I would use the 3/8th length 22 gauge needle. My vet said the longer needles run the risk of hitting important organs if you are novice at shots. Mareks is a subcutaneous shot, correct? So it wouldn't need to be very long at all, but the 22 gauge will ensure you can inject it quickly. I bought the 22 gauge for my Mycoplasma because the smaller one was too small for the vaccine's viscosity. Works great now.What size needles do you guys use for your Marek’s vaccine. I have made the decision to go ahead and vaccinate these guys and then just order another bottle for the ones that are coming. I’m thinking I will go with a 22G but should I do a 1/2 inch or 3/8 inch? I can only find needles in the 22G in 3/4 inch. There is a 20G in a 3/8 inch. I just don’t want the needle too big for these little guys.