A Google search reveals many claims that sharing earphones is unhealthy.

Marie Claire magazine provides an example:

Kelly Reynolds, Ph.D., associate professor of environmental health at the University of Arizona also told Buzzfeed US that 'studies have shown that earbuds cause an 11-fold increase in bacteria in the ears' and 'when you share headphones, you’re doubling the microbial flora in your ears and introducing new bacteria'. [...]

Bad earphone hygiene can also cause a whole host of nasty ailments too: Middle ear infections, fungus, swimmer's ear, blackheads, pimples to name a few. And Reynolds noted that because our wax also contains strep bacteria, if we have a cut or open wound near to the wax, it could also cause a serious skin infection.

I'm failing to find any scientific studies regarding this matter.

Picking one claim, does sharing earphones increase the risk of middle ear infections?