Nearly six in 10 new COVID-19 infections are spread by people who show no symptoms of the virus, according to a new study.
The Centers for Disease Control and Prevention (CDC) study found that asymptomatic people made up about 24% of all transmissions. Meanwhile, pre-symptomatic individuals, those who have contracted the virus but do not yet have symptoms, accounted for 35% of new cases.
“In the absence of effective and widespread use of therapeutics or vaccines that can shorten or eliminate infectivity, successful control of SARS-CoV-2 cannot rely solely on identifying and isolating symptomatic cases; even if implemented effectively, this strategy would be insufficient,” the CDC researchers wrote in their study. “These findings suggest that effective control also requires reducing the risk of transmission from people with infection who do not have symptoms.”
Jay Butler, CDC deputy director for infectious diseases and a lead author of the study, said the study findings support public health guidelines that call for masks and social distancing.
“There was still some controversy over the value of community mitigation — face masks, social distancing, and hand hygiene — to limit spread,” Dr. Butler told Business Insider. “This study demonstrates that while symptom screening may have some value, mitigation, as well as strategically planned testing of persons in some setting, will be a significant benefit.”
Said the study: “The baseline assumptions for the model were that peak infectiousness occurred at the median of symptom onset and that 30% of individuals with infection never develop symptoms and are 75% as infectious as those who do develop symptoms. Combined, these baseline assumptions imply that persons with infection who never develop symptoms may account for approximately 24% of all transmission. In this base case, 59% of all transmission came from asymptomatic transmission, comprising 35% from presymptomatic individuals and 24% from individuals who never develop symptoms. Under a broad range of values for each of these assumptions, at least 50% of new SARS-CoV-2 infections was estimated to have originated from exposure to individuals with infection but without symptoms.”
The study concludes: “In this decision analytical model of multiple scenarios of proportions of asymptomatic individuals with COVID-19 and infectious periods, transmission from asymptomatic individuals was estimated to account for more than half of all transmissions. In addition to identification and isolation of persons with symptomatic COVID-19, effective control of spread will require reducing the risk of transmission from people with infection who do not have symptoms. These findings suggest that measures such as wearing masks, hand hygiene, social distancing, and strategic testing of people who are not ill will be foundational to slowing the spread of COVID-19 until safe and effective vaccines are available and widely used.”
But the CDC said “this study has limitations.”
“First, we used a simplistic model to represent a complex phenomenon, ie, the average infectiousness of SARS-CoV-2 infections over time. We used this model deliberately to test assumptions about the timing of peak infectiousness and transmission among asymptomatic individuals so that we could vary only these 2 critical parameters and assess their relative effects. Therefore, these results lack quantitative precision, but they demonstrate the qualitative roles of these 2 components and show that across broad ranges of possible assumptions, the finding that asymptomatic transmission is a critical component of SARS-CoV-2 transmission dynamics remains constant.”