Do not vaccinate in manure stained areas. The location of the injection must be clean and dry to prevent bacteria from entering the injection site. Fresh shorn sheep are good to vaccinate, as well as dry newborn lambs. Long wool makes it very difficult to see if the vaccine and needle are properly placed or if any vaccine leaks back from the injection site. Improper syringe and needle handling can alter the effect of a vaccine or induce a local reaction at the injection site.
Disposable syringes are just that - disposable.
Normal cleaning, disinfecting or even boiling will affect the integrity of any syringe. Small microscopic cracks in the rubber plungers can contain bacteria. All rubber will crack with age and needs to be maintained and replaced as needed. The use of alcohol or disinfectants will often alter and greatly reduce the effectiveness of vaccines. Needles can be purchased for less than 30 cents each and need to be replaced frequently.