Severe Acute Respiratory Syndrome Coronavirus 2 Transmission in Georgia, USA, February 1-July 13, 2020

Abstract

The serial interval and effective reproduction number for coronavirus disease (COVID-19) are heterogenous, varying by demographic characteristics, region, and period. During February 1-July 13, 2020, we identified 4,080 transmission pairs in Georgia, USA, by using contact tracing information from COVID-19 cases reported to the Georgia Department of Public Health. We examined how various transmission characteristics were affected by symptoms, demographics, and period (during shelter-in-place and after subsequent reopening) and estimated the time course of reproduction numbers for all 159 Georgia counties. Transmission varied by time and place but also by persons' sex and race. The mean serial interval decreased from 5.97 days in February-April to 4.40 days in June-July. Younger adults (20-50 years of age) were involved in most transmission events occurring during or after reopening. The shelter-in-place period was not long enough to prevent sustained virus transmission in densely populated urban areas connected by major transportation links.

Keywords: 2019 novel coronavirus disease; COVID-19; Georgia; SARS-CoV-2; United States; coronavirus disease; reproduction number; respiratory infections; serial interval; severe acute respiratory syndrome coronavirus 2; shelter-in-place; transmission; viruses; zoonoses.

Figure 1

Estimated serial interval distribution for 3 periods in study of severe acute respiratory syndrome coronavirus 2 transmission in Georgia, USA: early transmission and shelter-in-place (February–April 2020); after reopening (May); and further reopening (June–July). k and θ indicate the scale and shape parameters for the gamma distribution. The y-axis represents the estimated probability density of having a certain serial interval.

Figure 2

Patterns of severe acute respiratory syndrome coronavirus 2 transmission by patient age (A), sex (B), and race (B), based on 4,080 tracked pairs of coronavirus disease cases from Georgia, USA, during February–July 2020. The matrix graphs show numbers of transmission pairs as a percentage of the total, with primary case-patients as columns and their secondary case-patients as rows. Darker colors indicate a higher percentage of fraction of tracked pairs observed. In panel A, marginal totals are shown as density curves to illustrate the age distribution of case-patients.

Figure 3

Patterns of severe acute respiratory syndrome coronavirus 2 transmission according to patient sex and age, based on 4,080 tracked pairs of coronavirus disease cases in Georgia, USA, February–July 2020. A) Male-to-male transmission; B) male-to-female transmission; C) female-to-female transmission; D) female-to-male transmission. The matrix graphs show numbers of transmission pairs as a percentage of the total, with primary case-patients as columns and their secondary case-patients as rows. Darker colors indicate a higher percentage of fraction of tracked pairs observed. Marginal totals are shown as density curves to illustrate the age distribution of case-patients.

Figure 4

Patterns of severe acute respiratory syndrome coronavirus 2 transmission in Georgia, USA, February–July 2020, by age group, in 3 successive periods. A) Early transmission and shelter-in-place (February–April); B) after reopening (May); C) further reopening (June–July). The matrix graphs show numbers of transmission pairs as a percentage of the total, with primary case-patients as columns and their secondary case-patients as rows. Darker colors indicate a higher percentage of fraction of tracked pairs observed. Marginal totals are shown as density curves to illustrate the age distribution of case-patients.

Figure 5

Examples of the 5 categories of severe acute respiratory syndrome coronavirus 2 spreading patterns in counties in Georgia, USA, February–July 2020. Shown are epidemic curves from the start of the outbreak until July 13, 2020, and effective reproduction number (R_t) estimates until June 15, in Gwinnett (A), Clayton (B), Sumter (C), Glynn (D), and Dawson (E) Counties. Tick marks indicate the first day of the month. The x-axis represents the date of symptom onset for patients with confirmed cases. The y-axis in the top plot shows the number of cases; the y-axis in the bottom plot shows the estimated median reproduction numbers. Error bars represent 2.5th–97.5th percentile ranges of R_ts. The gray area shows where R_t estimates were truncated on June 15.

Figure 6

Spatial patterns of transmission of severe acute respiratory syndrome coronavirus 2 in Georgia, USA, February–July 2020. A) Date of reaching the peak (local maximum of effective reproduction number) for the first wave; B) spatial distribution of the 5 categories of virus transmission patterns by June 15, 2020. The black lines represent interstate highways.

Figure 7

Distributions of estimated dates of first maximum, minimum, and second maximum in effective reproduction numbers for severe acute respiratory syndrome coronavirus 2 transmission in 87 counties in Georgia, USA, with 200 cumulative cases by July 13, 2020, and dates of key events possibly driving virus transmission.