Author and date	Allocation concealment	Baseline influenza vaccination rate treatment group (%)	Baseline influenza vaccination rate control group (%)
		Difference 2%	Or less
Abramson 2011	Unclear	43.4	44.4
Arthur 2002	Unclear	48.7	46.7
Barnas 1989	Unclear	5	5
Beck 1997	No	74	72
Clayton 1999	Unclear	0% for not vaccinated 100% for vaccinated	0% for not vaccinated 100% for vaccinated
Frank 2004	Yes	65	66
Ives 1994	Unclear	41.3	40.6
Karuza 1995	Unclear	47.5	46.5
Kiefe 2001	Unclear	40	40
Kim 1999	Unclear	79	80
Kouides 1998	Unclear	57.6	58
Krieger 2000	Yes	0% for not vaccinated 100% for vaccinated	0% for not vaccinated 100% for vaccinated
McCaul 2002	Unclear	0	0
McDowell 1986	Unclear	0	0
McMahon 1995b (McMahon Wyoming)	Unclear	Participants who received a personal letter 23.8 Participants who received a form letter 20.5	Participants who received no letter 21.6
Moran 1995	Unclear	16.7	16.6
Nuttall 2003	Unclear	0	0
Roca 2012	Unclear	50.9	49.1
		Difference	3% to 4%
Dietrich 1989	Unclear	36	39
Herman 1994	Unclear	31.3	34.3
Lemelin 2001	Unclear	46.1	49.4
Lukasik 1987	No	7.3	4.5
MacIntyre 2003	Yes	61	64
McMahon 1995b (McMahon Montana 1994)	Unclear	Participants who received a personal letter 41.2 Participants who received a form letter 46	Participants who received no letter 42.3
Siriwardena 2002	Unclear	48.6	44.7
		Difference	5% or more
Chan 2002	Unclear	31.8 solo 42.5 group practice	37.8 solo 30.1 group practice
Puech 1998	Yes	32	38
Marrero 2006	Unclear	36	14