At first, the ultrasound pushed the mixture into the upper layers of skin, where the shape of the proteins caused vaccine-filled bubbles to form. As ultrasound kept hitting the skin, those bubbles burst and released the vaccine. As the experiment went on, the action of the bubbles breaking also cleared some dead skin cells, making the skin more permeable and allowing more and more vaccine molecules make it through.

A needle pushes vaccine molecules all the way into the muscles beneath the skin, while the ultrasound technique just delivers the vaccine to the upper layers of skin. But this more shallow process is sufficient for immunisation, says Dunn-Lawless.

In tests with live mice, the researchers found that while the ultrasound method delivered 700 times fewer molecules of vaccine than conventional jabs, the animals produced more antibodies. The researchers say that the mice didn’t show signs of pain and there was no visible damage to their skin.

Neat. I'm wondering about the effectiveness with thicker skin in humans.