Effect of seasonal transmission on excess morbidity in primigravidae and multigravidae at different background transmission rates. The left panel illustrates a scenario of low background transmission between seasonal (or epidemic) outbreaks (λ0 = 0.01), whereas the right panel depicts a situation with moderate level of year-round transmission (λ0 = 0.2). The top row (a, b) shows the excess morbidity from primigravidae; the middle row (c, d) shows the excess morbidity of multigravid women, clearly indicating that higher background transmission significantly reduces excess morbidity from multigravid women during epidemic outbreaks. The bottom row (e, f) compares the maximum in excess morbidity from primi- and multigravidae (E
, respectively) for the two background transmission rates with a stable transmission setting under equivalent maximum transmission rates, clearly demonstrating the effect of unstable transmission on multigravid women. Parameter values: μ = 1/40, m = 1/14, c = 1, B = 1/40, α = 0.1, H = 10, k = 10.