Community transmission was responsible for the virus' spread earlier this year in Wuhan, the pandemic's origin, according to a new study.
Researchers from the Wuhan Center for Disease Control & Prevention and the University of Illinois reported that a reanalysis of throat swabs from Wuhan patients with flu-like symptoms found that some were positive for COVID-19, suggesting community transmission of the virus in Wuhan in early January 2020.
The COVID-19 pandemic was caused by the SARS-CoV-2 virus.
Although SARS-CoV-2 shares 80% sequence identity and the same cell receptor with the SARS coronavirus that caused an epidemic in 2003, the clinical outcome of SARS-CoV-2 not only includes SARS-like viral pneumonia, but also causes milder illnesses, the researchers noted.
Some people also present with asymptomatic infection—an analysis by the China CDC indicated that 80% of confirmed COVID-19 cases were characterized as the mild or moderate types, without breathing difficulty or hypoxia, they added.
The first cases of COVID-19 were reported in Wuhan in December 2019. The rapid increase in reported cases suggested that community transmission of SARS-CoV-2 was established in Wuhan and nearby regions no later than the end of January 2020. Because the use of rapid molecular diagnostic assays was not widespread before Jan. 23, it was difficult to monitor the transmission of SARS-CoV-2 throughout the community, the researchers said.
In order to better understand how the disease spread in Wuhan, they retrospectively investigated the presence of SARS-CoV-2 among local patients with flu symptoms, which were defined as outpatients with a sudden onset of a fever of more than 100 degrees Fahrenheit, a cough, or sore throat.
They re-analyzed 640 throat swabs collected from these patients between Oct. 6, 2019, and Jan. 21, 2020. This time period coincided with the winter peak of influenza and other respiratory illnesses.
They noted that the number of patients with flu symptoms in all age groups had increased dramatically, starting in early December and reaching a peak by the New Year. In particular, cases in the 5- to 14-year-old group increased more than 24-fold during this period. Further, flu-like symptom data for the 2019-2020 winter was significantly higher in comparison to previous years, suggesting a need to distinguish between influenza-infected and suspected COVID-19 patients.
In their re-analysis of the throat swabs, the researchers found that nine of the 640 were, in fact, infected with SARS-CoV-2. The onset date of the earliest case was Jan. 4, one week after the outbreak was reported by hospitals.
"Although the weekly sample size was small, it seems that COVID-19 was gradually expanding among the flu-like illness cases during January," the authors wrote. "In the last week of observation, the frequency of SARS-CoV-2-positive cases had exceeded that of influenza virus among the group of patients older than 30 years. This finding is consistent with the recent epidemiological estimations about the early transmission dynamics of SARS-CoV-2."
Interestingly, they added, the nine patients came from six different districts of the Wuhan metropolitan and surrounding areas, providing additional evidence for community transmission in this region.
This story first appeared in our sister publication, Genomeweb.