Table 2.

Sources of bias

Source of bias	Description
A	Vaccination affects the probability of NFARI, i.e. ρ β≠1. This may result from virus interference [6].
B1	Health status affects the probability of NFARI, i.e. η β≠1.
B2	Health status affects the probability of FARI, i.e. η γ≠1.
BS	Health status affects the probability of FARI and NFARI, and the risk ratios comparing a healthy and a frail person are the same for both types of ARI, i.e. η β=η γ≠1. This is a special case of $B1\bigcap B2$.
C	Vaccination affects the probability of seeking medical care for FARI, while it does not affect the probability of seeking medical care for NFARI, i.e. Ψ F≠1. This may result from reduced severity of symptoms in vaccinated influenza patients.
D	Health status affects the probabilities of seeking medical care against FARI and NFARI, i.e. λ≠1.