Table 1. Characteristics of studies reporting proportion of asymptomatic infection among persons positive for COVID-19, by study population (i.e. contacts, general population, and other populations).

No studies were blinded for participants or assessors.

First Author, Country	Type of study	Inclusion criteria	Definition of asymptomatic	Duration of follow-up from 1st testing (Days)
General Population
Lavezzo, E.	Cohort	The entire population of the town of Vò, in lockdown from 23 February to 8 March after 1st COVID-19 was reported on 21 February.	Not requiring hospitalization and/or did not report fever (yes/no or a temperature above 37°C) and/or cough and/or at least two of the following symptoms: sore throat, headache, diarrhoea, vomit, asthenia, muscle pain, joint pain, loss of taste or smell, or shortness of breath.	Range:
Italy				7 to 132
Snoeck, Ch.	Cohort	Random selection of adult (>18 years old) population of Luxembourg, stratified by age, gender and electoral districts (2000 participants invited from 18,000 panel members, 1,840 accepted)	None of the symptoms listed in self-reported online questionnaire for the 2 weeks prior to test and in the follow-up	Range:
Luxembourg*				13 to 37
Contacts
Park, S.	Cohort	All occupants (workers and residents) of a building, closed on 9 March 2020, immediately after an outbreak of COVID-19 was reported.	No symptoms.	For all:
South Korea				141
Schwierzeck, V.	Cohort	HCWs, patients and their accompanying person with contact to SARS-CoV-2 infected individuals (exposure assessed with a risk-based questionnaire), in the Paediatric Dialysis Unit of University Hospital of Munster.	None of the typical COVID-19 symptoms	For all:
Germany				10
Zhang, J.	Cross sectional	All close contacts of an index patient (supermarket employee) and all persons who visited the ZH supermarket tested in the week prior to the index patients last day working (January 16–30, 2020), in Shandong province.	No symptoms	Not followed
China
Other Populations
Healthcare Workers in Settings Other Than Nursing Homes
Lai, X.	Cross-sectional	335 randomly selected HCWs of Fever clinic and of other Departments of Tonji Hospital, Wuhan	None of the following: fever, myalgia or fatigue, cough, sore throat, muscle ache, diarrhoea, headache, dyspnoea, dizziness, sputum production, nausea and vomiting, haemoptysis	Not followed
China
Lombardi, A.	Cohort	All the consecutive HCWs who were tested at the Ca' Granda Ospedale Maggiore Policlinico in Milan, February 24 to March 31, 2020	None of the following in the 14 days preceding the test: fever, cough, dyspnoea, asthenia, myalgia, coryza, sore throat, headache, ageusia or dysgeusia, anosmia or parosmia, ocular symptoms, diarrhoea, nausea, and vomit.	Range:
Italy				52 to 88
Romao, V.C.	Cohort	All staff (symptomatic/asymptomatic) of rheumatology department, Hospital de Santa Maria, Lisbon, tested on 15–16 March 2020.	None of the following: fever, cough, dyspnoea, chest tightness, malaise, fatigue, headache, rhinorrhoea, sore throat, anosmia, dysgeusia, arthralgia, myalgia, nausea/vomiting/diarrhoea, dizziness.	Range:
Portugal*				27 to 51 days
Obstetric Patients Presenting to Hospitals
Andrikopolou, M. & Sutton, D.	Cohort	All women, admitted to the labour unit of the New York–Presbyterian Allen Hospital and Columbia University Irving Medical Centre (March 22 to Apr 19, 2020) for delivery or antepartum/postpartum indications	No Covid-19 specific symptoms at testing	For all:
USA				Up to 14 days
Bianco, A.	Cross-sectional	All women who were scheduled for a planned delivery within the Mount Sinai Health system (April 4 to 15, 2020)	None of the following fever or feel hot, cough, shortness of breath, sore throat, vomiting, diarrhoea, rash.	Not followed
USA
Goldfarb, I.	Cohort	All women admitted to labour and delivery units of 2 academic and 2 community hospitals affiliated with Mass General Brigham Health (April 18 to May 5, 2020)	None of the following: fever -subjective or documented-, new cough, shortness of breath, sore throat, muscle aches, new rhinorrhoea, or new anosmia	Time of hospitalization for delivery
USA
London, V.	Cohort	All pregnant women admitted to antepartum, labour and delivery units at one tertiary care hospital in Brooklyn, New York (March 15 to April 15, 2020).	None of the following: fever, cough, shortness of breath, sore throat, nausea, vomiting	For all:
USA				6
Ochiai, D.	Cohort	All obstetric patients admitted for delivery during universal screening admitted to Keio University Hospital, Tokyo (April 6 to April 27, 2020)	No symptoms of COVID-19	Range:
Japan				7 to 13
Liver Transplant Patients Presenting to Hospitals
Ossami, R.	Cohort	All liver transplant outpatients visiting clinic, Berlin, tested between 23 March and 23 April 2020.	None of the following: cough, fever, sore throat, dyspnoea, new/changed sputum, new fatigue, exhaustion, new onset of diarrhoea.	For all:
Germany				14
Congregate Setting
Baggett, T.	Cross sectional	All adults residing in one homeless shelter in Boston on 2 April and 3 April 2020 (excluded if previously tested for COVID19).	No symptoms at testing.	Not followed
USA
Ly, T.	Cross-sectional	Homeless people, people living in precarious condition and asylum-seekers residing in four shelters, four hotels, and three residences, as well as the employees of these centres, in Marseille (March 26 to April 17,2020)	No fever or respiratory symptoms (cough, rhinorrhoea, dyspnoea, sore throat)	Not followed
France*
Nursing Homes
Dora, A.	Cohort	All residents at skilled nursing facility (Veteran Affairs Greater Los Angeles Healthcare System), regardless of symptoms (March 29 to April 23,2020)- Note: the testing was serial (approximately weekly).	None of the following at time of test of during follow-up, from retrospective notes review: fever, myalgia, headache, cough, dyspnoea, nausea, emesis, diarrhoea, poor appetite.	Range:
USA				14 to 264
Graham, N.	Cohort	All residents of four nursing homes in central London, tested 15 April to 1 May 2020	None of the following (from note review): new fever, cough and/or breathlessness; newly altered mental status or behaviour, anorexia, diarrhoea or vomiting.	Up to:
UK				7
Kimball, A. & Arons, M.	Serial cross- sectional	All residents on 13 March 2020 at a skilled nursing facility in King County, Washington, after a case of COVID-19 was reported on 1 March 2020.	No symptoms or only stable chronic symptoms (e.g., chronic cough without worsening).	For all:
USA3				7
Patel, M.,	Cohort	Population 1: All residents of Facility where there had been a patient of COVID-19, regardless of symptoms.	No symptoms at testing and during follow-up.	For all:
USA		Population 2: All staff members who worked on the ward where the index case lived.		30
Roxby, A.	Cohort	All residents and staff at an assisted living community in Seattle after notification of two COVID-19 cases from 5 March to 9 March 2020. First round of testing was carried out on 10 March.	No symptoms at the time of testing or 14 days prior.	For all:
USA				20
Public Facing Workers
Lan, F.	Cross-sectional	All workers older than 18-year-old sent by a grocery retail store for city mandated group-testing in the greater Boston area.	None of the following: fever/chills, headache, running nose, sore throat, cough (acute, new onset, dry or productive), shortness of breath loss of taste or smell, diffuse body ache, fatigue/ feeling run down, nausea, diarrhoea	Not followed
USA*

Abbreviations: IQR, Interquartile range; HD, hemodialysis, HCW, healthcare workers

*Pre-print studies.

¹In Park S et al. time in quarantine (not from diagnosis).

²In Lavezzo et al. 1^st test done 21 to 29 Feb 2020; 2^nd test done 7 March 2020.

³Same cohort of patients, reported in different studies.

⁴Reviewed March 26 to April 20.