As long as the paint is good (quality) there is no need for further protection. The best way to protect your walls and ceiling from moisture damage is to use ventilation during and after showering. This alone is enough.
If you want more protection use a semi-gloss paint instead of a satin. The semi-gloss is more resistant to water. A clear coat can also be applied. If you choose this, use a clear acrylic urethane with a semi-gloss finish. This will stick well to paint.
Personally, I would just ventilate then repaint when necessary.