Table 2. Vaccine effectiveness estimates for seasonal influenza vaccination on laboratory-confirmed influenza in persons 65 years and older, Stockholm and Finland, 1 October 2016–15 January 2017 (n =358,583 and 1,144,894, respectively).
| Cases | Person-years | Populationa | Crude hazard rate ratio (95% CI) |
Adjusted hazard rate ratio (95% CI) |
Vaccine effectivene % (95% CI) |
|
|---|---|---|---|---|---|---|
| Stockholm County, Swedenb | ||||||
| Unvaccinated | 654 | 83,263 | 201,113 | Ref | Ref | Ref |
| Vaccinated for 1 day or morec |
380 | 20,736 | 157,470 | 0.90 (0.79–1.03) |
0.72 (0.63–0.89) |
28 (16–37) |
| Vaccinated for 15 days or more |
322 | 14,345 | 153,762 | 0.94 (0.82–1.08) |
0.76 (0.65–0.89) |
24 (11–35) |
| Finlandd | ||||||
| Unvaccinated | 3,674e | 247,456 | 613,202 | Ref | Ref | Ref |
| Vaccinated for 1 day or morec |
2,171 | 85,674 | 531,692 | 0.73 (070–0.77) |
0.68 (0.64–0.73) |
32 (27–37) |
| Vaccinated for 15 days or more |
2,006 | 65,357 | 527,664 | 0.73 (0.70–0.78) |
0.67 (0.63–0.72) |
33 (28–38) |
CI: confidence interval.
a By vaccination status at the end of each individual’s follow-up.
b Models were adjusted for age, sex, comorbidity status, socioeconomic status, previous seasonal vaccination and pneumococcal vaccination. As complete case analysis was used, the number of cases decreased due to missing data on socioeconomic status.
c The sensitivity analysis showed that there was no protection during the first week after vaccination, but that a significant vaccine effectiveness could be observed already during days 8 to 14 (see text).
d Models were adjusted for age, sex and previous seasonal vaccination.
e The number of vaccinated/unvaccinated differs from Table 1 because 384 people were vaccinated after having a laboratory-confirmed influenza. For Table 2, they are counted as unvaccinated and then their follow-up was stopped because they turned out to be a case.