Aletho News


Asymptomatic Spreaders, Typhoid Mary, SARS, MERS and COVID

By Christopher Brett – Fossils of Lanark – April 30, 2020

“The next pandemic virus will be present in Canada within 3 months after it emerges in another part of the world, but it could be much sooner because of the volume and speed of global air travel.  …  Given the increase, different patterns and speed of modern travel, a  new virus once arriving in Canada could spread quickly in multiple directions throughout the country. … The first peak of illness in Canada could occur within 2 to 4 months after the virus arrives in Canada. The first peak in mortality is expected to be approximately 1 month after the peak in illness.”

The Canadian Pandemic Influenza Plan for the Health Sector, 2006
Dr. Theresa Tam and Karen Grimsrud, Co-Chairs

At the beginning of February I was surprised by Prime Minister Justin Trudeau’s assertions that it was safe to continue to fly to China, that we would only  be testing those who self-reported symptoms, and that his plan for testing was science based. I was also surprised that Canada’s Chief Public Health Officer Dr. Theresa Tam asserted that there was no risk from asymptomatic spreaders. It has come as no surprise to me that we have now closed our borders to China and most other countries, that we have stepped up our testing and begun contact tracing, and that studies have shown that asymptomatic spreading of COVID-19 is the norm.

I had six  main reasons for objecting to Canada continuing passenger flights to China:

– First, we were the only country continuing to fly to China.
– Second, it was all over the news that by the time Wuhan was placed under quarantine over half the population of Wuhan had fled to other parts of China.
– Third, the virus had spread to many other parts of China, including most major cities, by February 1st
– Fourth, we were not testing people when they got on the planes in China for coronavirus, we were not testing the passengers when they disembarked from the planes, there were no penalties for breaking the quarantine,  there no checks being made of the passengers to ensure that they were adhering to the quarantine, and there were no spot tests of the people coming from China. A few people arriving from China were advised to self-quarantine, but not everyone.
– Fifth, in November, 2014 during the Ebola crisis, Prime Minister Stephen Harper banned people  from Ebola-stricken West Africa from traveling to Canada. As a consequence of his actions no Ebola case arose in Canada. The USA did not ban people from West Africa and confirmed a case of Ebola diagnosed in the United States in a man who traveled from West Africa to Dallas, Texas. That patient died. Earlier Saudi Arabia had announced a travel ban aimed at preventing Liberians, Sierra Leoneans and Guineans from visiting Islam’s holy sites. No Ebola case arose in Saudi Arabia.
– Sixth, a ban works.

My main concern with  Dr. Theresa Tam’s assertion  that there was no risk from asymptomatic spreaders is that I have been aware of Typhoid Mary for over fifty-five years as she was often mentioned in side bars and fillers in newspapers when I was young. Typhoid Mary is the poster child for  asymptomatic spreaders. Her real name was Mary Mallon. She was employed as a cook in various households and kitchens in the New York area over the period from 1907 to 1915. She was the first person in the United States identified as an asymptomatic carrier of  typhoid fever and is believed to have infected 51 people, at least  three of whom died. (Some estimates put the death total at fifty.) Eventually she was arrested and put in quarantine to stop her working and spreading the disease. Interestingly, Marineli et al. (2013) mention that “By the time she died New York health officials had identified more than 400 other healthy carriers of Salmonella typhi.”

Intriguingly there is a fair amount of information on diseases having been transmitted by  asymptomatic carriers of diseases.    In addition to typhoid, Wickipedia mentions  C. difficile, influenzas,  tuberculosis, and HIV. Transmission of diseases by asymptomatic carriers appears to be the norm,  rather than the exception, for infectious diseases.

Dr. Theresa Tam stated that she followed and implemented The Canadian Pandemic Influenza Plan for the Health Sector , 2006 (“Canada’s Pandemic Plan”), of which she was the co-author. If she had followed the plan she should have noticed that “Transmission by asymptomatic persons is possible but it is more efficient when symptoms, such as coughing, are present and viral shedding is high (i.e. early in symptomatic period).” and that the “potential for asymptomatic infection and spread from asymptomatic individuals greatly limits the effectiveness and feasibility of most traditional public health control measures.”

If she had done a bit of research Dr. Tan might also have located an article by Fraser  et al. (2004) discussing factors that make an infectious disease outbreak controllable, in which they argue that “Direct estimation of the proportion of asymptomatic and presymptomatic infections is achievable by contact tracing and should be a priority during an outbreak of a novel infectious agent.” noting that “no confirmed cases of transmission from asymptomatic patients have been reported to date in detailed epidemiological analyses of clusters of SARS cases, which suggests that, for SARS, there is a period after symptoms develop during which people can be isolated before their infectiousness increases. Actions taken during this period to isolate or quarantine ill patients can effectively interrupt transmission.”

She might also have noted a paper by Myoung-don Oh et al. (2018) analyzing the 2015 MERS coronavirus outbreak in Korea in which they mention that “the potential for transmission from asymptomatic rRT-PCR positive individuals is still unknown. Therefore, asymptomatic [persons who test] positive for MERS-CoV should be isolated and should not return to work until two consecutive respiratory-tract samples test negative.”

Another paper that Dr. Tan might have located without much trouble is a 2018 report by the World Health Organization providing guidance for asymptomatic persons who test positive for Middle East respiratory syndrome coronavirus (MERS-CoV). She should have noted the paper in part because Katherine Defalco, Public Health Agency of Canada, Ottawa, Canada contributed to the WHO’s report. In this report WHO state that the potential for transmission from asymptomatic  positive MERS-CoV  persons is currently unknown  but still recommended that “asymptomatic RT-PCR positive persons should be isolated , followed up daily for development of any  symptoms and tested at least weekly – or earlier, if symptoms develop – for MERS-CoV. The place of isolation (hospital or home) shall depend on the  health – care system’s isolation  bed capacity, its capacity to  monitor asymptomatic RT- PCR positive persons daily outside a health-care setting, and  the  conditions of the household and its occupants.” WHO also recommended that “When providing home isolation of asymptomatic RT-PCR  positive persons, the person and family  should be provided with clear instructions on:

•  adequate physical separation from potential householdor social contacts, especially those with risk conditions for severe MERS-CoV illness (e.g. separate room and toilet);
•  having  food in the room and avoid sharing food or  being in the same room with others as much as possible;
•  avoidance of visitors and travel; …

WHO also cautioned that “sometimes it is difficult to classify a case as ‘asymptomatic’ because although the person may not have any symptoms at the time of testing, he or she may develop illness during the course of infection.”

In contrast to WHO’s recommendations for asymptomatic MERS-CoV coronavirus persons, Canada did no testing to find asymptomatic COVID-19 coronavirus carriers. Instead we were told that they posed no threat, and that it was only those that developed symptoms who required testing. As noted above, recent studies have shown that asymptomatic spreading of COVID-19 is the norm. More importantly, Lai et al. (2020) report that “the transmission of COVID-19 through asymptomatic carriers via person-to-person contact was observed in many reports” citing reports published on the web by Rothe on January 30, 2020; by Liu on February 12, 2020; by Yu on February 18; and by Bai on February 21, 2020. Surprisingly, despite these warnings no effort was made in Canada in February or March to test for asymptomatic carriers.  In fact, we still don’t test for asymptomatic carriers.

There have been further reports of asymptomatic spreading. On March 23 Qian et al. reported a COVID-19 family cluster in China caused by a presymptomatic case. On April 1st Wei et al, reported on an investigation of all 243 cases of COVID-19 reported in Singapore during January 23–March 16 and identified seven clusters of cases in which presymptomatic transmission is the most likely explanation for the occurrence of secondary cases. Ten of the cases within these clusters were attributed to presymptomatic transmission and accounted for 6.4% of the 157 locally acquired cases reported as of March 16.

Dr. Tam was on the CBC News the other night reporting that only ten cases in Canada could be traced to origins in China. However, we only tested those who self-reported symptoms. As we never  tested for  asymptomatic spreaders , we will never know how many asymptomatic spreaders from China (or other countries) were in Canada. Further, we have only traced a fraction of those who have developed the disease, and will only know that many people contracted the disease while in Canada, without knowing the source for their disease.


References and Suggested Reading

Anonymous, 2003
Learning from SARS – Renewal of Public Health  in Canada. A report of the National Advisory Committee on SARS and Public Health October 2003. 234 pages

Anonymous, 2020a
Asymptomatic carrier

Anonymous, 2020b
Mary Mallon

Anonymous, 2020c
Subclinical infection

Y. Bai, L. Yao, T. Wei, F. Tian, D.Y. Jih, L. Chen, et al., 2020 Feb 21
Presumed asymptomatic carrier transmission of COVID-19
J Am Med Assoc (2020 Feb 21), 10.1001/jama.2020.2565

Filio Marineli, Gregory Tsoucalas, Marianna Karamanou, and George Androutsos, 2013
Mary Mallon (1869-1938) and the history of typhoid fever.  Ann Gastroenterol. 2013; 26(2): 132–134.

Fraser C, Riley S, Anderson R et al. 2004
Factors that make an infectious disease outbreak controllable. Proc Natl Acad  Sci USA 2004;101(16):6146–51.

Lai, Chih-Cheng; Liu, Yen Hung; Wang, Cheng-Yi; Wang, Ya-Hui; Hsueh, Shun-Chung; Yen, Muh-Yen; Ko, Wen-Chien; Hsueh, Po-Ren (2020-03-04).
Asymptomatic carrier state, acute respiratory disease, and pneumonia due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2): Facts and myths”. Journal of Microbiology, Immunology and Infection. doi:10.1016/j.jmii.2020.02.012. ISSN 1684-1182.

Y.C. Liu, C.H. Liao, C.F. Chang, C.C. Chou, Y.R. Lin , 2020 Feb 12
A locally transmitted case of SARS-CoV-2 infection in Taiwan.
New England J Med (2020 Feb 12), 10.1056/NEJMc2001573

Myoung-don Oh, Wan Beom Park, Sang-Won Park, Pyoeng Gyun Choe, Ji Hwan Bang, Kyoung-Ho Song, Eu Suk Kim, Hong Bin Kim, and Nam Joong Kim, 2018
Middle East respiratory syndrome: what we learned from the 2015 outbreak in the Republic of Korea.  Korean J Intern Med. 2018 Mar; 33(2): 233–246.
Published online 2018 Feb 27. doi: 10.3904/kjim.2018.031

Qian G, Yang N, Ma AHY, et al. Epub March 23, 2020
A COVID-19 Transmission within a family cluster by presymptomatic infectors in China. Clin Infect Dis 2020.

C. Rothe, M. Schunk, P. Sothmann, G. Bretzel, G. Froeschl, C. Wallrauch, et al. (2020 Jan 30)
Transmission of 2019-nCoV infection from an asymptomatic contact in Germany
N Engl J Med, 10.1056/NEJMc2001468

Theresa Tam and Karen Grimsrud, Co-Chairs, 2006
The Canadian Pandemic Influenza Plan for the Health Sector.  Public Health Agency of Canada.
550 pages.

W. E. Wei; , Z. Li; C. J. Chiew, S. E. Yong, M. P. Toh, V. J. Lee,  2020, April 10& April 1
Presymptomatic Transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020
Morbidity and Mortality Weekly Report (MMWR), 69(14);411–415. On April 1, 2020, this report was posted online as an MMWR Early Release.

World Health Organization,   2018
Management of asymptomatic persons who are RT-PCR positive for Middle East respiratory syndrome coronavirus (MERS-CoV): Interim guidance . 3 January  2018
WHO/MERS/IPC/15.2 Rev.1 Geneva:

P. Yu, J. Zhu, Z. Zhang, Y. Han, L. Huang, 2020 Feb 18
A familial cluster of infection associated with the 2019 novel coronavirus indicating potential person-to-person transmission during the incubation period
J Infect Dis (2020 Feb 18), 10.1093/infdis/jiaa077

Peter Zimonjic, Rosemary Barton, Philip Ling, 2020
‘Was it perfect? No’: Theresa Tam discusses Canada’s early pandemic response. ‘Could we have done more at the time? You can retrospectively say yes,’ Canada’s top doctor says CBC News Posted: Apr 27, 2020

May 1, 2020 - Posted by | Science and Pseudo-Science |

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