Rheumatoid arthritis and the incidence of influenza and influenza-related complications: a retrospective cohort study

Table 1 Baseline characteristics of patients identified from MarketScan

	RA patients (cases)		Non-RA patients (controls)		p value^a
	N	%	N	%	p value^a
Total no. of patients	46,030	100	46,030	100	–
Age (years)
Mean (SD)	56.61 (14.79)				–
Median (IQR)	57 (19)				–
18–29	1780	3.87	1780	3.87	1
30–39	4119	8.95	4119	8.95	1
40–49	8064	17.52	8064	17.52	1
50–59	12,735	27.67	12,735	27.67	1
60–69	10,428	22.65	10,428	22.65	1
≥70	8904	19.34	8904	19.34	1
Medical conditions
Cardiovascular disease	8330	18.10	6489	14.10	<0.0001
Chronic lung disease	2470	5.37	1604	3.48	<0.0001
Diabetes	5284	11.48	5475	11.89	0.0501
Cancer	9152	19.88	9362	20.34	0.0842
HIV/AIDS	44	0.10	47	0.10	0.753
Transplant	926	2.01	421	0.91	<0.0001
Renal dysfunction	474	1.03	425	0.92	0.1005
Neurological disorders	3465	7.53	183	0.40	<0.0001
Depression	2555	5.55	2542	5.52	0.8514
Medications at baseline
DMARDs	6413	13.93	642	1.39	<0.0001
Biologics	3276	7.12	104	0.23	<0.0001
Influenza vaccination	3500	7.60	4667	10.14	<0.0001
High-dose corticosteroids^b	10,056	21.85	4990	10.84	<0.0001
Low-dose corticosteroids^b	7903	17.17	1182	2.57	<0.0001
Antibiotics	28,847	62.67	24545	53.32	<0.0001
Statins	7408	16.09	7908	17.18	<0.0001
RA patients who had received no active medication
No DMARDs, biologics or corticosteroids^c	25,894	56.25	NA	NA	–
Sex
Male	15,230	33.09	15230	33.09	1
Female	30,800	66.91	30800	66.91	1

Note: ^ap value of 1 indicates cases with matched controls; ^bPharmacy data were used to compute an average daily dose during a 6-month period prior to cohort entry. The average daily dose was unique for each individual course of corticosteroid therapy. ‘High dose’ referred to values that were greater than or equal to the average daily dose, while ‘low dose’ referred to values that were less than the average daily dose; ^cFrom 1 year prior to cohort entry until the end of the study; SD = standard deviation; IQR = interquartile range.