COVID-19 Recognition: Asymptomatic Infection of COVID-19

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Asymptomatic and pre-symptomatic infection of COVID-19 is now thought to have a more significant role in the spread of the current pandemic than initially thought, making containment of the virus more difficult, without the infrastructure and widespread for COVID-19.

Asymptomatic Infection

Mathematical modelling results of this pandemic has proposed asymptomatic infection as a cause of rapid spread of the virus [1,2,3]. This is further supported by analysis of contact-tracing research from China and Singapore (see summary diagram from BBC below) [4], which showed that asymptomatic individuals tested positive for COVID-19. [4,5,6]

Source: BBC

Image from Pung et al. (2020), published in Lancet: Transmission estimates from three outbreak clusters of COVID-19 in Singapore, linked to a tour group from China, a company conference and a church.

Pre-Symptomatic Infection

Some individuals who were initially thought to be asymptomatic were actually pre-symptomatic and eventually developed COVID-19 signs and symptoms, however in these series, none of the pre-symptomatic individuals had severe infections [6,7]. This pre-symptomatic period can be 1-3 days long, during which COVID-19 transmission could occur [7].

SARS-CoV-2 viral shedding may begin 2 to 3 days before the appearance of the first symptoms.

Asymptomatic, pre-symptomatic and undocumented transmission - know the difference

Unfortunately, asymptomatic and pre-symptomatic transmission of COVID-19 will be a problem in many countries fighting to contain and eradicate this virus. Firstly, asymptomatic infections are not recognised unless they are confirmed by testing, which is also dependent on individuals reported and presenting to medical centres. In light of this, how much of what we now presume to be asymptomatic or pre-symtomatic is actually due to undocumented infection [8]?

Nevertheless this “silent infection” of COVID-19 should not be forgotten and countries should be vigilant in their public health and medical management protocols [3]. This not only puts pressure on countries with limited medical resources and infrastructure, and may also burden the individuals and their household who don’t have easy access to medical attention, for multiple socioeconomic reasons.


1. Bai Y, Yao L, Wei T, et al. Presumed Asymptomatic Carrier Transmission of COVID-19. JAMA. Published online February 21, 2020. doi:10.1001/jama.2020.2565

2. He, X., Lau, E.H.Y., Wu, P. et al. Temporal dynamics in viral shedding and transmissibility of COVID-19. Nat Med (2020).

3. Nishiura H, Kobayashi T, Miyama T, et al. Estimation of the asymptomatic ratio of novel coronavirus infections (COVID-19). Int J Infect Dis. 2020;94:154‐155. doi:10.1016/j.ijid.2020.03.020

4. Pung R, Chiew CJ, Young BE, et al. Investigation of three clusters of COVID-19 in Singapore: implications for surveillance and response measures. Lancet. 2020;395(10229):1039‐1046. doi:10.1016/S0140-6736(20)30528-6

5. Chan JF, Yuan S, Kok KH, et al. A familial cluster of pneumonia associated with the 2019 novel coronavirus indicating person-to-person transmission: a study of a family cluster. Lancet. 2020;3 95(10223):514-523. doi:10.1016/S0140-6736(20)30154-9

6. Hu Z, Song C, Xu C, et al. Clinical characteristics of 24 asymptomatic infections with COVID-19 screened among close contacts in Nanjing, China. Sci China Life Sci. 2020;63(5):706‐711. doi:10.1007/s11427-020-1661-4

7. Wei WE, Li Z, Chiew CJ, Yong SE, Toh MP, Lee VJ. Presymptomatic Transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020. MMWR Morb Mortal Wkly Rep 2020;69:411–415. DOI:

8. Ruiyun Li, Sen Pei, Bin Chen, Yimeng Song, Tao Zhang, Wan Yang, Jeffrey Shaman, 2020, Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV-2), Science, 01 May 2020, Vol. 368, Issue 6490, pp. 489-493, DOI: 10.1126/science.abb3221

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