system
Given the following post, respond as if you're commenting on Reddit:
human
Why is there a vaccine for FIV (Feline immunodeficiency virus), but not HIV?

These answers seem to be very good, but I'm finding a hard time understanding them. Any chance of a TL;DR?
gpt
You are assuming the FIV vaccine is efficacious. 

The following is from Alice Wolf, DVM, DACVIM, DABVP: 

"[The FIV vaccine] has never been evaluated in a situation of natural exposure. Once you vaccinate, our antibody tests will be falsely positive. PCR tests are not sufficiently accurate to make this determination.

If it so easy to protect against the millions of strains of lentivirus such as FIV and HIV, why don't we have a safe and efficacious vaccine against HIV yet? Answer: It's not so simple."

Study: 
 Limited efficacy of an inactivated feline immunodeficiency virus vaccine
Vet Rec. April 2006;158(16):561-2.
S P Dunham1; J Bruce; S Mackay; M Golder; O Jarrett; J C Neil
1Institute of Comparative Medicine, Faculty of Veterinary Medicine, University of Glasgow, Bearsden Road, Glasgow G61 1QH.
Companion Notes

Report on the efficacy (limited) of an inactivated feline immunodeficiency virus vaccine against experimental challenge:

Study design

- introduction on vaccination against feline immunodeficiency virus (FIV):

- inactivated vaccine available (Fel‑O‑VaX FIV; Fort Dodge Animal Health)

- vaccine is licensed in the USA, Canada and Australia and has 2 isolates

- FIV Petaluma (subtype A)

- FIV Shizuoka (subtype D)

- reported to provide protection versus several isolates (subtypes A, B and C)

- “challenge viruses used…were poorly characterized”

- “or may not have been representative of isolates… in the field”

- authors use UK subtype A isolate, FIV Glasgow‑8 for assessing vaccine efficacy

- causes viral loads in experimentally infected cats similar to those in field cats

- experimental vaccines tested did not prevent infection after challenge

- vaccines did reduce viral loads post-challenge

- study population: 11, 13‑week‑old, specific pathogen‑free domestic cats

- procedure:

- cats were randomized into 2 groups

- group 1 of 6 cats

- immunized sc 3 times every 3 weeks (manufacturer’s recommendations)

- group 2 of 5 cats

- immunized sc 3 times every 3 weeks with buffered saline

- blood samples taken before vaccination and at 2nd and 3rd vaccination

- also on day of challenge and at 2, 4, 8, 12, and 16 weeks post-challenge

- all cats were challenged 4 weeks after the 3rd vaccination

- infectious doses of FIV Glasgow‑8 IM (imitating a cat bite)

Results:

- all 11 cats became infected post-challenge

- confirmed by viral isolation and plasma viral RNA (real‑time PCR)

- vaccinates had markedly higher viral loads 2 weeks post-challenge
(as compared to unvaccinated kittens)

- authors caution vaccine may enhance infection

- vaccinates and controls had similar viral loads by 4 weeks post-challenge

- vaccinates and controls had similar proviral loads in spleen and lymph nodes

“The lack of protection afforded by the vaccine against a viral challenge relevant to that likely to be encountered in the UK underlies the need for caution when using FIV vaccines in domestic cats.”