The WAIFW matrix (Who Acquires Infection From Whom) is a central parameter in modelling the spread of infectious diseases. The calculation of the basic reproductive number (R0) depends on the assumptions made about the transmission within and between age groups through the structure of the WAIFW matrix and different structures might lead to different estimates for R0 and hence different estimates for the minimal immunization coverage needed for the elimination of the infection in the population. In this paper, we estimate R0 for varicella in Belgium. The force of infection is estimated from seroprevalence data using fractional polynomials and we show how the estimate of R0 is heavily influenced by the structure of the WAIFW matrix.