Dengue fever vaccine is probably the best example. https://en.wikipedia.org/wiki/Dengvaxia_controversy https://en.wikipedia.org/wiki/Dengvaxia_controversy From

Dengue fever vaccine is probably the best example. https://en.wikipedia.org/wiki/Dengvaxia_controversy https://en.wikipedia.org/wiki/Dengvaxia_controversy From https://news.ycombinator.com/item?id=22511832 https://news.ycombinator.com/item?id=22511832 : > "The story of dengue vaccine development is a cautionary tale. The first time you get dengue is usually not too bad, but it can be deadly if you get it again. Part of that is hypothesized to be driven by your own immune system, which, once immunized against one strain, produces antibodies against it in the next infection. However, if it's a slightly different strain of dengue the second time around, the antibodies meant to neutralize the disease are believed to make the disease more virulent instead (antibody-dependent enhancement). The immune system makes it worse. So when a dengue vaccine that didn't fully immunize against all strains was developed (not deliberately, people tried to immunize against all strains but this is a case where less efficacy than expected made things worse), it didn't make things better. It ended up priming people to get dengue."