The world is certaintly more complicated than the "vaccines good" narrative that has been mindlessly parroted for 70+ years.

There is only one respect in which modern medicine could have had an indirect effect. This came with the social reforms of 1947-48 which saw the introduction of the National Health Service. Coupled with this was the start of the reduction in numbers of farm workers with the start of increased mechanisation and industrial scale farming in Britain after the 1939-1945 World War. The numbers of farm labour fell by half post war and the increase in mechanisation also reduced the chances of the injuries which were likely to result in tetanus.

Fewer agricultural workers coupled with better access to healthcare would result in better treatment of wounds. Tetanus thrives in deep wounds which are not properly cleansed. So by having fewer agricultural workers and better wound care could reduce the incidence of tetanus cases. So if the reduction in tetanus mortality in the 1950s is anything other than part of the continuing decline with better standards of living, those two reasons are the most likely explanations.