what does " Entry only" mean?

Context:

Entry only

VIRxSYS is convinced that the HIV vaccine it is planning would be safe because, as in their SIV vaccine, all the genes that would usually make it infectious and able to multiply itself would be removed. "It can't replicate," says Lemiale.

All that would be left of the dozen or so genes that HIV normally has would be the three " called gag, pol and rev " that enable it to infiltrate cells and embed itself into their DNA. This means the vaccine's version of the virus only undergoes one cycle of protein production, which enables it to get inside a few cells, but then can't spread further.

The hope is that the vaccine version of HIV would invade sentinel cells, known as dendritic cells - as the SIV-based vaccine did in the monkeys. These would then prime the immune system's T-cells to attack the real virus, should it turn up in the body.

More context, click:
http://www.newscientist.com/article/dn18551-fight-hiv-with-hiv-safe-virus-proposed-as-vaccine.html