Let t = (t 1 ,…,t n ) be the vector of observed times of symptom onset of observed cases {1, …, n}. We assume that the elements of t are ordered such that t i ≤t j for all i <j. For subsets {i k , … i k+j } ⊆ {1,…,n} with all permutations of observations within this subset are equivalent. We chose 1 possible ordering arbitrarily. We now define a transmission matrix V = (v i,j ), whose elements represent the probability that the person with time of symptom onset t i was infected by the person with time of symptom onset t j , thus