Table 1. Basic characteristics of the included studies.

Study (publication year)	Study design	Country	Single/multi-center study	Sample	Hand washing frequency	Disease
Godoy 2012, (14)	Case-control	Spain	Multi-center	3,087	1–4, 5–10, >10 times/day	Influenza A
Lau 2004, (15)	Case-control	China	Single-center	990	1–10, >10 times/day	Acute respiratory tract infection, influenza-like illness
Merk 2014, (37)	Cohort	Sweden	Single-center	2,864	2–4, 5–9, 10–19, ≥20 times/day	SARS
Huang 2007, (20)	RCT	America	Single-center	148	4, 7 times/day	Diarrhea
Mangklakeree 2014, (27)	RCT	Thailand	Single-center	454	≤3, >3 times/day	Influenza-like illness
Torner 2015, (26)	Case-control	Spain	Multi-center	478	1–4, ≥5 times/day	Influenza
Thumma 2009, (38)	Cross-sectional	America	Single-center	462	3, 3–6, >6 times/day	Upper respiratory, gastrointestinal symptoms
Chen 2011, (25)	Case-control	China	Single-center	132	<4, ≥4 times/day	Influenza A

RCT, randomized controlled trial; SARS, severe acute respiratory syndrome.