That's a rare event, however, and requires that two flu strains invade a single cell, replicate and then get their products mixed up in the packaging.

然而这是罕见的，而且需要两种流感病毒进入同一个细胞中，复制然后在包装过程中其产物混合。

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In the formula of the method, all components are dispersed to be mixed and are subject to melting by a double screw extruder, mixing, extrusion, cooling, drying, granulating and packaging.

该方法是将配方中各组分分散混合后，通过双螺杆机熔融、混炼、挤出、冷却、干燥、切粒和包装。

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