Pre-symptomatic transmission may account for 44% of the COVID-19 cases

Pre-symptomatic transmission may account for 44% of the COVID-19 cases

Last Reviewed : 12/23/2020
Pre-symptomatic transmission may account for 44% of the COVID-19 cases

As COVID-19 cases continue to rise, it became evident that disease transmission can occur before symptoms develop. Social distancing, wearing face masks and thorough hand washing are recommended to prevent transmission from other individuals without symptoms.

Multiple cases were reported that COVID-19 was transmitted from individuals before developing symptoms. In an article published by Centers for Disease Control and Prevention (CDC) in Emerging Infectious Diseases reported two people infected by a person in pre-symptomatic stage. Similarly, another study published in JAMA reported five cases that were potentially transmitted by individual without any symptoms. Multiple similar cases were reported in the recent past. However, it remained unclear what percentage of cases are transmitted by pre-symptomatic individuals.

In a study published in Nature Medicine on April 15, 2020, it was estimated that 44% of the cases were infected during the pre-symptomatic stage. This study looked at serial interval (duration between symptom onsets of successive cases in a transmission chain) and the incubation period (time between infection and onset of symptoms) of successively infected patients. As per the study, if the observed serial interval is shorter than the observed incubation period, this indicates that a significant portion of transmission may have occurred before infected persons have developed symptoms.

The study included 77 transmission pairs within and outside mainland China. It inferred that infectiousness started from 2.3 days before symptom onset and peaked at 0.7 days before symptom onset. Infectiousness declined within 7 days. The estimated proportion of pre-symptomatic transmission was 44%.

Another study that has not been peer-reviewed yet, reported similar data. This study concluded that pre-symptomatic transmission accounted for 48% cases for Singapore and 62% for Tianjin, China.

Based on the available data, pre-symptomatic transmission appears to be more common than previously thought. 44% in one study, 48% - 62% in another study demonstrates that a significant proportion of cases are transmitted by individuals in pre-symptomatic stage. These studies further underscore the importance of social distancing, wearing a face mask and washing hands regularly to prevent COVID-19 in a community setting.



  1. Tong, Z. D. et al. Potential presymptomatic transmission of SARS-CoV-2, Zhejiang Province, China, 2020. Emerg. Infect. Dis. (2020).
  1. Bai, Y. et al. Presumed asymptomatic carrier transmission of COVID-19. JAMA (2020).
  1. He, X. et al. Temporal dynamics in viral shedding and transmissibility of COVID-19. Nat Med (2020).
  2. Tapiwa Ganyani et al. Estimating the generation interval for COVID-19 based on symptom onset data. medRxiv 2020.03.05.20031815; doi:

Please leave your comments:

Related Articles