That is the vaccine for feline leukaemia virus (FeLV).
Primary course vaccination
In cats of eight weeks and over, administer two doses of Leucogen 3 to 4 weeks apart. To protect kittens, the vaccination of female cats prior to mating is recommended.
Where cats are exposed to infection, a single dose of Leucogen is recommended annually to boost immunity to feline leukaemia; otherwise earlier revaccination may be required.
I don’t vaccinate for this, not 100% that it works as well as they say it does. If the cat is not a wanderer and you don’t get other cats in the garden then the risk is not too high.