title: Hypothesis: the act of taking a Papanicolaou smear reduces the prevalence of human papillomavirus infection: a potential impact on the risk of cervical cancer.
abstract: BACKGROUND
  Based on data collected in a case-control study of hormonal contraceptive use and invasive cervical cancer among South African colored and black women, we have reported a five-fold reduction in incidence among women who in their lifetimes had previously received as few as two Papanicolaou (Pap) smears. Given the conditions prevailing in the community under study, a reduction in risk of this magnitude was surprising. We hypothesized that the act of performing a Pap smear may trigger an immune response to human papillomavirus (HPV) infection and thus reduce the risk of cervical cancer.
  
  
  METHODS
  Among 1540 control women we compared prevalence rates of infection with specific HPVs known to cause cervical cancer among those who previously received 0, 1, 2 or > or = 3 Pap smears in their lifetimes.
  
  
  RESULTS
  Overall, 16% of the controls were HPV-positive. Among women who received 0, 1, 2 and > or = 3 Pap smears the HPV positivity rates were 24, 17, 12, and 11%, respectively. Compared with never having received a smear, the confounder-adjusted relative risk estimates for the receipt of 1, 2, and > or = 3 smears were 0.7 (95% confidence interval, 0.5-1.0), 0.5 (0.3-0.8). and 0.5 (0.3-0.7), respectively (test for trend among Pap smear recipients: p = 0.04). The data were consistent within strata of age, parity, and race.
  
  
  CONCLUSIONS
  The data are compatible with the hypothesis that the minor trauma involved in taking a Pap smear may reduce the risk of cervical cancer by triggering an immune response to HPV infection.
authors.affiliation: Columbia University, USA. SS1832@columbia.edu
authors.forename: Samuel
authors.initials: S
authors.lastname: Shapiro
authors.forename: Henri
authors.initials: H
authors.lastname: Carrara
authors.forename: Bruce R
authors.initials: BR
authors.lastname: Allan
authors.forename: Margaret
authors.initials: M
authors.lastname: Hoffman
authors.forename: Lynn
authors.initials: L
authors.lastname: Rosenberg
authors.forename: Judith P
authors.initials: JP
authors.lastname: Kelly
authors.forename: Diane D
authors.initials: DD
authors.lastname: Cooper
authors.forename: Anna-Lise
authors.initials: AL
authors.lastname: Williamson
mesh_terms: D000328:Adult; D017677:Age Distribution; D016022:Case-Control Studies; D016001:Confidence Intervals; D015986:Confounding Factors, Epidemiologic; D005260:Female; D006801:Humans; D008875:Middle Aged; D065006:Papanicolaou Test; D027383:Papillomaviridae / Q000276:immunology*; D030361:Papillomavirus Infections / Q000276:immunology*; D011159:Population Surveillance; D015995:Prevalence; D018570:Risk Assessment; D012307:Risk Factors; D013019:South Africa / Q000453:epidemiology; D002583:Uterine Cervical Neoplasms / Q000453:epidemiology / Q000276:immunology* / Q000517:prevention & control* / Q000821:virology; D014626:Vaginal Smears / Q000706:statistics & numerical data*; D016387:Women's Health