I suspect, my dear Watson, that our friend Pirate has some experience with "the job" and knows that police officers have all sorts of jabs as protection against the various ailments suffered by their clients that may, in the heat of battle, be passed onto the officers. Such jabs may well, as with the armed services, be stuck in an arm (or in the case of at least one esoteric tropical disease in the stomach).
I have telegraphed our old friend LeStrange at Scotland Yard and unless I am mistaken he will be able to provide us with an answer within the hour.