Comment: The way it reads is (as posted)...

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The way it reads is (as posted)...

... is that a sample was taken from all who were know to have contracted mumps in that broad area. As long as that sample was random then it covers both those who got and those who did not get vaccines.

It can be that the sample itself is faulty.

However, even if this sample was totally "unrandom" and only included those who got vaccines, it would then still point out that vaccines are only slightly effective because of the high percentages that DID get mumps of those who were vaccinated.