Overview of included studies
| Author | Year | Country | Study inclusion period | Study type | Total N | Vaccination | Placebo/ No vaccination | Age range at time of surgical treatment (years) | Jadad Score | |
| Vaccinated | Unvaccinated | |||||||||
| Hildesheim (22) | 2016 | USA | 2004–2005 | Post-hoc analysis nested in larger RCT | 311 | 142 | 169 | 18–25 | 18–25 | 3 |
| Minors Score | ||||||||||
| Ghelardi (19) | 2018 | Italy | 2013–2017 | Prospective case-control trial | 344 | 172 | 172 | 18–45 | 18–45 | 21 |
| Garland (20) | 2016 | Australia | 2004–2006 | Post-hoc analysis nested in larger RCT | 454 | 190 | 264 | 15–25 | 15–25 | 19 |
| Kang (21) | 2013 | Korea | 2007–2010 | Retrospective case-control study | 737 | 360 | 377 | 21–45 | 20–45 | 24 |
| Joura (13) | 2012 | Austria | 2001–2003 | Post-hoc analysis nested in larger RCT | 1066 | 474 | 592 | 15–26 | 15–25 | 24 |