Researchers from Emory University and the Georgia Department of Public Health showed that 2019 coronavirus disease (COVID-19) transmission in the state of Georgia earlier this year changed over time and varied by age.
Peter Teunis (Emory University) and colleagues found that between February and July, the serial interval (time between consecutive cases) for symptoms onset decreased by approximately 1.5 days and that the main contributor to transmission was from an older age group ( 40 to 70 years) to a younger one (20 to 50 years).
The researchers also report that the layout of the shelters (where people are encouraged to stay indoors) was insufficient to prevent transmission in urban areas linked by major transport links.
The team also suggests that examining local patterns of transmission across populations and regions can help states help prevent and control COVID-19.
A pre-print version of the paper is available on the medRxiv * server while the article is being peer reviewed.
The rapid rise in confirmed COVID-19 cases and deaths
Since the first cases of COVID-19 were identified in Wuhan, China late last year, the causative agent of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has rapidly spread around the world and is an urgent global threat for public health.
As of October 27, 2020, the United States had the highest number of confirmed COVID-19 cases (8,773,407) and deaths (226,604) in the world, and the state of Georgia had 353,372 confirmed cases and 7,844 deaths.
On March 14, Georgia, 1,712 cases were confirmed and the government declared a public health emergency. Large gatherings were banned on March 24, and an on-site placement order was issued on April 3. On April 24, Georgia became the first state to allow some businesses to reopen, and by April 30, the housing order had been lifted.
On June 1, the government relaxed the size limit for public gatherings, allowed bars and nightclubs to reopen, sporting events to resume, and summer schools and camps to be held.
Spatial transmission patterns of COVID-19. (a) Date of reaching the peak (local maximum of Rt) for the first wave. (b) Spatial distribution of the five categories of transmission patterns of COVID-19 in Georgia by June 15, 2020. The black lines represent the interstate highways.
The number of new infections in Georgia continued to rise
As the number of new infections in Georgia and other states continued to rise in June and July, it became important to understand the impact of the layout of shelters and reopening on the transmission of COVID-19 in different regions.
When Teunis searched PubMed for articles published on and before October 11, 2020, they found that some studies had estimated the serial interval, while no study looked at the time-varying serial interval. Similarly, few studies had looked at patterns of transmission between groups with different characteristics, and none had looked at time-varying reproductive numbers for COVID-19 or the effects of district-level housing arrangements anywhere in the United States.
What did the current study include?
The team examined the characteristics of COVID-19 transmission in Georgia by estimating the series intervals and the number of reproductions at a given point in time (Rt) using epidemiological data based on reports of confirmed cases from the Georgia Department of Public Health.
Between February 1 and July 13, researchers identified a large number of linked cases (4,080 transmission pairs, [infectors and infectees]) and estimated the serial interval distribution for COVID-19 symptoms.
Using the serial interval distribution, the researchers were then able to estimate the Rt. By studying Rts over time, they were able to examine the spatial distribution of transmission across all 159 counties in Georgia, as well as the impact of the housing layout and gradual reopening.
Distribution of the estimated data of the first maximum, minimum, and second maximum in Rt for 87 counties with a cumulative 200 cases by July 13, 2020 and key events that may fuel COVID-19 transmission.
Transmission varied by area and changed over time and by age group
The transmission of COVID-19 in Georgia varied by area and changed over the period between February and July.
Transmission was more significant in counties near major cities and along major highways. The series interval fell from 5.97 days in February to April to 4.40 days in June to July.
The transmission patterns over time also changed by age group. COVID-19 mainly spread from adults to all ages, but the main contributor to transmission shifted from 40 to 70 years old in February to April to 20 to 50 years old in June to July.
“Younger adults (20 to 50 years old) were involved in the majority of the transmissions that took place during or after the reopening after on-site protection,” the researchers write.
The team also found that the three to four week shelter-in-place ordinance, while slowing transmission, was not enough to prevent transmission to urban and suburban districts that are linked by key transportation links.
What are the implications for the study?
“The contracting series intervals and the increasing prevalence by the younger generation show that COVID-19 transmission at the district level is changing over time,” say Teunis and colleagues.
“The spatial-temporal transmission patterns at district level also provide important evidence for effective COVID-19 prevention and control measures (e.g. protection on site) in various areas,” advises the team.
* Important NOTE
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or treated as established information.