Nearly Half Of COVID-19 Infections Occurred Before Symptoms: Study

A modeling study estimates that 44 percent of COVID-19 infections were transmitted during the pre-symptomatic stage.

AsianScientist (Apr. 16, 2020) – 44 percent of secondary cases of COVID-19 were likely infected during the pre-symptomatic stage, suggests a study published as a brief communication in Nature Medicine.

The modeling study, which compared clinical data on virus shedding with epidemiologic data, was a collaboration between researchers at Guangzhou Medical University and the World Health Organization Collaborating Centre for Infectious Disease Epidemiology and Control at the University of Hong Kong’s School of Public Health.

Dr. Eric Lau and colleagues investigated the temporal patterns of viral shedding in 94 patients with COVID-19 admitted to Guangzhou Eighth People’s Hospital in China. Throat swabs were collected from these patients from when symptoms first appeared until 32 days later. In total, 414 swabs were analyzed, and the authors found that the patients had the highest viral load at the onset of symptoms.

Separately, they modeled COVID-19 infectiousness profiles from a different sample of 77 ‘transmission pairs’ from publicly available data. Each transmission pair comprised two patients with COVID-19 with a clear epidemiological link, with one patient highly likely to have infected the other.

From the transmission pairs, the authors inferred that infectiousness started 2.3 days before symptoms appeared and peaked at 0.7 days before their appearance. They estimated that 44 percent of secondary cases were infected during the pre-symptomatic stage, with infectiousness predicted to decrease quickly within seven days.

“Significant presymptomatic transmission would probably reduce the effectiveness of control measures that are initiated by symptom onset, such as isolation, contact tracing and enhanced hygiene or use of face masks for symptomatic persons,” the authors wrote.

The observed transmission pattern of SARS-CoV-2 is in contrast to that of the 2003 SARS (severe acute respiratory syndrome) virus, for which infectiousness increased around seven to ten days after symptom onset. Thus, onward transmission for SARS could be substantially reduced by containment measures such as isolation and quarantine.

Assuming a basic reproductive number of 2.5, which means that SARS-CoV-2 spreads efficiently, contact tracing and isolation alone are less likely to be successful if more than 30 percent of transmission occurred before symptom onset, unless more than 90 percent of the contacts can be traced, the authors noted.

“More inclusive criteria for contact tracing to capture potential transmission events two to three days before symptom onset should be urgently considered for effective control of the outbreak,” the authors urged. Such criteria has already been in place in Hong Kong SAR and China since late February.

The article can be found at: He et al. (2020) Temporal Dynamics in Viral Shedding and Transmissibility of COVID-19.


Source: Nature; Photo: Unsplash.
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