SARS-CoV-2 is one of the most insidious viruses because it can spread from people showing no symptoms.

Consider this: The United States has had about 23 million cases of COVID-19 as of Jan. 13, 2021. According to the Centers for Disease Control and Prevention (CDC), at least 11.5 million of those cases were transmitted by someone without symptoms. The estimate is based on models and statistical assumptions in a new paper by CDC authors.1

“The findings presented here complement an earlier assessment and reinforce the importance of asymptomatic transmission: Across a range of plausible scenarios, at least 50% of transmission was estimated to have occurred from persons without symptoms,” the authors found. “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.”

Of course, this is further evidence that wearing masks and social distancing is all the more critical.

Veteran epidemiologist Arnold Monto, MD, of the University of Michigan said in a recent interview that asymptomatic transmission was a key distinction between the pandemic virus and the original severe acute respiratory syndrome (SARS) coronavirus that emerged in 2002.2

“Asymptomatic and pre-symptomatic spread were a real surprise — the magnitude of it,” he said. “You have to remember we didn’t really have a diagnostic test for SARS. (Polymerase chain reaction) was in its infancy at that point. Most of the cases were clinical cases that were recognized.

“Even so, SARS could not have been controlled the way it was if asymptomatic infection and transmission were as common as it is with this virus,” Monto added. “This was a real change from the SARS virus. This may have been something that evolved.”


  1. Johansson MA, Quandelacy TM, Kada S, et al. SARS-CoV-2 transmission from people without COVID-19 symptoms. JAMA Netw Open 2021;4:e2035057.
  2. AMA Ed Hub. Coronavirus vaccine update with Arnold S. Monto, MD. American Medical Association. Jan. 13, 2021.