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Asymptomatic carriers of COVID-19 in a confined adult community population in Quebec: A cross-sectional study

  • Lyne Cloutier
    Correspondence
    Address correspondence to Cloutier Lyne, Inf. PhD, Université du Québec à Trois-Rivières, Département des sciences infirmières, 3351 Boulevard des Forges, Trois-Rivières, Québec, Canada, G9A 5H7
    Affiliations
    Département des sciences infirmières, Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada
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  • Natacha Merindol
    Affiliations
    Département de chimie, biochimie et physique, Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada

    Centre Intégré Universitaire de Santé et Services Sociaux de la Mauricie-et-Centre-du-Québec, Trois-Rivières, Québec, Canada
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  • Geneviève Pépin
    Affiliations
    Département de biologie médicale, Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada
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  • Caroline Marcoux-Huard
    Affiliations
    Direction de santé publique et responsabilité populationnelle, Centre Intégré Universitaire de Santé et Services Sociaux de la Mauricie-et-du-Centre-du-Québec, Trois-Rivières, Québec, Canada
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  • Pier-Alexandre Vasil
    Affiliations
    Direction de santé publique et responsabilité populationnelle, Centre Intégré Universitaire de Santé et Services Sociaux de la Mauricie-et-du-Centre-du-Québec, Trois-Rivières, Québec, Canada
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  • Claudia Houle
    Affiliations
    Département de biologie médicale, Centre Intégré Universitaire de Santé et des Services Sociaux de la Mauricie-et-du-Centre-du-Québec, Trois-Rivières, Québec, Canada

    Département de microbiologie infectiologie et immunologie, faculté de médecine, Université de Montréal, Montréal, Québec, Canada
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  • Shweta Todkar
    Affiliations
    Département des sciences infirmières, Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada
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  • Marie-Claude Lehoux
    Affiliations
    Centre Intégré Universitaire de Santé et Services Sociaux de la Mauricie-et-Centre-du-Québec, Trois-Rivières, Québec, Canada
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  • Nathalie Houle
    Affiliations
    Département des sciences infirmières, Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada
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  • Hugo Germain
    Affiliations
    Département de chimie, biochimie et physique, Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada

    Centre Intégré Universitaire de Santé et Services Sociaux de la Mauricie-et-Centre-du-Québec, Trois-Rivières, Québec, Canada
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  • Alexis Danylo
    Affiliations
    Département de biologie médicale, Centre Intégré Universitaire de Santé et des Services Sociaux de la Mauricie-et-du-Centre-du-Québec, Trois-Rivières, Québec, Canada

    Département de microbiologie infectiologie et immunologie, faculté de médecine, Université de Montréal, Montréal, Québec, Canada
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Open AccessPublished:August 20, 2020DOI:https://doi.org/10.1016/j.ajic.2020.08.015

      Highlights

      • 1.82% of asymptomatic individuals may be SARS-CoV-2 carriers.
      • SARS-CoV-2 may spread asymptomatically in a population under social distancing restrictions.
      • None of the SARS-CoV-2-positive carriers had contact with COVID-19 patients.
      Several countries have undertaken social distancing measures to stop SARS-CoV-2 spread. Asymptomatic carriers’ prevalence is unknown and would provide essential information on hidden viral circulation. In our cross-sectional study, 1.82% of 330 asymptomatic confined individuals living in the community carried SARS-CoV-2 despite no contact with declared cases, raising concerns about unnoticed transmission.

      Graphical Abstract

      Key Words

      As of August 9th 2020, nearly 20 million individuals contracted COVID-19 and about 720,000 died.
      World Health Organisation
      Coronavirus Disease 2019 (COVID-19) Report – 86.
      The United States counts for one fourth of these cases. SARS-CoV-2 transmission occurs principally through respiratory droplets or direct contact.
      • Van Doremalen N
      • Bushmaker T
      • Morris DH
      • et al.
      Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1.
      Viral load peaks before or at the time of symptoms onset, raising the possibility that presymptomatic people significantly contribute to viral spread.
      • He X
      • Lau EHY
      • Wu P
      • et al.
      Temporal dynamics in viral shedding and transmissibility of COVID-19.
      ,
      • Tong ZD
      • Tang A
      • Li K-F
      • et al.
      Potential presymptomatic transmission of SARS-CoV-2, Zhejiang Province, China, 2020.
      Other individuals remain asymptomatic despite viral detection, suggesting that they may participate in transmission.
      • Lai C-C
      • Liu YH
      • Wang C-Y
      • et al.
      Asymptomatic carrier state, acute respiratory disease, and pneumonia due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2): Facts and myths, J Microbiol, Immunol.
      • Bai Y
      • Yao L
      • Wei T
      • et al.
      Presumed asymptomatic carrier transmission of COVID-19.
      • Rothe C
      • Schunk M
      • Sothmann P
      • et al.
      Transmission of 2019-nCoV infection from an asymptomatic contact in Germany.
      Studies conducted on individuals from the same households have convincingly shown that presymptomatic or asymptomatic SARS-CoV-2 carriers might transmit to their family members.
      • Jiang X-L
      • Zhang X-L
      • Zhao X-N
      • et al.
      Transmission potential of asymptomatic and paucisymptomatic SARS-CoV-2 infections: a three-family cluster study in China.
      • Li C
      • Ji F
      • Wang L
      • et al.
      Asymptomatic and human-to-human transmission of SARS-CoV-2 in a 2-family cluster, Xuzhou, China.
      • Ye F
      • Xu S
      • Rong Z
      • et al.
      Delivery of infection from asymptomatic carriers of COVID-19 in a familial cluster.
      In a long-term care facility, 23 (30.3%) residents carried the virus, including 13 asymptomatic of the 76 that were tested.
      • Kimball A
      • Hatfield KM
      • Arons M
      • et al.
      Asymptomatic and presymptomatic SARS-CoV-2 infections in residents of a long-term care skilled nursing facility - King County, Washington, March 2020.
      An investigation on the Diamond Princess cruise ship in Japan identified 17.9% of SARS-CoV-2 infected cases as asymptomatic.
      • Mizumoto K
      • Kagaya K
      • Zarebski A
      • Chowell G
      Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the diamond princess cruise ship, Yokohama, Japan, 2020.
      All these reports were carried out in a specific context of strong contacts between individuals.
      Very few studies have assessed the proportion of asymptomatic individuals in the reality of a community that followed social distancing measures. In an isolated city of Italy, screening 3 thousand individuals lead to the identification of 88 carriers, including 50-75% of asymptomatic individuals.
      • Romagnani S.
      Dobbiamo cambiare rotta, tampone a chi non ha sintomi.
      ,
      • Day M.
      Covid-19: identifying and isolating asymptomatic people helped eliminate virus in Italian village.
      Nishiura et al. reported 13 SARS-CoV-2 positive individuals, including 4 (30.8%) asymptomatic, corresponding to a prevalence of 0,7% out of 564 citizens evacuated from Wuhan.
      • Nishiura H
      • Kobayashi T
      • Suzuki A
      • et al.
      Estimation of the asymptomatic ratio of novel coronavirus infections (COVID-19).
      These investigations depict a wide range of possible rates for asymptomatic carriers and their proportion in a confined environment remains unknown, especially in the North American context. Despite early social distancing measures, Quebec is one of the most impacted nation with 735 confirmed cases per 100,000 individuals and a fatality rate of 675 per million.

      INSPQ, Institut National de santé publique du Québec. Données COVID-19 au Québec. 2020. Available at: https://www.inspq.qc.ca/covid-19/donnees. Accessed August 10, 2020.

      Our cross-sectional study was held between April 14th and 17th, 2020 in the Mauricie region, Québec, Canada. We aimed to identify the proportion of asymptomatic carriers in a confined community of adults. The term confinement is used herein to reflect social restrictions instructed by Quebec's government, progressively implemented starting on March 11th (Fig 1). On April 17th, the last day of this study, 723 positive cases were identified for a total of 17,521 with a prevalence of 0.204% and an incidence of 0.008% in the Quebec's Province.
      Fig 1
      FigTimeline of new cases count and of socio-sanitary measures implementation. In the province of Quebec, the first case was declared on February 27th, 2020. On March 11th, travelers were isolated at home for 14 days upon return, and gatherings of more than 250 people were forbidden. On March 13th, the health emergency state was declared. On March 14th, nonessential visits to hospitals and long-term care facilities were prohibited. Between March 15th and April 5th, borders, public places, daycares, schools, businesses, nonessential services were closed, social distancing measures were implemented, gatherings of more than 5 individuals and travels between regions were prohibited. This study was conducted between April 14th (612 positive cases on that day for a total of 14 860, a prevalence of 0,173% and an incidence of 0,007%) to April 17th (723 positive cases for a total of 17,521; a prevalence of 0,204% and an incidence of 0,008%) (data have been extracted from public health resources

      institut National de santé publique du Québec. Covid-19-données-ligne du temps. 2020. Available at: https://www.inspq.qc.ca/covid-19/donnees/ligne-du-temps. Accessed August 10, 2020.

      ).

      THE STUDY

      A total of 2,250 employees between 18 and 69 years old from the Université du Québec à Trois-Rivières (UQTR) residing in the region were solicited by email on April 13th 2020 after exactly 1 month of confinement. Informed consent was obtained from all participants by using an online secured platform. The study was approved by the Ethical committees of the 3 implicated institutions. Recruited participants were asked to fill out an online questionnaire to evaluate inclusion and exclusion criteria. Individuals presenting COVID-19 symptoms were excluded (supplemental methods). A second form was used within 24 hours post-test. The protocol included a recommendation to seek medical attention if any COVID-19-related symptoms emerged. A dedicated clinic where nurses collected a single upper respiratory tract specimen (oropharyngeal and nares swab) was set up (supplemental methods). Detection of SARS-CoV-2 was performed the day of collection by direct rRT-PCR using See Gene Allplex 2019-nCoV Assay, approved by Health Canada
      • Merindol N
      • Pépin G
      • Marchand C
      • et al.
      SARS-CoV-2 detection by direct rRT-PCR without RNA extraction.
      (supplemental methods). Out of 2,250 potential participants, swabs were collected from 330 individuals, including 210 women (63.6%) and 120 men (36.4%) of age 45.7 ± 11.2 years [18.6-69.8] (Table 1). We detected 6 SARS-CoV-2-infected individuals (1.82%; ranging from 0% to 4%; CI = 87.5%) at Ct values ranging from 36.7 to 38.6. For that same period, a mean of 18.03% of tested symptomatic people carried the virus in Quebec's Province (Fig).

      INSPQ, Institut National de santé publique du Québec. Données COVID-19 au Québec. 2020. Available at: https://www.inspq.qc.ca/covid-19/donnees. Accessed August 10, 2020.

      Table 1Attitudes towards social distancing measures and perception of self conduct towards the measures of confinement (Likert scale 1-7)
      Last 2 weeks of MarchFirst 2 weeks of April
      Positive cases (n = 6)Negative cases (n = 306)Positive cases (n = 6)Negative cases (n = 306)
      M ± ETM ± ETM ± ETM ± ET
      Agreement towards social distancing measures1.17 ± 0.411.25 ± 0.601.17 ± 0.411.18 ± 0.41
      Perception of self-conduct towards social distancing measures1.17 ± 0.411.32 ± 0.691.33 ± 0.521.24 ± 0.53
      Attitudes towards measures of social distancing and confinement were analysed through a Likert scale ranging from 1 to 7; 1 being complete agreement. Results presented in Table 1 show very little variation in the scores. All participants being very much in agreement with the measures issued by the government during the 2 periods. The vast majority also stated that they were abiding by those rules. All differences noted between groups and within groups were not statistically significant.
      Four of 6 positive-participants had travelled to North and Central America within the past 6 months and their trip lasted 5-16 days. The last return was on March 16th 2020. None of the SARS-CoV-2-positive cases reported symptom at time of post-test questionnaire. In a nonofficial statement to the local press, 1 participant reported experiencing cough and general fatigue a few days later.
      • Trahan B.
      Dépistage à l'UQTR: un nombre très faible de porteurs asymptomatiques, Le Nouvelliste.
      None of the SARS-CoV-2-positive individuals reported having contacts with COVID-19 patients. Both negative and positive individuals agreed with and respected confinement rules, and left their home only a few times a week (Table 1).

      CONCLUSION

      Our study identified 1,82% of asymptomatic carriers in a confined community. Their presence could be explained by 3 scenarios. First, they could be in fact presymptomatic individuals, as viral load peaks at the time of, or before symptoms onset.
      • Mizumoto K
      • Kagaya K
      • Zarebski A
      • Chowell G
      Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the diamond princess cruise ship, Yokohama, Japan, 2020.
      ,
      • Zhang J
      • Tian S
      • Lou J
      • Chen Y
      Familial cluster of COVID-19 infection from an asymptomatic.
      Our study was not designed to assess this possibility if symptoms developed later than 24-hour post-test. Second, following symptomatic infection, some cases experience prolonged viral nucleic acids detection by rRT-PCR.
      • Wu Y
      • Guo C
      • Tang L
      • et al.
      Prolonged presence of SARS-CoV-2 viral RNA in faecal samples.
      Consequently, some individuals could be postsymptomatic cases with residual viral shedding that were not screened at the beginning of the pandemic.
      The third possibility is true asymptomatic infection. The rate of this kind of infection is unknown. Presymptomatic people significantly contribute to viral spread, but we do not know if true asymptomatic and late postsymptomatic participate in transmission at this point. All positive cases had high Ct values, which most probably reflects low viral shedding, although data are sparse on correlation between Ct values and contagiousness.
      1,82% of asymptomatic carriers is high when compared to the 0,204% prevalence. However, there were 18.03% of carriers among the thousands of screened symptomatic individuals in the Province, ten times more than in our asymptomatic cohort. Our sample size is relatively small. At the time we designed our study, supplies in lab materials and staff availability were problematic, we chose a convenience sample size. Rates of asymptomatic carriers could be different in high-density urban area and in population with different socio-economic characteristics.
      Despite its limitations, this is the first methodically designed study that evaluates the rate of asymptomatic carriers in a confined population. Six SARS-CoV-2 asymptomatic carriers that agreed with social distancing measures is a significant number and could suggest that prevalence is underestimated. This could represent ∼100,000 asymptomatic carriers at the scale of the adult population of Quebec's Province at the time of this study. Interestingly, a preliminary analysis of the seroprevalence on 7,691 blood donors from Québec reported a similar rate of positivity.

      Héma-Québec. Étude de séroprévalence des donneurs de sang: 2,23% de la population adulter du Québec aurait contracté la COVID-19. 2020. Available at: https://www.hema-quebec.qc.ca/publications/communiques/archives/2020/communiques-2020/etude-seroprevalence-resultats.fr.html. Accessed August 10, 2020.

      Our results raise concerns about the possibility of viral spread through asymptomatic transmission, specifically in the context of an apprehended resurgence of SARS-CoV-2 infections following deconfinement. Since public health focuses on symptomatic cases, it is urgent to determine if asymptomatic cases contribute to transmission and to elucidate the dynamics of SARS-CoV-2 in distinct populations.

      Acknowledgments

      The research team wishes to acknowledge the Université du Québec à Trois-Rivières (UQTR) for providing human, financial and structural resources for the project and the Centre Intégré Universitaire de Santé et de Services Sociaux de la Mauricie et du Centre-Du-Québec (CIUSSS MCQ) for providing testing resources. We also thank participants and all volunteer nurses and personnel that worked at the clinic. Finally, we thank SeeGene that graciously provided the lab test kits for this study.

      Appendix. SUPPLEMENTARY MATERIALS

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