Study of secondary COVID-19 cases underscores importance of physical distancing
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Less than 4% of about 3,400 close contacts of people with COVID-19 in the early days of the pandemic in China were infected with SARS-CoV-2, researchers wrote in Annals of Internal Medicine.
Most of the secondary infections occurred at home, researchers said, and patients with more clinically severe disease were more likely to infect their close contacts. According to experts unaffiliated with the study, the findings reinforce the need for physical distancing and other basic prevention efforts to slow transmission.
Lei Luo, PhD, of the Guangzhou Center for Disease Control and Prevention in China, and colleagues studied 3,410 close contacts of 391 COVID-19 index cases in Guangzhou. Tracing took place between Jan. 13 and March 6. The researchers gathered data on the setting of the exposure, test results and clinical characteristics of COVID-19 cases.
Luo and colleagues found that among the close contacts, 127 (OR = 3.7%; 95% CI, 3.1%–4.4%) were secondarily infected with COVID-19. Of 119 symptomatic cases, 20 were mild, 87 were moderate and 12 were severe or critical. The remaining patients (OR = 6.3%; 95% CI, 2.1%–10.5%) were asymptomatic.
The researchers found that the most common source of exposure for those who acquired secondary infections was household environments (10.3%), followed by health care settings (OR = 0.09; 95% CI, 0.04–0.20) and public transportation (OR = 0.01; 95% CI, 0– 0.08). The secondary infection rate increased with the severity of the index cases, from 0.3% (95% CI, 0%–1%) for asymptomatic, to 3.3% (95% CI, 1.8%–4.8%) for mild, 5.6% (CI, 4.4%–6.8%) for moderate and 6.2% (95% CI, 3.2%–9.1%) for severe or critical cases. Index cases with expectoration were associated with a higher risk for secondary infection (13.6% vs. 3% for index cases without expectoration [OR = 4.81; 95% CI, 3.35–6.93]).
William Schaffner, MD, professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center, said the timing of the Luo study is critical to understanding its significance.
“Notice how workplace does not show up at all among the places of secondary exposure,” he said. “Guangzhou was on lockdown when the secondary infections occurred. The home, with its four walls, is the ideal environment for the spread of the virus.”
Schaffner, who is also medical director of the National Foundation for Infectious Diseases, added that the study “reinforces the importance of social distancing and masking in public and avoiding groups outside the home, since those activities might bring it into the home,” he said.
Luo and colleagues’ findings corroborate what is already known about coronavirus, added Aaron E. Glatt, MD, FACP, FIDSA, FSHEA, hospital epidemiologist and chair of medicine at Mount Sinai South Nassau and spokesperson for the Infectious Diseases Society of America.
“We are not going to get COVID from most things around our houses,” he told Healio Primary Care. “But it's very important that people understand that there is a potential to get COVID when someone coughs and/or sneezes. Routine handwashing remains important.”