Secondary attack rate and Basic reproductive rate; what is the difference?

Secondary attack rate (SAR) is the proportion of susceptibles infected by getting exposed by one diseased individual in one incubation period. 

Basic reproductive rate; whereas is the potential for a contagious disease to spread from person to person in a population (in which all are susceptible). The basic reproduction rate (BRR) takes into account the population dynamics as we can know by this formula:
BRR= a x b x d
where; a is the risk of transmission per contact; b is the number of such contacts that an average person in the population would normally have per time unit and d is the duration of infectivity of an infected person, measured in the same time unit as b was. 

We find that the formula is very trivial (which i hv tried to keep for explaining the basic concept) and tells us that if the risk of transmission per contact is high, the number of contacts is also high among the people
in a population and the duration of infectivity of the diseased person is high then the BRR or the potential for a contagious disease to spread in a population will by high.

These all things are not taken into account by the Secondary attack rate which is very much representative of a fixed (static) model estimate whereas BRR is representative of a dynamic model estimate.