There are some questions + answers out here that explain how to interpret coefficients from count data regressions (e.g. negative binomial), both as incidence rate ratios or marginal effects. Bottom line: Unlike in linear models, the coefficients are modelled at the mean of the other variables, such that the obtained effect is on average.
But there is one thing I can't get my head around: When I use dummy variables, how exactly is on average defined? Suppose I have
$$y = b_0 + b_1 X + b_2 D$$
with $D$ being a dummy and $b_1$ is the parameter I am interested in. Suppose also $D=1$ in exactly half of all observations. What is the mean observation now, at which $b_1$ is evaluated? Mean of $X$ and $0.5 D$?