Absolutely, it is possible to be attuned to Reiki online. I have carried out many distance attunements and all my students have been able to feel the energy during the course as if they were in the room with me. They have all experienced their attunements in a similar way to being physically present in a classroom with me, and all have gone on to heal others without difficulty. I have received several attunements at distance (not for Reiki, admittedly, but for other energy healing modalities) and there was no mistaking their effectiveness. If you can heal from a distance (which you can) you can attune from a distance. This is a quantum world we live in. The only drawback for me is that the hands-on practice is harder to teach and requires the student to be self-motivated do some work at home and have a volunteer to practice on. It's not the perfect way to learn, but the advantage is you can find a great teacher and work with them, no matter where you are in the world.

I have often healed at distance with reiki, this involves vibrational resonance and mental cooperation. To practice reiki, we first need to be initiated and this usually involves the physical presence of a reiki master, and when in the physical presence of someone there is a third activating energy, a kind of localized fluid substance, that certain sensitive people perceive as the colored aura and that can accumulate in certain substances such as water, coton, wax, paper, which however disintegrates under the influence of light and doesn't seem to travel very far in the ambient air. It's a fluid that magnetisers use.

So to conclude, personally i've never initiated anyone at a distance because i believe, for the reason explained above, that physical presence is necessary. However i have heard that there are some reiki masters that initiate a whole crowd of people all in one go, which suggests it may be possible to initiate at a short distance, at least within view of the reiki master because of the accumulative effect of a group which magnifies the intermingled auras of the individuals present.

I have taught students online, it involves distant atunements.

When I used to teach online I had to use yahoo messenger.