Measures of association between 2 types of human papillomavirus (HPV), designated type 1 and type 2, in the absence of interactions. The prevalence odds ratio (POR) (black columns) compares odds of a prevalent HPV type 2 infection in HPV type 1-infected versus -uninfected individuals. The incidence rate ratio (IRR) by infection status (striped columns) compares the HPV type 2 incidence rate in HPV type 1-infected versus -uninfected individuals. The IRR by immunity status (white columns) compares the HPV type 2 incidence rate in HPV type 1-immune versus -nonimmune individuals. The POR and IRRs are calculated in the full population including both single and partnered individuals (“Overall Crude”), in the full population with adjustment for sexual activity level (“Overall Adjusted”), in persons of each sexual activity level (“Low Sexual Activity” and “High Sexual Activity”), only among persons in current sexual partnerships (“Partnered”), and only among persons in current sexual partnerships with HPV type 2-infected partners (“At-Risk Partnered”). All associations (except for the overall crude association) were adjusted for sexual activity level. The y-axis is on a base-2 log scale.