Table 2.
Vaccine Effectiveness for the Prevention of Influenza-Associated Severe Outcomes, Including Severe Acute Respiratory Infection, Intensive Care Unit Admission, Organ Failure, and Death
| Influenza Cases, n [n Vaccinated (% Vaccinated)] | Noninfluenza Controls, n [n Vaccinated (% Vaccinated)] | Unadjusted OR for Vaccination (95% CI)a | Adjusted OR for Vaccination (95% CI)b | Vaccine Effectiveness,c % (95% CI) | P Value for Interaction | |
|---|---|---|---|---|---|---|
| Severe acute respiratory infection (full population) | 309 [138 (44.7%)] | 329 [201 (61.1%)] | .51 (.37–.7) | .68 (.47–.98) | 32 (2–53) | |
| Age groups (years) | ||||||
| 18–49 | 117 [26 (22.2%)] | 95 [45 (47.4%)] | .32 (.18–.57) | .38 (.20–.73) | 62 (27–81) | .0789 |
| 50–64 | 109 [53 (48.6%)] | 99 [60 (60.6%)] | .62 (.35–1.07) | .8 (.43–1.48) | 20 (−48 to 57) | |
| ≥65 | 83 [59 (71.1%)] | 135 [96 (71.1%)] | 1 (.55–1.83) | 1.03 (.54–1.97) | −3 (−97 to 46) | |
| Race/ethnicityd | ||||||
| Black non-Hispanic | 76 [32 (42.1%)] | 84 [46 (54.8%)] | .6 (.32–1.12) | .67 (.34–1.33) | 33 (−33 to 66) | .9083 |
| White non-Hispanic | 170 [86 (50.6%)] | 206 [136 (66%)] | .53 (.35–.8) | .7 (.44–1.12) | 30 (−12 to 56) | |
| Other race/Hispanic | 63 [20 (31.7%)] | 39 [19 (48.7%)] | .49 (.22–1.11) | .56 (.23–1.38) | 44 (− 38 to 77) | |
| ICU admission | ||||||
| Non-ICU admission | 180 [79 (43.9%)] | 172 [110 (64%)] | .44 (.29–.68) | .54 (.33–.87) | 46 (13–67) | .1612 |
| ICU admission | 129 [59 (45.7%)] | 157 [91 (58%)] | .61 (.38–.98) | .88 (.51–1.51) | 12 (−51 to 49) | |
| Acute organ failuree | ||||||
| No acute organ failure | 199 [87 (43.7%)] | 185 [118 (63.8%)] | .44 (.29–.66) | .59 (.37–.94) | 41 (6–63) | .3582 |
| Acute organ failure | 110 [51 (46.4%)] | 144 [83 (57.6%)] | .64 (.39–1.05) | .82 (.47–1.43) | 18 (−43 to 53) | |
| In-hospital death | ||||||
| No death | 288 [135 (46.9%)] | 308 [188 (61.0%)] | .56 (.41–.78) | .73 (.5–1.07) | 27 (−7 to 50) | .1136 |
| Death | 21 [3 (14.3%)] | 21 [13 (61.9%)] | .10 (.02–.46) | .20 (.04–.97) | 80 (4–96) | |
| Virus type and main subtype/lineage | ||||||
| Influenza A | 207 [99 (47.8%)] | 329 [201 (61.1%)] | .58 (.41–.83) | .72 (.48–1.09) | 28 (−9 to 52) | … |
| A(H1N1)pdm09 | 166 [83 (50%)] | 329 [201 (61.1%)] | .64 (.44–0.93) | .83 (.53–1.31) | 17 (−31 to 47) | … |
| Influenza B | 100 [38 (38%)] | 329 [201 (61.1%)] | .39 (.25–0.62) | .48 (.27–.87) | 52 (13–74) | … |
| Victoria | 77 [30 (39%)] | 329 [201 (61.1%)] | .41 (.24–.68) | .55 (.28–1.09) | 45 (−9 to 72) | … |
Abbreviations: CI, confidence interval; ICU, intensive care unit; OR, odds ratio.
aThe unadjusted OR was calculated with a logistic regression model with influenza case vs noninfluenza control status as the dependent variable and vaccination status (vaccinated vs unvaccinated) as the independent variable.
bThe adjusted OR was calculated with a multivariable logistic regression model with influenza case vs noninfluenza control status as the dependent variable, vaccination status (vaccinated vs unvaccinated) as the primary independent variable, and the following covariates: study site, age in years, sex, race/ethnicity, calendar time (categorized as tertiles generated based on site-specific influenza activity using disease-onset dates of influenza cases), insurance status, enrollment location (ICU vs outside the ICU), days from illness onset to specimen collection for influenza testing, chronic medical conditions (including [a] cardiovascular and pulmonary diseases, [b] kidney and gastrointestinal diseases, [c] neurological, psychiatric and gastrointestinal diseases, and [d] hematological, malignancies, autoimmune, and other immunosuppressive conditions), and frailty score (which ranged from 0 [not frail] to 5 [very frail]).
cVaccine effectiveness, reported as a percentage, was calculated as follows: (1 − adjusted OR for vaccination) × 100.
dThe “Other race” group was combined with the Hispanic group because of low counts in these groups.
eAcute organ failure was defined as receipt of any of the following during the hospitalization in which the patient was enrolled: noninvasive or invasive mechanical ventilation, vasopressors, acute renal replacement therapy, or extracorporeal membrane oxygenation.