HIV is hard to vaccinate against partly because its surface recognition sites:

1. A virus enters a cell by binding sites on the cell surface that are specific to it. 2. A vaccine works by teaching antibodies to recognise the virus surface. 3. The recognition sites on HIV change at a rapid rate because of mutations. 4. The antibodies will not recognise the new variations of the surface patterns. 5. So the difficulty is a moving target, not concealment or disappearance. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 5 'Structure and Function of Plasma Membranes', sections 5.1-5.4_