Influenza studies characteristics, influenza incidence rates and incidence rate ratios
| Author, Year | Country | Study design | AIIRD, Controls, Sample size | Influenza definition/ascertainment | IR AIIRD | IR Controls | IRR, 95% CI | Level of evidence |
| Dirven 201211 | The Netherlands | Cross-sectional | 879 RA | Influenza symptoms (patient-report) | 5.9% of the cohort | NA | NA | 4 |
| Blumentals 201210 | USA | Cohort (Marketscan) | 46 030 RA; 46 030 controls | ICD-9 | 409.3/100 000 PY | 306.1/100 000 PY | 1.2 (1.1 to 1.4) | 2b |
| Mohammad 201713 | Sweden | Cohort | 186 AAV; 744 age and gender-matched controls | ICD-10 | *5010/100 000 PY | *1530/100 000 PY | *3.3 (2.2 to 4.8) | 2b |
| Bello 201212 | Italy | Cross-sectional | 159 RA, PsA, AS, SpA on bDMARD | Influenza-like illness (patient-report) | 17% of the cohort | NA | NA | 4 |
*Combined incidence of influenza and pneumonia.
AAV, ANCA-associated vasculitis; AIIRD, autoimmune inflammatory rheumatic disease(s); AS, ankylosing spondylitis; bDMARD, biologic disease-modifying antirheumatic drug(s); IR, incidence ratio; IRR, incidence rate ratio; NA, non-available/non-applicable; PsA, psoriatic arthritis; PY, patient years; RA, rheumatoid arthritis; SpA, spondyloarthropathy.