A new study published in the peer reviewed journal: JAMA Network shows that infected individuals who show no symptoms might be contributing significantly to transmission of SARS-CoV-2, the virus that causes COVID-19, given that they account for 40.5% of confirmed infections worldwide.
COVID-19, the disease caused by SARS-CoV-2, was first reported in December 2019.1 Globally, as of January 28, 2021, there have been 100 455 529 confirmed cases, including 2 166 440 deaths.2 The disease course of COVID-19 ranges from asymptomatic to mild respiratory infections to pneumonia and even to acute respiratory distress syndrome.3
Patients with no symptoms at screening point were defined as having asymptomatic infections, which included infected people who have not yet developed symptoms but go on to develop symptoms later (presymptomatic infections), and those who are infected but never develop any symptoms (true asymptomatic or covert infections).4,5
Owing to the absence of symptoms, these patients would not seek medical care and could not be detected by temperature screening. Presymptomatic transmission will also make temperature screening less effective.6 Only extensive testing and close contact tracing could lead to identification of more asymptomatic infections.7
Unlike SARS, which had little known transmission from asymptomatic patients, evidence showed that asymptomatic patients were a potential source of transmission of COVID-19.3,6 A previous study8 showed that the upper respiratory viral loads in asymptomatic patients were comparable to those in symptomatic patients.
Meanwhile, the highest viral load in throat swabs at the time of symptom onset indicated that infectiousness peaked on or before symptom onset.9 Moreover, studies showed that asymptomatic infections might have contributed to transmission among households, nursing facilities, and clusters.10-13
As the pandemic has been contained in many countries and regions, travel restrictions have been lifted and public places have reopened. Asymptomatic infections should be considered a source of COVID-19 infections that play an important role in the spread of the virus within community as public life gradually returns to normal. The management of asymptomatic carriers was essential for preventing cluster outbreaks and transmission within a community.
However, comprehensive evaluation of the percentage of asymptomatic infections among the tested population and the population with confirmed COVID-19 (confirmed population) is limited. Current results from different studies3,5,7,8,10,11 varied considerably owing to different study design and study population.
Thus, we conducted a meta-analysis to better understand the global percentage of asymptomatic infections among the tested and confirmed COVID-19 populations. Our results could be useful for strategies to reduce transmission by asymptomatic infections.