The authors of this study evaluated the transmission of SARS-CoV-2 from individuals with no symptoms.
This decision analytical model assessed the relative amount of transmission from presymptomatic, never symptomatic and symptomatic individuals across a range of scenarios. The proportion of transmission from people who remain asymptomatic and the infectious period were varied according to published best estimates.
For all estimates, data from a meta-analysis of 8 studies from China set a median incubation period of 5 days. The infectious period duration was maintained at 10 days, and peak infectiousness was varied between 3 and 7 days (±2 days relative to the median incubation period). The overall proportion of SARS-CoV-2 was varied at a wide range between 0% and 70%.
Transmission from asymptomatic individuals was estimated to account for more than half (approximately 59%) of all transmission. Presymptomatic individuals accounted for 35% of transmission and 24% came from individuals who were never symptomatic. Even under a broad range of scenarios, each showed that at least 50% of new SARS-CoV-2 infections were estimated to have originated from exposure to infected individuals without symptoms.
To arrive at the results above, the study utilized 2 baseline assumptions: Peak infectiousness occurred at the median of symptom onset and 30% of individuals with infection never develop symptoms and are 75% as infectious as those who are symptomatic.
The authors acknowledged that they used a simplistic model to represent a complex phenomenon (i.e., the average infectiousness of SARS-CoV-2 infections over time). However, the model was used to deliberately test assumptions about the timing of peak infectiousness and transmission among asymptomatic individuals varying only these 2 critical parameters. The results lack quantitative precision, but they demonstrate the qualitative roles of these 2 components. Across broad ranges of possible assumptions, the finding that asymptomatic transmission is a critical component of SARS-CoV-2 transmission dynamics that remains constant.
The exact proportions of presymptomatic and never symptomatic transmission are unknown. This also applies to the incubation period estimates, which are based on individual exposure and onset windows that are difficult to observe with precision and therefore include uncertainty even when leveraging estimates across multiple studies. Moreover, they likely vary substantially in different populations.
Now that COVID-19 is widely recognized, individuals with COVID-19 symptoms are more likely to isolate themselves and further reduce the proportion of transmission from symptomatic individuals, shifting a greater proportion of transmission to those who are asymptomatic. Therefore, estimates in this study represent the lower end of the proportion of asymptomatic transmission in the presence of interventions to reduce symptomatic transmission.
Determining the proportion of SARS-CoV-2 transmission that occurs from persons without symptoms is foundational to developing control practices and policies. Measures to reduce transmission from individuals who do not have COVID-19 symptoms have become controversial and politicized. Since more than half of transmission of SARS-CoV-2 comes from asymptomatic individuals, these findings suggest COVID-19 cannot be contained by symptom-based screening alone.
Measures such as mask wearing and social distancing empower individuals to protect themselves and, if infected, to reduce risk to their communities. These measures can also be supplemented by strategic testing of people who are not ill, such as those who have exposures to known cases or are at high risk of exposing others (e.g., congregate facility staff, those with frequent contact with the public). Multiple measures that effectively address transmission risk in the absence of symptoms are imperative to control this pandemic. Moreover, it has not been determined if asymptomatic individuals who are infected with the novel coronavirus after being vaccinated can transmit the virus. Given the results of this study, both unvaccinated and vaccinated individuals need to take all recommended precautions to prevent the spread of this novel coronavirus.