Number of events, absolute risk, excess risk and HRs estimated from Cox proportional hazards models comparing patients with IJDs to matched comparators for outcomes hospitalisation listing influenza (during 2015/2016–2018/2019 influenza seasons) or COVID-19 (during the first wave of COVID-19 2020), and death listing influenza (influenza seasons 2015–2019) or COVID-19 (during the first wave of COVID-19 2020)
| Influenza, seasons 2015–2019 | COVID-19 2020* | ||||||||||
| Outcome | Events (n) (average risk, %) in IJD cohort | Events (n) (average risk, %) in the general population | Crude excess risk per 100 patients | HR 1† | HR 2‡ | Outcome | Risk (%) in IJD cohort | Risk (%) in the general population | Crude excess risk per 100 patients | HR 1† | HR 2‡ |
| All IJD | All IJD | ||||||||||
| Hospitalisation listing influenza | 1066 (0.25) | 2011 (0.1) | 0.15 | 2.38 (2.21 to 2.56) | 1.44 (1.33 to 1.56) | Hospitalisation listing COVID-19 | (0.5) | (0.3) | 0.2 | 1.77 (1.61 to 1.95) | 1.32 (1.19 to 1.46) |
| Death listing influenza | 64 (0.015) | 109 (0.006) | 0.009 | 2.63 (1.93 to 3.58) | 1.46 (1.07 to 2.01) | Death listing COVID-19 | (0.10) | (0.07) | 0.03 | 2.09 (1.73 to 2.52) | 1.18 (0.97 to 1.44) |
| RA | RA | ||||||||||
| Hospitalisation listing influenza | 743 (0.35) | 1312 (0.15) | 0.2 | 2.41 (2.21 to 2.64) | 1.47 (1.34 to 1.62) | Hospitalisation listing COVID-19 | (0.7) | (0.4) | 0.3 | 2.02 (1.78 to 2.28) | 1.40 (1.23 to 1.60) |
| Death listing influenza | 50 (0.02) | 78 (0.009) | 0.011 | 2.73 (1.91 to 3.90) | 1.56 (1.08 to 2.27) | Death listing COVID-19 | (0.30) | (0.11) | 0.19 | 2.28 (1.85 to 2.81) | 1.27 (1.02 to 1.59) |
*COVID-19 data taken from Bower et al,11 from Cox models fitted identically to those of ‡. Note that season lengths differ for COVID-19, and for influenza, the former was from March to September (6 months) 2020, whereas the latter was from September to May (8 months) each year 2015–2019.
†HR 1 accounts for age, sex and region via the matching (and for influenza season via adjustment for influenza analyses) .
‡HR 2 additionally adjusts for socioeconomic factors (education, civil status and country of birth), influenza hospitalisation in the previous year (in influenza analyses) and comorbidities (history of the following diseases: cancer, diabetes, heart failure, ischaemic heart disease, lung disease, stroke, surgery, venous thrombotic event and kidney failure).
IJD, inflammatory joint disease; RA, rheumatoid arthritis.