Why are there several kinds of HPV vaccines? What is the difference between bivalent, tetravalent and nonavalent vaccines?

The bivalent vaccine is only suitable for female vaccination, and can prevent two kinds of human papillomavirus, 16 and 18. The inoculation time was 0, 1 and 6.

Tetravalent is suitable for men and women over nine years old, and can effectively prevent four kinds of human papillomavirus, namely, 6,1,16 and 18, and can prevent more than 70% of cervical cancer and more than 70% of condyloma acuminatum.

Nine-valent vaccine is also suitable for men and women over nine years old. There are 9 kinds of human papillomavirus that can be prevented, namely 6, 1 1, 16, 18, 3 1, 3, 45, 52, 58, which can prevent 90% of viruses.