Background: Persistent infection with certain types of human papillomavirus (HPV) is believed to be a prerequisite for the development of cervical neoplasia. Persistence may depend on certain characteristics, such as viral load, which has so far been given little attention. We investigated the association between HPV 16 viral load and cervical carcinoma in situ.
Methods: We did a nested case-control study of women participating in cytological screening in Sweden. We used a sensitive quantitative PCR assay to estimate HPV 16 load in multiple smears for each woman, taken during a period of up to 26 years before diagnosis. We calculated C, values, which decrease as the number of viral DNA copies increases.
Findings: 2081 smears from 478 cases and 1754 smears from 608 controls were tested. Among cases, we found a consistently increased load of HPV 16 already 13 years or more before diagnosis, and when many smears were still cytologically normal. Women with high HPV 16 viral loads were at least 30 times the relative risk of HPV-16-negative women more than a decade before diagnosis. The increase in relative risk was constant over time. About 25% of women (95% CI 0.12-0.32) infected with a high viral load before age 25 years developed cervical carcinoma in situ within 15 years.
Interpretation: Cervical carcinoma in situ associated with HPV 16 occurs mainly in HPV-16-positive women who have consistently high viral loads long term. Women at high risk could be identified by use of a quantitative HPV test in addition to cytological screening.