Multivariable logistic regression on H3-SIV
Of the 6,532 paired nasal swabs and pig sera collected over the study period, 65% of the swine originated from Guangdong province, followed by Jiangxi (17%), with relatively fewer isolates from other provinces (Table 1). We isolated 63 H3N2 viruses from swabs and H3N2 antibodies were detected in 1,340 pigs (Table 1), with 7 pigs having both virus isolates and antibodies. Samples collected from farms (n = 5,832) which provided ≥20 samples were included in our logistic regression. Multivariable logistic regression was fitted to explain H3-subtype virus infections (Table 2).
In the multivariable logistic regression model (Table 2), pigs with anti-H3 HAI titers ≥1:40 were significantly associated (adjusted OR [aOR] = 0.33, p = 0.012) with a lower risk of H3 virus detection in the abattoir (i.e. presumed infection during transportation). This reflected the protection against infection provided by the humoral immune response. Swine from source farms with high H3 seroprevalence (>30%) showed significantly higher H3 virus isolation rates (aOR = 2.24, p-value = 0.015). No significant difference was observed in the virus isolation among pigs with different length of stay at the slaughterhouse and transport distance.