在极少情况下，疫苗中的病毒可能变异成一种可导致瘫痪的类型，这就是人们所称的疫苗衍生脊髓灰质炎病毒(VDPV)。

In very rare instances, the virus in the vaccine can mutate into a form that can paralyse - this is what is known as a vaccine-derived poliovirus or VDPV.

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然后遍历元素声明的代替链或类型层次结构，以决定元素声明是否满足代替或类型衍生要求。

They then traverse a substitution chain of element declarations or a type hierarchy to determine whether the element declaration satisfies the substitution or type derivation requirements.

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因为我们可以通过能给类发送的消息知道这个类，所以这意味着衍生类和基类属于同种类型。

Since we know the type of a class by the messages we can send to it, this means that the derived class is the same type as the base class.

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