Viral transmission in asymptomatic cases of SARS-CoV-2 infection
Authors: Isabelle Grall, Chakib-Ahmed Alloui, Yacine Tandjaoui-Lambiotte, Antoine Deslandes, Delphine Seytre, Julie Chappe, Etienne Carbonnelle, Segolène Brichler, Anne Jacolot, Jean-Ralph Zahar, Alexandra Lomont
Journal: Journal of Infection (2020)

Numerous reports have suggested transmission from asymptomatic SARS-Cov2 patients1Huang L. Zhang X. Zhang X. et al.Rapid asymptomatic transmission of COVID-19 during the incubation period demonstrating strong infectivity in a cluster of youngsters aged 16-23 years outside Wuhan and characteristics of young patients with COVID-19: a prospective contact-tracing study.J Infect. 2020; 80: e1-e13https://doi.org/10.1016/j.jinf.2020.03.006Abstract Full Text Full Text PDF PubMed Scopus (249) Google Scholar in the community,2Rothe C. Schunk M. Sothmann P. et al.Transmission of 2019-nCoV infection from an asymptomatic contact in Germany.N Engl J Med. 2020; 382: 970-971https://doi.org/10.1056/NEJMc2001468Crossref PubMed Scopus (2797) Google Scholar within the family3Chan J.F.-W. Yuan S. Kok K.-.H. 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; 395: 514-523https://doi.org/10.1016/S0140-6736(20)30154-9Abstract Full Text Full Text PDF PubMed Scopus (6149) Google Scholar or in public places.4Mizumoto 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.Euro Surveill. 2020; 25https://doi.org/10.2807/1560-7917.ES.2020.25.10.2000180Crossref PubMed Scopus (1562) Google Scholar Most of these reports highlighted the role of pre symptomatic carriers for transmission.5Wei W.E. Li Z. Chiew C.J. et al.Presymptomatic Transmission of SARS-CoV-2—Singapore, January 23–March 16, 2020.MMWR Morb Mortal Wkly Rep. 2020; 69: 411-415https://doi.org/10.15585/mmwr.mm6914e1Crossref PubMed Scopus (0) Google Scholar Also, the viral load in mild and asymptomatic patients seems similar to that found in symptomatic patients.6Zou L. Ruan F. Huang M. et al.SARS-CoV-2 viral load in upper respiratory specimens of infected patients.N Engl J Med. 2020; 382: 1177-1179https://doi.org/10.1056/NEJMc2001737Crossref PubMed Scopus (3423) Google Scholar Asymptomatic patients are sources for transmission7Wölfel R. Corman V.M. Guggemos W. et al.Virological assessment of hospitalized patients with COVID-2019.Nature. 2020; https://doi.org/10.1038/s41586-020-2196-xCrossref PubMed Scopus (4646) Google Scholar; however the main transmission routes remain undefined. In this prospective work, we wanted to evaluate the daily viral spreading and environmental contamination around patients infected by SARS-CoV-2 and without any respiratory symptoms. Between April 10, 2020 and May 10, 2020, patients hospitalized in our Rehabilitation Care ward, tested positive for COVID-19, without any respiratory symptoms at diagnosis and having given their prior consent were included. The exclusion criteria included any patient with influenza-like illness or any respiratory symptoms with or without fever in the last month prior to inclusion. Daily and over a period of 8 consecutive days, we sampled early in the morning and before any cleaning 3 surfaces (mobile phone, doorbell, and patient over bed table) and we collected the surgical masks (175 mm x 95 mm, 3 layers, model PLANETYPE) worn 30 min by the patients during the clinical examination (patients were prompted to talk and answer questions.). In case of symptoms onset we stopped the follow-up. The swabbing of the surfaces was carried out by the same operator using a pre-impregnated e-swab (reference ESWABR1,COPAN).. Each mask was cut over an area of 5 cm2 facing the patient's mouth/nose. The internal face was collected and then vortexed for 30 s at maximum speed with 3 ml of Phosphate-Buffered-Saline (PBS). The process was performed by the same operator for all samples. The analysis was then carried out from 200 µL of the recovered sample. RNA extraction was performed on a NucliSENSⓇ easyMAGⓇ (Biomerieux, Marcy-l'Etoile,France) device, following strictly the manufacturer's recommendations. RT-PCR was performed on a ABI QuantStudio 7 (Thermofisher) device, using the commercial RealStarⓇ SARS-CoV-2 RT-PCR Kit 1.0 (Altona Diagnostics, Hamburg, Germany) test. Briefly, 10 µL of RNA is added to the 20 µL RT-PCR mix. Two targets are detected, one specific for beta-coronavirus, and one specific for the SARS-COV-2 strains. Internal Control was added in the lysis buffer to validate both extraction and amplification steps. Ten consecutive patients (P1 to P10) were included in the study period (Table 1). The median threshold cycle (Ct) value of the RT-PCR performed at diagnosis was 16 (13–33). Three patients were transferred within 2 days of completion. Only one patient (P9) became symptomatic with coughing at day 5 after inclusion. The most frequent contaminated samples were the phone and over bed table in 44% of cases whereas masks and ring bell were respectively contaminated in 38% and 28% of cases. Except for one patient (P8), the viral spreading through the mask was high in the first few days after diagnosis and decreased in the following days (Fig. 1). Also, environment of 1 asymptomatic patient (P8) was contaminated for a longer period of time compared to the other.Table 1Patients characteristics.Age (years)GenderDate of admissionDate of diagnosisRespiratory signs within the last 21 daysSample conditionClinical signs at diagnosisCt threshold on respiratory samplesClinical signs during the 3 weeks of follow up% of days with at least one positive environmental sampleNb of days with at least one positive mask sampleP-164Female20/03/202014/04/2020NoCase contactNo19No12.5%1P-261Female03/12/201915/04/2020NoCase contactNo16No50%2P-346Male11/02/202014/04/2020NoCase contactNo17No12.5%1P-468Female06/04/202014/04/2020NoCase contactNo33No0%0P-566Female29/01/202014/04/2020NoCase contactNo16NoP-648Female27/12/201815/04/2020NoCase contactHeadache14NoP-775Female15/01/202015/04/2020NoCase contactNo13NoP-873Male01/04/202021/04/2020NoCase contactNo19No100%6P-961Male07/02/202024/04/2020NoCase contactNo13Cough and diarrhea (Day-5)100%0P-1081Female09/04/202029/04/2020NoCase contactNo13No14%3 Open table in a new tab The median Ct thresholds for different positive samples was respectively 34, 34, 31.5 36 for masks, phone ring bell and over bed table (Fig. 1). Our work on a small number of patients suggests despite a high viral load at diagnosis, an overall low viral spreading and environmental contamination, during a period of 8-days of follow up. Asymptomatic patients seems to contaminate more frequently the surfaces compared to their masks in view of the fact that sampled masks were rarely contaminated beyond the third day of follow up. Also, the thresholds cycles obtained from different samples suggest the presence of low viral concentrations. Unlike previous studies, we tried to focus on strictly asymptomatic patients. Indeed, except one patient (P9) all remained asymptomatic during the follow-up. According to recent data, there is more evidence of transmission from individuals in the incubation period that could occur as early as five days before the onset of symptoms.8He X. Lau E.H.Y. Wu P. et al.Temporal dynamics in viral shedding and transmissibility of COVID-19.Nat Med. 2020; : 1-4https://doi.org/10.1038/s41591-020-0869-5Crossref PubMed Scopus (2853) Google Scholar However, recent study suggests a high risk of transmission from asymptomatic individuals in the community and nursing homes9Arons M.M. Hatfield K.M. Reddy S.C. et al.Presymptomatic SARS-CoV-2 infections and transmission in a skilled nursing facility.N Engl J Med. 2020; https://doi.org/10.1056/NEJMoa2008457Crossref PubMed Scopus (1512) Google Scholar Also rare are the reports that suggests transmission from asymptomatic patients in the hospital setting. Indeed, asymptomatic carrier state can be further divided into asymptomatic infection and pre-symptomatic infection after extended follow-up based on the nature of disease progression. In this context, we could consider our (P9) patient as a pre symptomatic one. To our knowledge, the transmissibility of asymptomatic cases has not been quantified and the main transmission routes remain as yet undetermined. Recent authors concluded that the relative transmissibility from asymptomatic case could be significantly smaller than that of symptomatic cases.10He D. Zhao S. Lin Q. et al.The relative transmissibility of asymptomatic cases among close contacts.Int J Infect Dis. 2020; https://doi.org/10.1016/j.ijid.2020.04.034Abstract Full Text Full Text PDF Scopus (150) Google Scholar Knowing whether asymptomatic patients are at risk of transmission in the hospital setting is of major importance, as far as prevention measures are concerned. Indeed, as up to 18% of patients infected with SARS-CoV-2 are asymptomatic4Mizumoto 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.Euro Surveill. 2020; 25https://doi.org/10.2807/1560-7917.ES.2020.25.10.2000180Crossref PubMed Scopus (1562) Google Scholar and the diagnosis cannot be ruled out on the basis of either PCR or CT scan results or both.11Meng H. Xiong R. He R. et al.CT imaging and clinical course of asymptomatic cases with COVID-19 pneumonia at admission in Wuhan, China.J Infect. 2020; https://doi.org/10.1016/j.jinf.2020.04.004Abstract Full Text Full Text PDF Scopus (191) Google Scholar It is therefore crucial to address the issue of the risk of transmission in the hospital setting from asymptomatic patients. Regarding our quantitative and qualitative results, it suggest a higher risk related to environmental contamination. Our study has several limitations. Firstly, we cannot exclude that our included patients were infected before the inclusion date, however they were enrolled first day after a contact with an recent known case index. Secondly, our results are based on RT-PCR tests that only imply the potential infectivity and we were unable to analyze the live virus isolation. Thirdly, it would have been important to study the daily throat viral load in parallel to ensure that there was no correlation with environmental contamination. Finally, our study is monocentric with a low number of cases which may limit the extent of the message. In conclusion, data on strictly asymptomatic carriers acting as potential viral transmitters in the hospital are scarce. We need larger studies involving a greater number of patients in order to better define the transmission routes in this specific population.