The burden of influenza complications in different high-risk groups: a targeted literature review

Figures & data

Table 1.  People recommended for influenza vaccination because of high risk of complications.

High-risk group

By age

2–4 years

≥ 65 years

Chronic respiratory disease including asthma, COPD, and cystic fibrosis

Chronic heart disease including congestive heart failure and acute coronary syndromes

Chronic neurological disease including multiple sclerosis and Parkinson’s disease

Chronic kidney or liver disease or hematologic disorders

Metabolic disorders including diabetes and morbid obesity

Immunocompromised individuals including post-transplant, HIV infection, during chemotherapy

Other

Pregnant women

People living in nursing homes

COPD, chronic obstructive pulmonary disease; HIV, human immunodeficiency virus.

Sources: United Kingdom6; US7; all countries8.

Table 2.  Probability of a clinical case of influenza.

Age	Risk group (vaccination rate)	Annual influenza rate	Country	Years	Study quality			n	Reference
					Design	# Sites and population representative?*	Influenza defined
65+	Otherwise healthy	2.4–5.0%	US	1999–2003	PC	Multiple; Cannot tell	LCI	608	Falsey et al.25
	Living in the community (0%)	6.55%	UK, NL	1991–1994	MA	Multiple; Cannot tell	LCI	1098	Turner et al.5
	Living in the community (58.9–64.3%)	0.23–1.52%	ES	2002–2005	PC	Multiple; Cannot tell	ILI	11,240	Vila-Córcoles et al.24
	Registered with general practitioner	0.99%	UK	1991–1996	GPRD	Multiple; Yes	ILI	507,556	Meier et al.43
	Living in residential care (very high)	4.85%	US, JA, UK, FR, EU	1985–1999	MA	Multiple; Cannot tell	LCI	18,566	Turner et al.5
All	CHF or CLD	2.4–7.2%	US	1999–2003	PC	Multiple; Cannot tell	LCI	540	Falsey et al.25
	SCT	0.9%	EU, AU	1997–1998	PC	Multiple; Yes	LCI	1973	Ljungman et al.44
	SCT, allogeneic	6.55%	ES	1999–2003	PC	Single; Yes	LCI	172	Martino et al.26
	SCT, autologous	3.05%	ES	1999–2003	PC	Single; Yes	LCI	240	Martino et al.26

*Was the population representative of the age, risk group, country and site(s) studied in the analysis?

AU, Australia; CHF, congestive heart failure; CLD, chronic lung disease; ES, Spain; EU, Europe; GPRD, General Practice Research Database; ILI, influenza-like illness; LCI, laboratory-confirmed influenza; MA, meta-analysis of clinical trial data; PC, prospective cohort; SCT, stem cell transplant; UK, United Kingdom; US, United States.

Figure 1.  Framework for assessment of the burden of influenza complications. ICU, intensive care unit; LRTI, lower respiratory tract infection.

Table 3.  Probability of pneumonia or lower respiratory tract infection or chest infiltrates, given influenza.

Age	Risk group	Probability pneumonia or LRTI	Country	Years	Study quality				n	Reference
					Design	# Sites and population representative?*	Influenza defined	Outcome well defined
65+	GP visit, NHS high risk	1.3%	UK	1991–1996	CrS using GPRD	Multiple; Yes	ILI	Yes	7407	Meier et al.43
	GP visit, NHS low risk	1.0%	UK	1991–1996	CrS using GPRD	Multiple; Yes	ILI	Yes	10145	Meier et al.43
	Healthy, community	0%	US	1999–2003	PC	Multiple; Can’t tell	LCI	Yes	24	Falsey et al.25
All	CHF or CLD, community	10%	US	1999–2003	PC	Multiple; Can’t tell	LCI	Yes	20	Falsey et al.25
	Diabetes, no AV	2.6%	US	2000–2006	CrS using CDB	Multiple; Can’t tell	ID	Yes	6171	Orzeck et al.45
	Diabetes, AV-OS	2.1% (ns)	US	2000–2006	CrS using CDB	Multiple; Can’t tell	ID	Yes	2919	Orzeck et al.45
	Hospitalized	27%	US	2002–2004	RC	Single; Yes	ID	Yes	207	Babcock et al.33
	Hospitalized	30%	US	1999–2003	PC	Single; Yes	LCI	Yes	144	Falsey et al.25
	High-risk hospitalized	52.3%	US	1999–2003	PC	Single; Yes	LCI	Yes	193	Murata et al.46
	Hospitalized	48.6%	US	1999–2000	PC	Single; Yes	LCI	Yes	35	Oliveira et al.37
	SCT, no AV	35%	US	1989–2009	RC	Single; Yes	LCI	Yes	58	Boudreault et al.28
	SCT, AV-RM, AM, OS, or ZN	4% (p < 0.01)	US	1989–2009	RC	Single; Yes	LCI	Yes	84	Boudreault et al.28
	SCT	29%	US	1989–2002	RC	Single; No	LCI	Yes	62	Nichols et al.47
	SCT	44%	EU + AU	1997–2000	PC	Multiple; Yes	LCI	Yes	36	Ljungman et al.44
	SCT, autologous	26%	ES	1999–2003	PC	Single; Yes	LCI	Yes	15	Martino et al.26
	SCT, allogeneic	55%	ES	1999–2003	PC	Single; Yes	LCI	Yes	24	Martino et al.26
	BMT, hospitalized	66%	US	1991–1994	PC	Single; Yes	LCI	Yes	12	Whimbey et al.48
	BMT, hospitalized	75%	US	1991–1992	PC	Single; Yes	LCI	Yes	8	Whimby et al.49
	Leukemia	80%	US	1993–1994	PC	Single; Yes	LCI	Yes	15	Yousuf et al.27
	HM/SCT, no AV	42%	US	2000–2002	RC	Single; Yes	LCI	Yes	71	Chemaly et al.29
	HM/SCT, AV – OS, or RV	10% (p < 0.005)	US	2000–2002	RC	Single; Yes	LCI	Yes	41	Chemaly et al.29
	SOT, hospitalized	42–83%	US	1990–2000	RC	Single; Yes	LCI	Yes	30	Vilchez et al.50
	IC, hospitalized	53%	FR	1998–2008	RC	Single; Yes	LCI	Yes	100	Schnell et al.32
	HIV infection	16.3%	US	1997–1999	RC	Single; Yes	LCI	Yes	43	Skiest et al.41

*Was the population representative of the age, risk group, country, and site(s) studied in the analysis?

AM, amantadine; AU, Australia; AV, antiviral drug therapy; BMT, bone marrow transplant; CDB, claims database; CHF, congestive heart failure; CLD, chronic lung disease; CrS, cross-sectional; ES, Spain; EU, Europe; FR, France; GP, general practitioner; GPRD, General Practice Research Database (UK); HIV, human immunodeficiency virus; HM, hematological malignancy; IC, immunocompromised; ID, influenza diagnosis; ILI, influenza like illness; JA, Japan; LCI, laboratory-confirmed influenza; LRTI, lower respiratory tract infection; NHS, National Health Service; NL, The Netherlands; PC, prospective cohort; OS, oseltamivir; RC, retrospective cohort; RM, rimantadine; RV, ribavirin; SCT, stem cell transplant; SOT, solid organ transplant; UK, United Kingdom; US, United States; ZN, zanamivir.

Table 4.  Probability of hospitalization, given influenza.

Age	Risk group	Probability of hospital stay	Country	Years	Study quality				n	Reference
					Design	# Sites and population representative?*	Influenza defined	Outcome well defined
65+	All risk levels	8.8%	US	1996–1997	CrS using CDB	Multiple; Yes	ID	Yes	170	Irwin et al.30
	Otherwise healthy	0%	US	1999–2003	PC	Multiple; Can’t tell	LCI	Yes	24	Falsey et al.25
65–74	NHS high risk	2.4%	UK	1990–1997	CC for EH	Multiple; Yes	LCI	Yes	87,332	Turner et al.5
	NHS low risk	0.44%	UK	1990–1997	CC for EH	Multiple; Yes	LCI	Yes	181,381	Turner et al.5
75+	NHS high risk	4.08%	UK	1990–1997	CC for EH	Multiple; Yes	LCI	Yes	102,425	Turner et al.5
	NHS low risk	1.13%	UK	1990–1997	CC for EH	Multiple; Yes	LCI	Yes	141,444	Turner et al.5
All	High risk or 65+ no AV	3.2%	NH, SH	1997–2000	MA	Multiple; Yes	LCI	Yes	401	Kaiser et al.51
	High risk or 65+ AV-OS	1.6% (p = 0.16)	NH, SH	1997–2000	MA	Multiple; Yes	LCI	Yes	368	Kaiser et al.51
	Chronic corticosteroids	20.7%	US	1996–1997	CrS using CDB	Multiple; Yes	ID	Yes	29	Irwin et al.30
	Cancer	20.8%	US	1996–1997	CrS using CDB	Multiple; Yes	ID	Yes	72	Irwin et al.30
	CHD	17.2%	US	1996–1997	CrS using CDB	Multiple; Yes	ID	Yes	128	Irwin et al.30
	CLD	2.9%	US	1996–1997	CrS using CDB	Multiple; Yes	ID	Yes	1283	Irwin et al.30
	Diabetes	12.1%	US	1996–1997	CrS using CDB	Multiple; Yes	ID	Yes	116	Irwin et al.30
	CHF or CLD	20%	US	1999–2003	PC	Multiple; Can’t tell	LCI	Yes	20	Falsey et al.25
	Asthma, COPD, diabetes or 65+	1.65%	IT	1998–1999	PC	Multiple; Can’t tell	ID	Yes	1211	Sessa et al.52
	Diabetes, no AV	3.4%	US	2000–2006	CrS using CDB	Multiple; Can’t tell	ID	Yes	6171	Orzeck et al.45
	Diabetes, AV - OS	2.0% (p < 0.05)	US	2000–2006	CrS using CDB	Multiple; Can’t tell	ID	Yes	2919	Orzeck et al.45
	HIV infection	14.0%	US	1997–1999	RC	Single; Yes	LCI	Yes	43	Skiest et al.41

*Was the population representative of the age, risk group, country, and site(s) studied in the analysis?

AV, antiviral drug therapy; CBD, claims database; CC, case control; CHD, coronary heart disease; CHF, congestive heart failure; CLD, chronic lung disease; COPD, chronic obstructive pulmonary disease; CrS, cross-sectional; EH, excess hospitalizations; HIV, human immunodeficiency virus; ID, influenza diagnosis; IT, Italy; LCI, laboratory-confirmed influenza; MA, meta-analysis of clinical trials; NH, northern hemisphere countries; NHS, National Health Service; OS, oseltamivir; PC, prospective cohort; RC, retrospective cohort; SH, southern hemisphere countries; UK, United Kingdom; US, United States.

Table 5.  Probability of intensive care unit admission given hospitalized for influenza.

Age	Risk group	Probability of intensive care	Country	Years	Study quality				n	Reference
					Design	# Sites and population representative?*	Influenza defined	Outcome well defined
75+	All risk levels	11.4–17.1%	US	2005–2008	PC	Multiple; Yes	LCI	Yes	2050	Dao et al.38
50–74	All risk levels	14.7–20.4%	US	2005–2008	PC	Multiple; Yes	LCI	Yes	1738	Dao et al.38
65+	All risk levels	4.2%	FR	2006–2007	HDB	Multiple; Can’t tell	ID	Can’t tell	1346	Tomas et al.31
All	All risk levels	17.4%	US	2002–2004	RC	Single; Yes	LCI	Yes	207	Babcock et al.33
	All risk levels	12.9–16.8%	US	2005–2008	PC	Multiple; Yes	LCI	Yes	5055	Dao et al.38
	All risk levels	16.0%	CA	2005–2006	PC	Multiple; Yes	LCI	Yes	327	McGeer et al.36
	All risk levels	16.0%	CA	2005–2007	PC	Multiple; Yes	LCI	Yes	607	Muller et al.39
	All risk levels	11.8%	US	1999–2003	PC	Single; Yes	LCI	Yes	144	Falsey et al.25
	All risk levels	28.6%	US	1999–2000	PC	Single; Yes	LCI	Yes	35	Oliveira et al.37
	ARS	16.0%	US	1999–2003	PC	Single; Yes	LCI	Yes	101	Murata et al.46
	No ARS	13.0%	US	1999–2003	PC	Single; Yes	LCI	Yes	92	Murata et al.46

*Was the population representative of the age, risk group, country, and site(s) studied in the analysis?

ARS, acute respiratory symptoms; CA, Canada; FR, France; HDB, hospital database; ID, influenza diagnosis; LCI, laboratory-confirmed influenza; PC, prospective cohort; RC, retrospective cohort; US, United States.

Table 6.  Probability of mechanical ventilation given intensive care unit or hospital admission for influenza.

Age	Risk group	Probability mechanical ventilation	Country	Years	Study quality				n	Reference
					Design	# Sites and population representative?*	Influenza defined	Outcome well defined
65+	Hospital admits, cancer	3.8%	US	1998–2001	CrS using HDB	Multiple; Can’t tell	ID	Yes	49,611	Cooksley et al.35
18–64	Hospital admits, cancer	5.8%	US	1998–2001	CrS using HDB	Multiple; Can’t tell	ID	Yes	13,532	Cooksley et al.35
All	Hospital admits	9.8%	US	1998–1999	RC	Single; Yes	LCI	Yes	132	Angelo et al.42
	Hospital admits	12.1%	US	2002–2004	RC	Single; Yes	LCI	Yes	207	Babcock et al.33
	Hospital admits	9.0%	CA	2005–2007	PC	Multiple; Yes	LCI	Yes	607	Muller et al.39
	Hospital admits	10.4%	US	1999–2003	PC	Single; Yes	LCI	Yes	144	Falsey et al.25
	Hospital admits, COPD	26.0%	US	2002–2004	RC	Single; Yes	LCI	Yes	46	Babcock et al.33
	Hospital admits, no COPD	8.0%	US	2002–2004	RC	Single; Yes	LCI	Yes	161	Babcock et al.33
	Hospital admits, ARS	10.0%	US	1999–2003	PC	Single; Yes	LCI	Yes	101	Murata et al.46
	Hospital admits, no ARS	12.0%	US	1999–2003	PC	Single; Yes	LCI	Yes	92	Murata et al.46
	Hospital admits, SCT	9.1%	US	1989–2009	RC	Single; Yes	LCI	Yes	142	Boudreault et al.28
	Hospital admits, SCT, no AV	12.0%	US	1989–2009	RC	Single; Yes	LCI	Yes	58	Boudreault et al.28
	Hospital admits, SCT, AV-RM, AM, OS, or ZN	4.0% (p = 0.25)	US	1989–2009	RC	Single; Yes	LCI	Yes	84	Boudreault et al.28
	Hospital admits, IC, PN	21.0%	FR	1998–2008	RC	Single; Yes	LCI	Yes	53	Schnell et al.32
	Hospital admits, IC, no PN	0%	FR	1998–2008	RC	Single; Yes	LCI	Yes	47	Schnell et al.32
	Intensive care unit admits	69.4%	US	2002–2004	RC	Single; Yes	LCI	Yes	36	Babcock et al.33
	Intensive care unit admits	78.4%	US	1999–2006	RC	Multiple; Yes	LCI	Yes	111	Li et al.34

* Was the population representative of the age, risk group, country, and site(s) studied in the analysis?

AM, amantadine; ARS, acute respiratory symptoms; AV, antiviral drug therapy; CA, Canada; COPD, chronic obstructive pulmonary disease; CrS, cross-sectional; FR, France; HDB, hospital database; IC, immunocompromised; ID, influenza diagnosis; LCI, laboratory-confirmed influenza; OS, oseltamivir; PC, prospective cohort; PN, pneumonia; RC, retrospective cohort; RM, rimantadine; SCT, stem cell transplant; US, United States; ZN, zanamivir.

Table 7.  Probability of death with influenza episode.

Age	Risk group	Probability death	Country	Years	Study type				n	Reference
					Design	# Sites and population representative?*	Influenza defined	Outcome well defined
65+	GP visit, NHS high risk	1.5%	UK	1991–1996	CrS using GPRD	Multiple; Yes	ILI	Yes	7407	Meier et al.43
	GP visit, NHS low risk	1.1%	UK	1991–1996	CrS using GPRD	Multiple; Yes	ILI	Yes	10,145	Meier et al.43
	Hospital admits	13.5%	CA	2005–2007	PC	Multiple; Yes	LCI	Yes	451	Muller et al.39
	Hospital admits	3.1%	FR	2006–2007	CrS using HDB	Multiple; Can’t tell	ID	Yes	1346	Tomas et al.31
	Hospital admits, cancer	9.5%	US	1998–2001	CrS using HDB	Multiple; Can’t tell	ID	Yes	49,611	Cooksley et al.35
18–64	Hospital admits, cancer	8.3%	US	1998–2001	CrS using HDB	Multiple; Can’t tell	ID	Yes	13,532	Cooksley et al.35
5–64	Hospital admits	0.3%	FR	2006–2007	CrS using HDB	Multiple; Can’t tell	ID	Yes	3258	Tomas et al.31
15–49	GP visit, NHS high risk	0.04%	UK	1991–1996	CrS using GPRD	Multiple; Yes	ILI	Yes	12,195	Meier et al.43
50–64	GP visit, NHS high risk	0.20%	UK	1991–1996	CrS using GPRD	Multiple; Yes	ILI	Yes	5402	Meier et al.43
All	Hospital admits	11.7%	CA	2005–2007	PC	Multiple; Yes	LCI	Yes	607	Muller et al.39
	Hospital admits	3.4%	US	2002–2004	RC	Single; Yes	LCI	Yes	207	Babcock et al.33
	Hospital admits	2.9–4.4%	US	2005–2008	PC	Multiple; Yes	LCI	Yes	5055	Dao et al.38
	Hospital admits	8.3%	CA	2005–2006	PC	Multiple; Yes	LCI	Yes	327	McGeer et al.36
	Hospital admits	7.0%	US	1999–2003	PC	Single; Yes	LCI	Yes	144	Falsey et al.25
	Hospital admits	14.3%	US	1999–2000	PC	Single; Yes	LCI	Yes	35	Oliveira et al.37
	Hospital admits, high risk	12.3%	CA	2005–2007	PC	Multiple; Yes	LCI	Yes	568	Muller et al.39
	Hospital admits, CLD	12.1%	CA	2005–2007	PC	Multiple; Yes	LCI	Yes	257	Muller et al.39
	Hospital admits, CHD	14.2%	CA	2005–2007	PC	Multiple; Yes	LCI	Yes	360	Muller et al.39
	Hospital admits, IC and Diabetes	13.2%	CA	2005–2007	PC	Multiple; Yes	LCI	Yes	372	Muller et al.39
	Hospital admits, NMD	29.8%	CA	2005–2007	PC	Multiple; Yes	LCI	Yes	47	Muller et al.39
	Hospital admits, ARS	5.9%	US	1999–2003	PC	Single; Yes	LCI	Yes	101	Murata et al.46
	Hospital admits, no ARS	5.4%	US	1999–2003	PC	Single; Yes	LCI	Yes	92	Murata et al.46
	SCT	12.0%	US	1989–2009	RC	Single; Yes	LCI	Yes	143	Boudreault et al.28
	SCT, no AV	17.0%	US	1989–2009	RC	Single; Yes	LCI	Yes	58	Boudreault et al.28
	SCT, AV – RM, AM, OS, or ZN	4.0% (p = 0.05)	US	1989–2009	RC	Single; Yes	LCI	Yes	84	Boudreault et al.28
	SCT,MV	69.0%	US	1989–2009	RC	Single; Yes	LCI	Yes	13	Boudreault et al.28
	SCT	23.0%	EU + AU	1997–2000	PC	Multiple; Yes	LCI	Yes	1973	Ljungman et al.44
	SCT, autologous	0%	ES	1999–2003	PC	Single; Yes	LCI	Yes	15	Martino et al.26
	SCT, allogeneic	12.5%	ES	1999–2003	PC	Single; Yes	LCI	Yes	24	Martino et al.26
	SCT	10.0%	US	1989–2002	RC	Single; No	LCI	Yes	62	Nichols et al.47
	SCT, PN	28.0%	US	1989–2002	RC	Single; No	LCI	Yes	18	Nichols et al.47
	Hospital admits, BMT, PN	50.0%	US	1992–1994	PC	Single; Yes	LCI	Yes	8	Whimbey et al.48
	HM or SCT, no AV	27.0%	US	2000–2002	RC	Single; Yes	LCI	Yes	71	Chemaly et al.29
	HM or SCT, AV – OS, RV	9.0% (p = 0.02)	US	2000–2002	RC	Single; Yes	LCI	Yes	41	Chemaly et al.29
	HM	7.7%	ES	1999–2001	PC	Single; Yes	LCI	Yes	56	Martino et al.53
	Hospital admits, leukemia, PN	33.0%	US	1993–1994	PC	Single; Yes	LCI	Yes	12	Yousuf et al.27
	IC, PN	19.0%	FR	1998–2008	RC	Single; Yes	LCI	Yes	53	Schnell et al.32
	IC, no PN	0%	FR	1998–2008	RC	Single; Yes	LCI	Yes	47	Schnell et al.32
	ICU admits	18.9%	US	1999–2006	RC	Multiple; Yes	LCI	Yes	111	Li et al.34
	ICU admits, COPD	16.4%	US	1999–2006	RC	Multiple; Yes	LCI	Yes	55	Li et al.34
	ICU admits, CAD	13.9%	US	1999–2006	RC	Multiple; Yes	LCI	Yes	36	Li et al.34
	ICU admits, diabetes	13.9%	US	1999–2006	RC	Multiple; Yes	LCI	Yes	36	Li et al.34
	ICU admits, hypertension	19.6%	US	1999–2006	RC	Multiple; Yes	LCI	Yes	56	Li et al.34
	ICU admits, hypothyroidism	13.6%	US	1999–2006	RC	Multiple; Yes	LCI	Yes	22	Li et al.34
	ICU admits, IC	52.4%	US	1999–2006	RC	Multiple; Yes	LCI	Yes	21	Li et al.34
	ICU admits, Transplant	28.6%	US	1999–2006	RC	Multiple; Yes	LCI	Yes	7	Li et al.34

*Was the population representative of the age, risk group, country, and site(s) studied in the analysis?

ARS, acute respiratory symptoms; AU, Australia; AV, antiviral therapy; BMT, bone marrow transplant; CA, Canada; CAD, coronary artery disease; CHD, chronic heart disease; CLD, chronic lung disease; COPD, chronic obstructive pulmonary disease; CrS, cross-sectional; ES, Spain; EU, Europe; FR, France; GP, general practitioner; GPRD, General Practice Research Database; HDB, hospital database; HM, hematologic malignancy; IC, immunocompromised; ICU, intensive care unit; ID, influenza diagnosis; ILI, influenza-like illness; LCI, laboratory-confirmed influenza; MV, mechanical ventilation; NHS, National Health Service; NMD, neuromuscular disease; OS, oseltamivir; PC, prospective cohort; PN, pneumonia; RC, retrospective cohort; RV, ribavirin; SCT, stem cell transplant; UK, United Kingdom; US, United States.

Table 8.  Length of stay in hospital, in intensive care unit, or on mechanical ventilation.

Age	Type of care, risk group	Length of stay (days)	Country	Years	Study quality				n	Reference
					Design	# Sites and population representative?*	Influenza defined	Outcome well defined
Length of total hospital stay
65+	Hospital admits	M, 9.1; MD, 7; SD, 12.3	FR	2006–2007	CrS using HDB	Multiple; Can’t tell	ID	Yes	1346	Tomas et al.31
50+	Hospital admits,	M, 10.5–10.8; MD, 7–8	US	1979–2001	CC for EH	Multiple; Yes	ICD-9-CM 480-487	Yes	n/a	Thompson et al.40
50+	Hospital admits,	M, 7.8–8.6; MD, 5–6	US	1979–2001	CC for EH	Multiple; Yes	ICD-9-CM 390-519	Yes	n/a	Thompson et al.40
5–49	Hospital admits,	M, 7.5; MD, 5	US	1979–2001	CC for EH	Multiple; Yes	ICD-9-CM 480-487	Yes	n/a	Thompson et al.40
5–49	Hospital admits,	M, 5.8; MD, 3	US	1979–2001	CC for EH	Multiple; Yes	ICD-9-CM 390-519	Yes	n/a	Thompson et al.40
5–64	Hospital admits	M, 3.2; MD, 2; SD, 5.6	FR	2006–2007	CrS using HDB	Multiple; Can’t tell	ID	Yes	5451	Tomas et al.31
All	Hospital admits, all	M, 7.4	US	1998–1999	RC	Single; Yes	LCI	Yes	132	Angelo et al.42
	Hospital admits	MD, 3–4; IQR, 2–6	US	2005–2008	PC	Multiple; Yes	LCI	Yes	5055	Dao et al.38
	Hospital admits	M, 8; SD, 5	US	1999–2003	PC	Single; Yes	LCI	Yes	144	Falsey et al.25
	Hospital admits	MD, 6; range, 1–103	CA	2005–2006	PC	Multiple; Yes	LCI	Yes	327	McGeer et al.36
	Hospital admits	M, 8.4; MD, 5	US	1996–1997	CrS using CDB	Multiple; Yes	LCI	Yes	103	Irwin et al.30
	Hospital admits, poor outcomes	M, 12.5	US	1998–1999	RC	Single; Yes	LCI	Yes	27	Angelo et al.42
	Hospital admits, good outcomes	M, 6.1	US	1998–1999	RC	Single; Yes	LCI	Yes	105	Angelo et al.42
	Hospital admits, ARS	M, 8.6; SD, 5.1	US	1999–2003	PC	Single; Yes	LCI	Yes	101	Murata et al.46
	Hospital admits, no ARS	M, 8.9 SD, 10	US	1999–2003	PC	Single; Yes	LCI	Yes	92	Murata et al.46
	Hospital admits, IC, PN	MD, 13; IQR, 6–23	FR	1998–2008	RC	Single; Yes	LCI	Yes	53	Schnell et al.32
	Hospital admits, IC, no PN	MD, 5; IQR, 1–8	FR	1998–2008	RC	Single; Yes	LCI	Yes	47	Schnell et al.32
	Hospital admits, HIV	M, 12; MD, 7; range, 2–30	US	1997–1999	RC	Single; Yes	LCI	Yes	6	Skiest et al.41
	Hospital admits, cancer	M, 6.1	US	1998–2001	CrS using HDB	Multiple; Can’t tell	ID	Yes	64,008	Cooksley et al.35
	ICU admits	MD, 9.2; IQR, 5.7–15.0	US	1999–2006	RC	Multiple; Yes	LCI	Yes	111	Li et al.34
	ICU admits	MD, 6; range, 1–103	CA	2005–2006	PC	Multiple; Yes	LCI	Yes	52	McGeer et al.36
Length of stay in intensive care unit
All	ICU admits	MD, 4; range, 1–64	US	2002–2004	RC	Single; Yes	LCI	Yes	36	Babcock et al.33
	ICU admits	MD, 3.0; IQR, 1.3–6.5	US	1999–2006	RC	Multiple; Yes	LCI	Yes	111	Li et al.34
	ICU admits	MD, 5; range, 1–22	CA	2005–2006	PC	Multiple; Yes	LCI	Yes	52	McGeer et al.36
	ICU admits	M, 28; SD, 260	US	1999–2000	PC	Single; Yes	LCI	Yes	10	Oliveira et al.37
	ICU admits	MD, 6; range, 4–14	CA	2005–2007	PC	Multiple; Yes	LCI	Yes	97	Muller et al.39
Duration of mechanical ventilation
All	ICU admits	MD, 4; range, 1–60	US	2002–2004	RC	Single; Yes	LCI	Yes	36	Babcock et al.33
	ICU admits	MD, 3.0; IQR, 2.0–6.8	US	1999–2006	RC	Multiple; Yes	LCI	Yes	87	Li et al.34
	ICU admits	M, 21.5; SD, 20.5	US	1999–2000	PC	Single; Yes	LCI	Yes	10	Oliveira et al.37
	ICU admits	MD, 5; range, 2–13	CA	2005–2007	PC	Multiple; Yes	LCI	Yes	55	Muller et al.39
	ICU admits	MD, 7; range, 1–22	US	1989–2009	RC	Single; Yes	LCI	Yes	13	Boudreault et al.28

*Was the population representative of the age, risk group, country, and site(s) studied in the analysis?

ARS, acute respiratory symptoms; CC, case control; CA, Canada; CDB, claims database; CrS, cross-sectional; EH, excess hospitalizations; FR, France; HDB, hospital database; HIV, human immunodeficiency virus; IC, immunocompromised; ICD-9-CM, International Classification of Diseases, 9th Revision, Clinical Modification (code); ICU, intensive care unit; ID, influenza diagnosis; IQR, interquartile range; LCI, laboratory-confirmed influenza; M, mean; MD, median; PC, prospective cohort; PN, pneumonia; RC, retrospective cohort; SD, standard deviation; US, United States.

Figure 2.  Overview of clinical outcomes and resource-use data associated with influenza complications by high-risk group. CV, cardiovascular; HIV, human immunodeficiency virus; ICU, intensive care unit; LCI, laboratory-confirmed influenza; LOS, length of stay; LRTI, lower respiratory tract infection. ^a Including those with HIV infection, post-transplant, and with cancer. ^b Rate for those hospitalized with a confirmed influenza diagnosis.

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