The Vaccine is the cause of some of the ESP like abilities Desmond is demonstrating.
The function of the vaccine is hindered by the electromagnetic abnormalities on the Island. Now that the discharge has happened, the actual power and/or use of the vaccine is becoming apparent.
Affects memory - this is what caused Claire's amnesia. This means that Desmond may have been on the Island much longer than he thinks - when Kelvin said Desmond was unconscious, he was actually quite awake, but he does not remember this.
Is the "cure" as mentioned in the Sri Lanka Video. Thomas Mittelwerk spoke about a virus which kills 30% of the population by hitting precise genetic targets that they have engineered into the virus. This will change one of the six numbers in the Valenzetti Equation(over-population) and therefore save humanity.
The CR label bears the numbers, tying it circumstantially to the Equation.
It is a harmless substance that can be injected, which explains Desmond's comment that the injections are useless, as he managed to find out that it was not a vaccine at all, but merely another part of a behavioral experiment.
The DHARMA Initiative "vaccine" allows one to continue to age normally while on the Island - this is why all the DHARMA folks continued to age normally (like Ben and his father), since they are regularly injecting themselves. If you stopped taking the vaccine, you stop aging, like the Others (in particular, Richard Alpert). This is also why Claire was injected repeatedly with "vaccine" by the others. To allow baby Aaron to grow and develop in utero until birth.
This would give a reason for the vaccine to be so widely referenced on the show, but would also explain why there are no perceptible consequences for those that are not using it.
It also explains Desmond's comment that the injections are useless.
It also explains why the others don't take it.
So why did Walt age then?
Because he aged off the Island and not on it.
Ji Yeon grew and developed normally on the island which is how Juliet managed to determine that the baby was Jin's. Sun never received any injections and it didn't affect the baby's development in the least.(D.O.C.)
A strain of this has led to the onset of the sickness i.e. they were testing a number of strains to see what they would do to people and one such strain led to the sickness. Afterward they altered it and this strain will lead to an onset of the sickness.
The substance is used to alter a person's genome in order to prevent genetic disorders/deformities. The Hanso Foundation also describes a Human Genome project with the previous stated goals.
It is a drug to make people more agreeable and obedient. (Claire's behavior in the Staff, Swan employees not rebelling against the Protocol)
It is a relabeled vaccine for a common disease (e.g. influenza, measles, etc.) that may or may not appear on the island (which may be one explanation for why Desmond told Claire that the injections are useless).
The Sickness does not exist; the "vaccine" is actually a substance which can cause/increase psychic abilities. It works best on children, which is why the Others want the babies.
CR (CerberusRepellant) - it marks Cerberus targets as friends or Dharma employees.
It is activated by the Discharge and causes the person to behave in an odd way, such as more subservient or honest so the scientists can find out what happened, or their memory of the events are erased so they will not blab about the project if they survive the Discharge. This could explain the way Charlie was acting when he arrived back to camp at the end of "Live Together, Die Alone, Part 1".
The Vaccine was created by Dharma to protect their people from the experiments they were doing on the island.
The Vaccine offsets the effects of the time-travel sickness as experienced by Minkowski and Desmond.
Faraday indicates that electromagentism make one more prone to the time sickness
Vaccine was used to allow Swan participants to function without getting sick due to the exposure to extreme electromagnetism and time-shift
The Vaccine is an antidote to the gas released from the Tempest
The Vaccine Keeps people grounded in time, Which is why the others haven't been affected by the turning of the wheel.
The Vaccine is actually a medicine against the radiation sickness caused by Jughead
The "CR" component may be a homage to Stanley Kubrick, who tried to incorporate "CRM114" into most of his creative works. This included a "Serum 114" in A Clockwork Orange, which was a liquid to be administered intravenously.