Table 2. Hazard ratios with 95% confidence intervals and vaccine effectiveness estimates for seasonal influenza vaccination on influenza outcome including inpatient and outpatient cases, Stockholm County, influenza seasons 2011/12–2014/15.
| Category | Total number | All cases | Outpatient | Inpatient | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Cases | HR (95% CI) | VE | Cases | HR (95% CI) | VE | Cases | HR (95% CI) | VE | ||
| 2011/12 | ||||||||||
| All | ||||||||||
| Unvaccinated | 1,883,612 | 5,109 | Ref | NA | 4,793 | Ref | NA | 316 | Ref | NA |
| Vaccinated | 205,415 | 374 | 0.81 (0.69–0.94) | 19% (6–31) | 210 | 0.69 (0.57–0.84) | 31% (14–43) | 164 | 1.07 (0.79–1.46) | 0 |
| Age ≥ 65 years | ||||||||||
| Unvaccinated | 140,143 | 263 | Ref | NA | 161 | Ref | NA | 102 | Ref | NA |
| Vaccinated | 176,622 | 299 | 0.90 (0.72–1.12) | 10% (0–28) | 149 | 0.86 (0.64–1.17) | 14% (0–36) | 150 | 0.94 (0.68–1.31) | 6% (0–32) |
| 2012/13 | ||||||||||
| All | ||||||||||
| Unvaccinated | 1,935,823 | 2,471 | Ref | NA | 1,885 | Ref | NA | 586 | Ref | NA |
| Vaccinated | 185,646 | 139 | 0.60 (0.48–0.77) | 40% (23–52) | 48 | 0.55 (0.37–0.81) | 45% (19–63) | 91 | 0.53 (0.39–0.73) | 47% (27–61) |
| Age ≥ 65 years | ||||||||||
| Unvaccinated | 163,988 | 202 | Ref | NA | 55 | Ref | NA | 147 | Ref | NA |
| Vaccinated | 162,678 | 106 | 0.51 (0.38–0.69) | 49% (31–62) | 31 | 0.62 (0.35–1.10) | 38% (0–65) | 75 | 0.48 (0.34–0.69) | 52% (31–66) |
| 2013/14 | ||||||||||
| All | ||||||||||
| Unvaccinated | 1,971,500 | 2,076 | Ref | NA | 1,850 | Ref | NA | 226 | Ref | NA |
| Vaccinated | 199,707 | 105 | 0.63 (0.48–0.83) | 37% (17–52) | 57 | 0.58 (0.41–0.83) | 42% (17–59) | 48 | 0.70 (0.44–1.11) | 30% (0–56) |
| Age ≥ 65 years | ||||||||||
| Unvaccinated | 166,024 | 129 | Ref | NA | 58 | Ref | NA | 71 | Ref | NA |
| Vaccinated | 170,752 | 74 | 0.54 (0.37–0.79) | 46% (21–56) | 33 | 0.59 (0.33–1.05) | 41% (0–67) | 41 | 0.51 (0.31–0.83) | 49% (17–69) |
| 2014/15 | ||||||||||
| All | ||||||||||
| Unvaccinated | 2,001,463 | 4829 | Ref | NA | 3,980 | Ref | NA | 849 | Ref | NA |
| Vaccinated | 205,709 | 829 | 0.85 (0.76–0.95) | 15% (5–24) | 298 | 0.83 (0.70–0.98) | 17% (2–30) | 531 | 0.84 (0.72–0.99) | 16% (1–28) |
| Age ≥ 65 years | ||||||||||
| Unvaccinated | 172,245 | 697 | Ref | NA | 212 | Ref | NA | 485 | Ref | NA |
| Vaccinated | 173,075 | 705 | 0.82 (0.71–0.93) | 18% (7-29) | 204 | 0.89 (0.69–1.15) | 11% (0–31) | 501 | 0.79 (0.68–0.93) | 21% (7-32) |
CI: confidence interval; HR: hazard ratio; NA: not applicable; Ref: reference value; VE: vaccine effectiveness.
International Classification of Diseases, 10th revision codes J09-J11 were used to identify influenza diagnoses [10].
Vaccine effectiveness calculated (1 – HR × 100).
Hazard ratios derived from Cox proportional hazards regression model; adjusted for sex, age (age groups 10-year intervals), comorbidity status, socioeconomic status, previous seasonal vaccination, pneumococcal vaccination and Pandemrix vaccination. As complete case analysis was used, the number of cases decreased due to missing in socioeconomic status.