Force of Infection (FOI) in farms and during transport
There were in total 4,226 paired pig sera and nasal swabs collected from swine originating from Guangdong, with 46 (1.09%) H3 viruses isolated and 936 (22.1%) having antibody titers ≥1:40 (sero-positive) to H3 SIV (Table 1). Among the 3,290 sero-negative samples, there were 40 (1.22%) H3 viruses isolated. The mean weekly FOI λt during transport for swine originating from Guangdong was estimated to be 0.0286 (95% CI = 0.0211-0.0391), while the weekly force of infection in farms were λf = 0.0125 (95% CI = 0.0118-0.0134), 0.0089 (95% CI = 0.0084-0.0095), and 0.0069 (95% CI = 0.0065-0.0074) respectively, assuming exposure durations of 20, 28 and 36 weeks in farms. Hence, the relative FOI during transport was 229-414% of that in farms, indicating a noticeable increased transmission intensity during transportation comparing to that within farms.