As of March 15, 2022, there had been almost 458.5 million confirmed cases of COVID-19 worldwide, including more than six million deaths (WHO, 2022). Asymptomatic and pre-symptomatic transmission seems to be one of the most important features of COVID-19 (Gao et al., 2021). Defined by the CDC as “when a person is infected with a virus and will never feel any symptoms at all,” asymptomatic has become a “catchall phrase” for those who aren’t exhibiting any of the most common markers of COVID-19 – dry cough, lack of taste or smell, fever – but still test positive and appear to be capable of spreading the virus to others (Volpe, 2022). The term “asymptomatic transmission” refers specifically to transmission of the virus from a person who did not develop disease symptoms (Gao et al., 2021). Estimating the influence of asymptomatic cases on outbreaks is challenging, and whether they act as “silent drivers” of COVID transmission remains controversial (Nogrady, 2020; Gao et al., 2021). Unlike other viruses, such as the virus that causes severe acute respiratory syndrome (SARS), there has been “little comprehensive research” on transmission rates among asymptomatic people with a COVID-19 infection (Lennon, 2021). Increased knowledge of these rates could go a long way in improving public health policies to manage the virus.

While asymptomatic people may be less infectious than symptomatic people, asymptomatic infections still account for a large proportion of all COVID-19 infections; moreover, asymptomatic individuals may have more close contacts than people with symptoms, as those who feel sick will often isolate (Gao et al., 2021). Consequently, the role of asymptomatic cases in community transmission of COVID-19 should not be neglected. Research from early in the pandemic suggested that the rate of asymptomatic infections could be “as high as 81%” (Nogrady, 2020). According to a more recent meta-analysis from December 2021, about four in ten COVID-19 cases are asymptomatic (Ma et al., 2021). Researchers from Peking University in Beijing conducted a review of 95 studies representing a total of almost 30 million individuals who were tested for COVID; of the studies, 25 were done in Asia, 32 were done in North America, and 35 were done in Europe (Ma et al., 2021). Researchers defined an asymptomatic case as a person who tested positive on a PCR test – considered to be the gold standard for COVID testing – while not feeling symptoms at the time (Ma et al., 2021). Then, they calculated the percentage of asymptomatic people among those who had confirmed infections, as well as among individuals undergoing testing. Of everyone tested in the studies, 0.25% had a COVID-19 infection and were asymptomatic; of those who received a positive test result, 40.5% were asymptomatic (Ma et al., 2021). The figure was closer to 50% for subgroups like pregnant women and travelers on airplanes and cruises (Ma et al., 2021).

Considering the proportion of asymptomatic cases and the transmission risk associated with such infections, asymptomatic cases are an important focus when it comes to measuring disease spread. One barrier to more comprehensive research on asymptomatic COVID transmission is that medical professionals are struggling to offer concrete guidelines on how to categorize “asymptomatic.” “Currently, there are no data available to define ‘asymptomatic,’ which can be different in different people, given that many have chronic respiratory symptoms as baseline, from congestive heart failure to allergies,” says Michael David, assistant professor of medicine and epidemiology in the Perelman School of Medicine at the University of Pennsylvania (Volpe, 2022). Active testing is therefore a key tool for control.

References 

Gao, W., Lv, J., Pang, Y., & Li, L.-M. (2021). Role of asymptomatic and pre-symptomatic infections in covid-19 pandemic. BMJ, 375, n2342. doi:10.1136/bmj.n2342 

Ing, A. J., Cocks, C., & Green, J. P. (2020). COVID-19: In the footsteps of Ernest Shackleton. Thorax, 75(8), 693–694. doi:10.1136/thoraxjnl-2020-215091 

Lennon, A. (2021, December 17). How many SARS-CoV-2 infections are asymptomatic? Medical News Today. https://www.medicalnewstoday.com/articles/4-in-10-people-with-a-sars-cov-2-infection-may-have-no-symptoms 

Ma, Q., Liu, J., Liu, Q., Kang, L., Liu, R., Jing, W., Wu, Y., & Liu, M. (2021). Global Percentage of Asymptomatic SARS-CoV-2 Infections Among the Tested Population and Individuals With Confirmed COVID-19 Diagnosis: A Systematic Review and Meta-analysis. JAMA Network Open, 4(12), e2137257. doi:10.1001/jamanetworkopen.2021.37257 

Nogrady, B. (2020). What the data say about asymptomatic COVID infections. Nature, 587(7835), 534–535. doi:10.1038/d41586-020-03141-3 

Volpe, A. (2022, January 18). Am I asymptomatic, or do I just really not want to have Covid-19? A guide. Vox. https://www.vox.com/22883820/am-i-asymptomatic-covid-symptoms-isolation-quarantine 

World Health Organization (WHO). WHO Coronavirus (COVID-19) Dashboard. (n.d.). Retrieved March 15, 2022, from https://covid19.who.int 

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