IJID One Health · Published 2026-07-01 · DOI 10.1016/j.ijidoh.2026.100148
Jwee Chiek Er, Osnes Magnus Nygård, Lars Erik Gangsei
Objectives: Influenza A(H1N1)pdm09 became established in Norwegian pig herds following human-to-pig transmission during the 2009 pandemic. Unlike most countries, Norway's swine population remains endemic with only this single influenza lineage, providing a unique epidemiological setting to investigate long-term temporal associations between human and sow-herd influenza dynamics. Methods: We analyzed 15 years (2009–2024) of parallel national human and sowherd surveillance data. Annual population-level exposure was approximated using an intensity-weighted human infection pressure metric integrating subtype predominance with seasonal peak laboratory positivity. Annual sow-herd prevalence, incidence, and seroreversion were derived from routine serological surveillance of stable breeding cohorts. Temporal associations were evaluated using lagged Pearson/Spearman cross-correlations (−2 to +2 years) and cluster-robust logistic regression models. Results: Temporal analyses suggested that increases in human infection pressure preceded subsequent changes in sow-herd epidemiological dynamics. The strongest association with herd incidence occurred at lag 0 (Pearson r = 0.66, P = 0.005; Spearman ρ = 0.51, P = 0.043). Sow-herd prevalence peaked at a lag of + 2 years (Pearson r = 0.47, P = 0.087), while Spearman correlations peaked at lag + 1 (ρ = 0.42, P = 0.122). Seroreversion showed the strongest inverse associations at lag + 2 (r = −0.46, P = 0.101) and lag + 1 (ρ = −0.48, P = 0.072). Congruently, cluster-robust logistic regression demonstrated a consistent pattern of associations. Conclusion: Over 15 years, variations in human influenza pressure consistently preceded sow-herd epidemiological shifts. These lagged associations are compatible with repeated human-to-pig spillover contributing to viral persistence. These findings highlight the utility of integrated One Health surveillance and suggest that mitigating human-to-pig exposure could complement herd-level biosecurity.
Abstract from DOAJ. Public domain (CC0 1.0).
Read the article at the publisher →
Er, J., Nygård, O., Gangsei, L. (2026). Human A(H1N1)pdm09 infection pressure predicts lagged influenza dynamics in Norwegian sow herds: a 15-year One Health surveillance study (2009–2024). IJID One Health. https://doi.org/10.1016/j.ijidoh.2026.100148