbreak in Harris County community and among inpatients in Texas Medical Center. Critical phases of SARS-CoV-2 outbreak (i.e., exponential growth + deceleration stages) in Harris County community (top row) and among TMC inpatients (bottom row). Dark gray areas indicate exponential growth and deceleration stages. Yellow areas indicate lag periods between transition to exponential growth in the community and in local hospitals. Light green areas indicate lag periods between transition to stationary/linear growth in the community and in local hospitals.

Figure 4

Daily growth rate in SARS-CoV-2 cases per 100,000 population among community residents^a and inpatients^b during outbreak waves in Harris County, stratified by age group. (A) May 2020 – September 2020, (B) September 2020 – May 2021, (C) Overall Period (April 2020 – June 2021), (D) Inpatient:community case ratio. ^aCommunity cases include all residents of Harris County with confirmed SARS-CoV-2 infections; ^b Inpatients include Harris County residents admitted to TMC hospitals with a positive SARS-CoV-2 test.

Figure 5

Community metrics informing response to COVID-19 pandemic.