Comparative analysis of comorbidity, demography, clinical characteristics and selected investigations in first wave versus second wave for patients hospitalised with pandemic H1N1 infection during the 2009–2010 pandemic (n=1520)
| Patient characteristic | First wave, n=601 (39.5%), n (%) | Second wave, n=919 (60.5%), n (%) | Unadjusted OR* (95% CI) | Age-adjusted OR (95% CI) |
| Age (years) | ||||
| <1 | 39 (6.5) | 82 (8.9) | 1.00 (reference) | – |
| 1–4 | 60 (10.0) | 78 (8.5) | 0.62 (0.37 to 1.03) | |
| 5–15 | 116 (19.3) | 105 (11.4) | 0.43 (0.27 to 0.68) | |
| 16–24 | 106 (17.6) | 139 (15.1) | 0.62 (0.39 to 0.99) | |
| 25–34 | 87 (14.5) | 155 (16.9) | 0.85 (0.53 to 1.35) | |
| 35–44 | 72 (12.0) | 123 (13.4) | 0.81 (0.50 to 1.31) | |
| 45–54 | 59 (9.8) | 109 (11.9) | 0.88 (0.54 to 1.44) | |
| 55–64 | 33 (5.5) | 82 (8.9) | 1.18 (0.68 to 2.06) | |
| ≥65 | 29 (4.8) | 46 (5.0) | 0.75 (0.41 to 1.38) | |
| p Value | p trend=0.010 | |||
| Ethnicity | ||||
| White | 180 (30.0) | 450 (49.0) | 1.00 | 1.00 |
| Other | 337 (56.1) | 173 (18.8) | 0.21 (0.16 to 0.26) | 0.21 (0.16 to 0.27) |
| Missing | 84 (14.0) | 296 (32.2) | – | – |
| p Value | <0.001 | <0.001 | ||
| Neurological disorders | 45 (7.5) | 42 (4.6) | 0.59 (0.38 to 0.91) | 0.60 (0.39 to 0.92) |
| p Value | 0.018 | 0.020 | ||
| Radiological pneumonia | 79 (13.1) | 175 (19.0) | 1.55 (1.16 to 2.07) | 1.41 (1.04 to 1.90) |
| p Value | 0.003 | 0.025 | ||
| C-reactive protein (mg/litre) | ||||
| ≤30 | 171 (28.5) | 222 (24.2) | 1.00 | 1.00 |
| 31–99 | 91 (15.1) | 171 (18.6) | 1.45 (1.05 to 2.00) | 1.39 (0.99 to 1.93) |
| 100 | 40 (6.7) | 122 (13.3) | 2.35 (1.56 to 3.54) | 2.22 (1.46 to 3.39) |
| Missing | 299 (49.8) | 404 (44.0) | – | – |
| p Value | p trend <0.001 | P trend <0.001 | ||
| Altered consciousness | 21 (3.5) | 53 (5.8) | 1.69 (1.01 to 2.83) | 1.72 (1.03 to 2.89) |
| p Value | 0.046 | 0.040 | ||
| Dyspnoea | 191 (31.8) | 384 (41.8) | 1.54 (1.24 to 1.91) | 1.46 (1.17 to 1.82) |
| p Value | <0.001 | 0.001 | ||
| Pre-admission antibiotics | 82 (13.6) | 198 (21.6) | 1.74 (1.31 to 2.30) | 1.69 (1.27 to 2.24) |
| p Value | <0.001 | <0.001 | ||
| Pre-admission antiviral drugs | 80 (13.3) | 92 (10.0) | 0.72 (0.53 to 0.99) | 0.73 (0.53 to 1.00) |
| p Value | 0.048 | 0.049 | ||
| Delayed admission | ||||
| No | 420 (69.9) | 423 (46.0) | 1.00 | 1.00 |
| Yes | 77 (12.8) | 150 (16.3) | 1.93 (1.42 to 2.63) | 1.91 (1.41 to 2.60) |
| Missing | 104 (17.3) | 346 (37.6) | – | – |
| p Value | <0.001 | <0.001 | ||
| Required supplemental oxygen on admission | 154 (25.6) | 281 (30.6) | 1.28 (1.02 to 1.61) | 1.21 (0.96 to 1.53) |
| p Value | 0.037 | 0.104 | ||
| Intravenous fluid replacement on admission | 179 (29.8) | 211 (23.0) | 0.70 (0.56 to 0.89) | 0.70 (0.55 to 0.88) |
| p Value | 0.003 | 0.002 | ||
| Oxygen saturation <94% on air | ||||
| No | 61 (10.2) | 315 (34.3) | 1.00 | 1.00 |
| Yes | 178 (29.6) | 439 (47.8) | 0.48 (0.35 to 0.66) | 0.49 (0.35 to 0.68) |
| Missing | 362 (60.2) | 165 (18.0) | – | – |
| p Value | <0.001 | <0.001 | ||
| Length of hospital stay | ||||
| <2 days | 137 (22.8) | 152 (16.5) | 1.00 | 1.00 |
| ≥2 days | 395 (65.7) | 650 (70.7) | 1.48 (1.14 to 1.93) | 1.41 (1.08 to 1.84) |
| Missing | 69 (11.5) | 117 (12.7) | – | – |
| p Value | 0.003 | 0.012 | ||
| Adverse outcomes (death or level 2 or 3 admission) | 80 (13.3) | 188 (20.5) | 1.67 (1.26 to 2.23) | 1.64 (1.23 to 2.18) |
| p Value | <0.001 | 0.001 | ||
| Level 2 or 3 admission | 72 (12.0) | 178 (19.4) | 1.76 (1.31 to 2.37) | 1.73 (1.29 to 2.33) |
| p Value | <0.001 | <0.001 | ||
↵* All comparisons are second wave compared with first wave.
No statistically significant differences were found by pandemic wave in the following patient characteristics: sex; comorbidities such as asthma, chronic obstructive pulmonary disease (COPD) and chronic pulmonary conditions other than asthma or COPD; hepatic disease, cardiovascular disease, diabetes, hypertension, immunocompromised status, or total comorbidity burden; recorded obesity, smoking status, pregnancy; inpatient treatment with antivirals or antibiotics; and mortality.