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