Sucrose is used as a preservative in vaccines, more specifically as a cryoprotectant:
Earlier empirical approaches stabilize liquid formulations of Ad5 viral vaccines were described by Evan et al.10. They identified freeze-thaw damage, surface adsorption and free radical oxidation as reasons of virus destruction. They tested a combination of buffering molecules, ions and sucrose and found a solution containing 5 w/v% sucrose as the most effective cryoprotectant. [...] Stewart et al. showed that a preservation of infectivity of adenovirus for a sucrose-stabilized liquid formulation for up to 6 weeks at 40 °C, hence confirming the stabilizing effect of sucrose.
The breakdown of sucrose in other parts of the body is much slower than in the small intestine, which has a specific molecule (sucrase) to speed-up its breakdown.
Through a process called hydrolysis, water assists in severing the glycosidic bond to separate the glucose and fructose molecules; one molecule of water is needed for each molecule of sucrose. However, this reaction naturally occurs very slowly. The presence of sucrase, an enzyme in the small intestine, accelerates this reaction.
I haven't found what level is safe in humans, but presumably the regulators took that into account. A quick search found that even if given orally, sucrose is lethal in rats above a certain amount. The adage that dose makes the poison applies here.
Also, I found a 1967 study in which 10 g of various dissacharides (including sucrose) were administered intravenously in humans, so presumably that much of it is safe even in injections.
Studies in man [...] Four adult subjects were given iv infusions of 10 g
in 100 ml of water of either lactose, sucrose, or maltose
over a 30-minute period.
I've only glossed over this study, but its main finding regarding sucrose is that its breakdown in the bloodstream is too slow to cause a detectable spike in measured bloodstream glucose. Also
A mean of 8.7 +/- 1.8 g of the lactose lactose and and 6.3 +/- 1.3 g of the sucrose was excreted in the 24-hour urine sample. In contrast, only 0.11 +/- 0.03g, or 1%, of the infused maltose was excreted into the urine.
What might have happened to the sucrose not excreted during that observation period they didn't say/study.