Table 2.

Separate and joint effects of the preventive exposures “Influenza vaccination” and “OFC” on the risk of ILI after recoding and results of analyses of interaction

Odds Ratios representing separate effects	OR (95% CI)
Non-OFC users	4.38 (2.54 – 7.53)
Users of OFC	1 (reference category)
Not Vaccinated	3.8 (2.15 – 6.71)
Vaccinated	1 (reference category)
Odds Ratios representing joint effects
OR11 non-OFC users and unvaccinated	5.75 (2.71 – 12.22)*
OR10 non-OFC users but vaccinated	1.48 (0.64 – 3.40)
OR01 users of OFC but unvaccinated	0.92 (0.33 – 2.53)
OR00 users of OFC and vaccinated	1 (reference category)
Measure of effect of interaction on additive scale
RERI	4.34 (1.46 - 7.26)

*Observed OR = 5.75, while expected OR in the absence of interaction on an additive scale was 1.40, calculated as [(1.48 – 1) + (0.92 – 1) + 1], where 1.48 = OR non-OFC users but vaccinated and 0.92 = OR users of OFC but unvaccinated.