The protective measures have not only helped to
keep this virus in check but also have driven down
cases of regular flu and “has”reduced case of stomach flu by 90%
请问为什么” “内不是have? 而是has

看了也充满疑惑，求解，想知道是不是笔误！

看起来是笔误

这段除了原PO说的地方之外还有其他的错误，看得懂就好，遇到这种句子不用太纠结，很多外媒的文章与撰文者写作功力都很参差不齐的～