Alcohol as a Disinfectant
Alcohol kills germs, but most cocktail ingredients do not have enough alcoholic strength to ignore the possibilities of food-borne illness in drinks. For example, adding a splash of vodka to a syrup might help it last longer before spoiling, but it’s not enough to make it bacteria-free and shelf-stable. Yes alcohol kills germs, but only at high enough concentrations.
According to the CDC, alcohol at an ABV of over 50% significantly kills bacteria, but 60-90 percent alcohol is optimum [link]. Please do not let the addition of alcohol substitute for safe food-handling.