Before Edward Jenner tested the possibility of using the cowpox vaccine as an immunisation for smallpox in humans in 1796, at least six people had done the same several years earlier. However, the findings of these six people regarding the cowpox vaccination are either hardly known or have even been forgotten. For the first time, the originally scattered information on the procedures used by these six people has been gathered and will be presented in this article. Detailed attention will be given to the works of the teacher Peter Plett (1766-1823), the only one to recognize the importance of his discovery for mankind. In 1790 and 1791/92, Plett reported his findings to the Medical Faculty of the University of Kiel. The faculty disregarded Plett's reports by neither responding to them nor changing their methods of immunisation, as the faculty at that time was still in favour of variolation. This article contains the available information and references concerning Plett's discovery, including the entire sources from 1802 and 1815 describing his findings.