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A novel swine-origin influenza A(H1N1)pdm09 virus has been circulating in humans since March–April, 2009. The 2009–2010 epidemic involved predominantly a single subtype of A(H1N1)pdm09 (at 96%, 46/48) in the sentinel sites of this study. However, A(H1N1)pdm09 started to circulate together with other type/subtype (49%, 33/68) at the first peak in the next epidemic season in 2010–2011: A(H1N1)pdm09/A(H3N2) (9%, 6/68), A(H1N1)pdm09/B (35%, 24/68), and A(H1N1)pdm09/A(H3N2)/B (4%, 3/68). Single infection of A(H1N1)pdm09 became a rare event (8%, 5/65) at the second peak of the same season in 2010–2011 compared with that at the first peak (50%, 34/68). Concurrently with this decline, single infections of others, A(H3N2) or B, became evident (6%, 4/65; 14%, 9/65, respectively). Triple infections were more common (29%, 19/65) at the second peak than at the first peak (4%). The A(H1N1)pdm09 detected in 2010–2011 produced less virus upon 72 hr of incubation in vitro after the inoculations at 104 and 3,300 copies/ml (2.3 × 109 and 2.3 × 109 copies/ml on average) than that in 2009–2010 (3.7 × 109 and 1.3 × 1010 copies/ml on average; P |
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