Reaction videos could be a fair use
Two YouTube channel owners -- MxR and Potastic Panda -- make videos where they react to online content (reaction videos). They recently received a copyright infringement notice from Jukin Media alleging that they had used unlawfully four copyrighted works and demanding a $6,000 payment (see YouTube link).
Fair use however is a fact specific determination that examines four factors (see 17 USC § 107):
1. The purpose and character of your use (transformative use) 2. The nature of the copyrighted work 3. The amount and substantiality of the portion taken 4. The effect of use upon the potential market for the copyrighted work
Moreover, fair use is affirmative defense which means that the burden is on the accused infringer to assert it and demonstrate that the use was a fair use. This means the accused has to have the resources to fight the copyright infringement claim.
If you do happen to have the resources, then you could prevail on a fair use defense. In 2017, two YouTubers -- Ethan and Hila Klein -- prevailed on a fair use defense when they were accused of using another YouTuber's video in their reaction video (Hosseinzadeh v. Ethan Klein and Hila Klein, case 1:16-cv-03081 (S.D.N.Y. August 23, 2017) (see Google Scholar link). As I mentioned, fair use is a fact specific determination, and thus the Klein case does not mean that all reaction videos are fair use. Rather, it means that reaction videos -- like the Kleins -- that contain criticism and commentary on copyrighted works could be found to be a fair use.
In this case, the accused reaction videos can be found here (video 1, video 2, video 3). In the first video, the Jukin work consists of a clip of a cat sticking its tongue out and moving its paws (see link). In the second video, the Jukin work consists of a volcano eruption (see link). In the third video, the Jukin works appear twice and consist of a girl backflops off waterslide (see link at 1013 and 1026) and guy lighting leaves on fire (see link at 1018 and 1031). The question is whether these uses are transformative.