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Distribution of the percentage of sows according to their serological HI titre. HI serological titres (log2 transformed) of sera taken from sows one week post farrowing as regards subtypes H1avN1, H1huN2 and H3N2 and for different parity groups: gilts (a), parity 1–2 (b), parity 3–4 (c) and parity 5 and more (d).
