ヒト−ヒト感染、ヒトヒト感染

human to human transmission、human-to-human transmission
不可算名詞

Human to human transmission is clearly the main driver, so there is no need to panic about cats as an important source of virus. Obviously, if you think you have Covid-19 and share a house with a cat, then it would be sensible to limit close interactions with your furry friend until you are better.
https://www.theguardian.com/world/2020/apr/01/cats...

日本語にハイフネーション記号に関する正書法がないことを反映して、この語にも「ヒト-ヒト感染」のような異なる形態が存在する。