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Phase III trials have demonstrated the efficacy of human papillomavirus (HPV) vaccines in preventing transient and persistent high-risk (hr) HPV infection and precancerous lesions. A mathematical model of HPV type 16 infection and progression to cervical cancer, parameterised to represent the infection in Finland, was used to explore the optimal age at vaccination and pattern of vaccine introduction. In the long term, the annual proportion of cervical cancer cases prevented is much higher when early adolescents are targeted. Vaccinating against hr HPV generates greater long-term benefits if vaccine is delivered before the age at first sexual intercourse. However, vaccinating 12 year olds delays the predicted decrease in cervical cancer, compared to vaccinating older adolescents or young adults. Vaccinating males as well as females has more impact on the proportion of cases prevented when vaccinating at younger ages. Implementing catch-up vaccination at the start of a vaccination programme would increase the speed with which a decrease in HPV and cervical cancer incidence is observed. Human papillomavirus (HPV) is a common sexually transmitted agent (Trottier and Franco, 2006) and persistent infection with high risk (hr) types of HPV, most notably types 16 and 18, is the most important risk factor for cervical cancer (Ho et al, 1998). Rates of invasive cervical cancer (ICC) in fertile aged women in Finland declined significantly from 1960 to 1990 (Figure 1), mostly as a result of the national screening programme. However, cervical cancer incidence in Finnish women aged 30 -39 years is now four times higher than 15 years ago (2.9/100 000 in 1986 -1990; 9.8/ 100 000 in 2000: Finnish Cancer Registry, 2006. This is probably due to an increase in incidence and prevalence of hr HPV types in young (23 -28 year old) Finnish women (Laukkanen et al, 2003;Lehtinen et al, 2006).In phase III trials, two HPV virus-like particle vaccines have been shown effective in preventing incident and persistent HPV16 and 18 infection and associated precancerous lesions, with reported efficacies in the region of 90 -100% (Koutsky et al, 2002;Harper et al, 2004Harper et al, , 2006Villa et al, 2005;Mao et al, 2006). The vaccines could prevent around 70% of all cervical cancer (Munoz et al, 2003). One vaccine has recently been licensed for use in the US and in Europe. However, there remain important questions about how a HPV vaccine should be used at a population level (Lowndes and Gill, 2005). These include: the age chosen for vaccination, whether the vaccine is given to female subjects only or to female and male subjects and whether a catch-up vaccination campaign should accompany the introduction of routine vaccination. As mathematical models provide a framework for exploring these questions (Garnett, 2002), we have examined these questions with a model of single-type HPV using the observed epidemiology of HPV in Finland (Barnabas et al, 2006). MATERIALS AND METHODSIn earlier work, we used sexual behaviour data along with HPV seroprevalenc...

To improve understanding of long-range transport of perfluoroalkyl substances to the High Arctic, samples were collected from a snow pit on the Devon Ice Cap in spring 2008. Snow was analyzed for perfluoroalkyl acids (PFAAs), including perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs), as well as perfluorooctane sulfonamide (FOSA). PFAAs were detected in all samples dated from 1993 to 2007. PFAA fluxes ranged from <1 to hundreds of ng per m per year. Flux ratios of even-odd PFCA homologues were mostly between 0.5 and 2, corresponding to molar ratios expected from atmospheric oxidation of fluorotelomer compounds. Concentrations of perfluorobutanoic acid (PFBA) were much higher than other PFCAs, suggesting PFBA loading on the Devon Ice Cap is influenced by additional sources, such as the oxidation of heat transfer fluids. All PFCA fluxes increased with time, while PFSA fluxes generally decreased with time. No correlations were observed between PFAAs and the marine aerosol tracer, sodium. Perfluoro-4-ethylcyclohexanesulfonate (PFECHS) was detected for the first time in an atmospherically - derived sample, and its presence may be attributed to aircraft hydraulic system leakage. Observations of PFAAs from these samples provide further evidence that atmospheric oxidation of volatile precursors is an important source of PFAAs to the Arctic environment.

The patterns of transmission, clearance, and progression of HPV infection and the related precancerous lesions are key to accurately model cervical cancer epidemiology and prevention. We have developed an age-structured dynamic model of the transmission of HPV-16 infection. This mathematical model accounts, for the first time, for the effect of infection and precancerous lesions duration on the natural history of HPV-16 infection and precancerous lesions. The model's output has been fitted to contemporaneous sets of data from Turin, Italy, to estimate parameters that have had been indirectly tested by comparing them with other estimates reported in the literature. The average probability of HPV-16 infection transmission per sexual partnership was about 40%. The HPV-16 clearance and progression rates decreased as the length of time with infection increased, clearance ranging between 1.6 per woman-year (in the first 6 months of infection) and 0.036 (after more than 6 years of infection), and progression between 0.072 and 0.018 per woman-year. The rate of clearance of precancerous lesions (CIN2+) was inversely dependent on age, while the progression of CIN2+ toward invasive cervical cancer increased as the precancerous lesions persisted. The present study also suggests that an exclusive role of women's age in shaping the rate of progression to cancer is unlikely. These results should inform future analyses. Including more accurately the role of the duration of infection and precancerous lesions as determinants of the cervical cancer occurrence in models of cervical cancer control may influence predictors of the effectiveness of intervention strategies.