Can I still receive the 9-valent vaccine after having received the 4-valent vaccine?

Generally speaking, "quadrivalent" refers to the quadrivalent HPV vaccine, and "nonavalent" refers to the nonavalent HPV vaccine. It is possible to receive the nonavalent HPV vaccine after having received the quadrivalent HPV vaccine, although certain conditions must be met and it is not mandatory. Detailed explanation is as follows:

The quadrivalent HPV vaccine covers HPV types 6, 11, 16, and 18, while the nonavalent HPV vaccine adds types 31, 33, 45, 52, and 58, thereby offering protection against more high-risk subtypes. Currently, such a booster vaccination is not prohibited, but it is recommended to wait at least 12 months after completing the full course of quadrivalent HPV vaccine before receiving the nonavalent HPV vaccine, to avoid immune interference and ensure effective antibody production.

It should be noted that the nonavalent HPV vaccine has an age restriction—9 to 45 years old in China—and individuals outside this age range are not eligible for vaccination. Prior to vaccination, one should also assess whether there is a history of allergy to any vaccine components. From a cost-effectiveness perspective, if an individual has already completed the quadrivalent HPV vaccine series and has no special needs, such as high-risk exposure, it is not necessary to compulsorily receive the nonavalent HPV vaccine. Regular screening remains an important complement to cervical cancer prevention.

Additionally, it should be noted that neither the quadrivalent nor the nonavalent HPV vaccine can guarantee 100% protection. Therefore, even after receiving the HPV vaccine, regular cervical cancer screening should still be conducted to promptly detect and manage any potential cervical lesions.