Authorization for inoculation or other immunological treatment for occupational disease shall be given only in cases in which a work related activity has resulted in probable exposure of the worker to a potential infectious occupational disease. In no case shall such inoculation or immunological treatment be authorized until such time as a work related activity has resulted in such probable exposure. Inoculation or other treatment required as a condition for employment or otherwise obtained prior to the worker's performing a work related activity resulting in probable exposure to an occupational disease shall not be authorized. For purposes of this section, probable exposure is an incident which gives rise to a clear and immediate likelihood of contracting an occupational disease process.