02/06/2020
FACT-CHECK: People without symptoms CAN transmit COVID-19!
📘 DEFINITIONS: "asymptomatic" and "pre-symptomatic" 📕
A COVID-19 patient without symptoms can be ASYMPTOMATIC or PRE-SYMPTOMATIC. Asymptomatic cases never develop symptoms at all. On the other hand, pre-symptomatic patients just haven't developed symptoms as of the moment. The incubation period for COVID-19 can take up to 14 days [1]. There is NO reliable way yet for us to tell if a COVID-19 case without symptoms is asymptomatic or pre-symptomatic.
📈 HOW MANY asymptomatic or pre-symptomatic cases are there? 📉
There isn't an exact, fixed number. However, we do have some case studies:
🔹 Among the 634 cases reported on the Diamond Princess cruise ship, 50% were reported to have no symptoms. Later estimates showed that 18% of all cases were truly asymptomatic [2].
🔹 50% of positive cases in Iceland were reported to have no symptoms at the time of testing. At the time, 5% of the whole population had already been tested [3].
🔹 In a nursing home in Washington State, USA, 27 out of the 48 (56%) who tested positive were asymptomatic at testing. Eventually, 24 out of the 27 “asymptomatics” developed symptoms and were reclassified as “presymptomatic” [4].
🧐 CAN YOU CATCH COVID-19 from someone without symptoms? 🧐
YES, both asymptomatic and pre-symptomatic COVID-19 patients are contagious. As early as February, pre-symptomatic transmission was reported in China [5]. Studies have found that the viral load (or amount of virus detected) is already high before symptoms develop in pre-symptomatic cases, and that the viral load is similar among cases with or without symptoms. It is likely, though not certain, that viral load affects transmissibility or contagiousness [4, 6, 7].
One study estimates that pre-symptomatic cases may account for 44% of transmission, meaning almost half of the infections occurred before the patient developed symptoms [8]. Another study estimates that cases without symptoms may be responsible for up to 69% of all infections [9].
Because COVID-19 can spread so easily from people without symptoms, measures like thermal scanning and symptom checking, while important, are not enough [4]. We need to INCREASE our testing capacity for proactive mass testing: we should actively and quickly find and isolate asymptomatic or pre-symptomatic carriers among people with exposure such as traced contacts, frontline healthcare workers, and high-risk communities.
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REFERENCES
1. World Health Organization. (2020). Coronavirus disease 2019 (COVID-19)
Situation Report – 73. Retrieved fromhttps://www.who.int/docs/default-source/coronaviruse/situation-reports/20200402-sitrep-73-covid-19.pdf
2. Mizumoto, K., Kagaya, K., Zarebski, A., & Chowell, G. (2020). Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020. Eurosurveillance, 25(10). http://doi.org/10.2807/1560-7917.es.2020.25.10.2000180
3. John, T. (2020). Iceland lab's testing suggests 50% of coronavirus cases have no symptoms. Cable News Network. Retrieved from https://edition.cnn.com/2020/04/01/europe/iceland-testing-coronavirus-intl/index.html
4. Gandhi, M., Yokoe, D., & Havlir, D. (2020). Asymptomatic Transmission, the Achilles’ Heel of Current Strategies to Control Covid-19. New England Journal Of Medicine. http://doi.org/10.1056/nejme2009758
5. Ye, F., Xu, S., Rong, Z., Xu, R., Liu, X., & Deng, P. et al. (2020). Delivery of infection from asymptomatic carriers of COVID-19 in a familial cluster. International Journal Of Infectious Diseases, 94, 133-138. http://doi.org/10.1016/j.ijid.2020.03.042
6. Zou, L., Ruan, F., Huang, M., Liang, L., Huang, H., & Hong, Z. et al. (2020). SARS-CoV-2 Viral Load in Upper Respiratory Specimens of Infected Patients. New England Journal Of Medicine, 382(12), 1177-1179. http://doi.org/10.1056/nejmc2001737
7. Joynt, G., & Wu, W. (2020). Understanding COVID-19: what does viral RNA load really mean?. The Lancet Infectious Diseases. http://doi.org/10.1016/s1473-3099(20)30237-1
8. He, X., Lau, E., Wu, P., Deng, X., Wang, J., & Hao, X. et al. (2020). Temporal dynamics in viral shedding and transmissibility of COVID-19. Nature Medicine, 26(5), 672-675. http://doi.org/10.1038/s41591-020-0869-5
9. Emery, J. et al. (2020). The contribution of asymptomatic SARS-CoV-2 infections to transmission - a model-based analysis of the Diamond Princess outbreak. Centre for Mathematical Modelling of Infectious Diseases. Retrieved from https://cmmid.github.io/topics/covid19/asymp-transmission.html