Notifiable Diseases Surveillance Research Articles

Prevalence of hemorrhagic fever with renal syndrome in Yiyuan County, China, 2005-2014.

BackgroundHemorrhagic fever with renal syndrome (HFRS) is highly endemic in mainland China, where human cases account for 90 % of the total global cases. Yiyuan County is one of the most serious affected areas in China. Therefore, there is an urgent need for monitoring and predicting HFRS incidence in Yiyuan to make the control of HFRS more effective.MethodsThe study was based on the reported cases of HFRS from the National Notifiable Disease Surveillance System. The demographic and spatial distributions of HFRS in Yiyuan were established. Then we fit autoregressive integrated moving average (ARIMA) models and predict the HFRS epidemic trend.ResultsThere were 362 cases reported in Yiyuan during the 10-year study period. The human infections in the fall and winter reflected a seasonal characteristic pattern of Hantaan virus (HTNV) transmission. The best model was ARIMA (2, 1, 1) × (0, 1, 1)12 (AIC value 516.86) with a high validity.ConclusionThe ARIMA model fits the fluctuations in HFRS frequency and it can be used for future forecasting when applied to HFRS prevention and control.

Increasing Incidence of Ehrlichiosis in the United States: A Summary of National Surveillance of Ehrlichia chaffeensis and Ehrlichia ewingii Infections in the United States, 2008-2012.

Human ehrlichiosis is a potentially fatal disease caused by Ehrlichia chaffeensis and Ehrlichia ewingii. Cases of ehrlichiosis are reported to Centers for Disease Control and Prevention through two national surveillance systems: Nationally Notifiable Diseases Surveillance System (NNDSS) and Case Report Forms. During 2008-2012, 4,613 cases of E. chaffeensis infections were reported through NNDSS. The incidence rate (IR) was 3.2 cases per million person-years (PYs). The hospitalization rate (HR) was 57% and the case fatality rate (CFR) was 1%. Children aged < 5 years had the highest CFR of 4%. During 2008-2012, 55 cases of E. ewingii infection were reported through NNDSS. The national IR was 0.04 cases per million PY. The HR was 77%; no deaths were reported. Immunosuppressive conditions were reported by 26% of cases. The overall rate for ehrlichiosis has increased 4-fold since 2000. Although previous literature suggests E. ewingii primarily affects those who are immunocompromised, this report shows most cases occurred among immunocompetent patients. This is the first report to show children aged < 5 years with ehrlichiosis have an increased CFR, relative to older patients. Ongoing surveillance and reporting of tick-borne diseases are critical to inform public health practice and guide disease treatment and prevention efforts.

Approaches to the Surveillance of Foodborne Disease: A Review of the Evidence.

Foodborne disease surveillance aims to reduce the burden of illness due to contaminated food. There are several different types of surveillance systems, including event-based surveillance, indicator-based surveillance, and integrated food chain surveillance. These approaches are not mutually exclusive, have overlapping data sources, require distinct capacities and resources, and can be considered a hierarchy, with each level being more complex and resulting in a greater ability to detect and control foodborne disease. Event-based surveillance is generally the least resource-intensive system and makes use of informal data sources. Indicator-based surveillance is seen as traditional notifiable disease surveillance and consists of routinely collected data. Integrated food chain surveillance is viewed as the optimal practice for conducting continuous risk analysis for foodborne diseases, but also requires significant ongoing resources and greater multisectoral collaboration compared to the other systems. Each country must determine the most appropriate structure for their surveillance system for foodborne diseases based on their available resources. This review explores the evidence on the principles, minimum capabilities, and minimum requirements of each type of surveillance and discusses examples from a range of countries. This review forms the evidence base for the Strengthening the Surveillance and Response for Foodborne Diseases: A Practical Manual.

Effect of Latitude and Seasonal Variation on Scrub Typhus, South Korea, 2001-2013.

In South Korea, scrub typhus is one of the most common rickettsial diseases. The number of scrub typhus patients has increased in South Korea, a total of 69,210 cases were reported from 2001 to 2013. The seasonality and relation of scrub typhus cases to latitude were analyzed in this article using data obtained from the National Notifiable Diseases Surveillance System website of the Korea Centers for Disease Control and Prevention. The incidence of scrub typhus tended to increase in the later months of the year, especially in October-December. In general, lower latitudes were associated with a later peak incidence. Our results suggest for the first time that the monthly observed incidence tended to increase in the later months of the year as the latitude decreased, and on a yearly basis in Korea.

Summary of Notifiable Infectious Diseases and Conditions - United States, 2013.

The Summary of Notifiable Infectious Diseases and Condition-United States, 2013 (hereafter referred to as the summary) contains the official statistics, in tabular and graphic form, for the reported occurrence of nationally notifiable infectious diseases and conditions in the United States for 2013. Unless otherwise noted, data are final totals for 2013 reported as of June 30, 2014. These statistics are collected and compiled from reports sent by U.S. state and territory, New York City, and District of Columbia health departments to the National Notifiable Diseases Surveillance System (NNDSS), which is operated by CDC in collaboration with the Council of State and Territorial Epidemiologists (CSTE). This summary is available at http://www.cdc.gov/mmwr/mmwr_nd/index.html. This site also includes summary publications from previous years.

Tracking Pertussis and Evaluating Control Measures through Enhanced Pertussis Surveillance, Emerging Infections Program, United States.

Despite high coverage with pertussis-containing vaccines, pertussis remains endemic to the United States. There have been increases in reported cases in recent years, punctuated by striking epidemics and shifting epidemiology, both of which raise questions about current policies regarding its prevention and control. Limited data on pertussis reported through the National Notifiable Disease Surveillance System have proved insufficient to answer these questions. To address shortcomings of national pertussis data, the Emerging Infections Program at the US Centers for Disease Control and Prevention launched Enhanced Pertussis Surveillance (EPS), which is characterized by systematic case ascertainment, augmented data collection, and collection of Bordetella pertussis isolates. Data collected through EPS have been instrumental in understanding the rapidly evolving epidemiology and molecular epidemiology of pertussis and have contributed essential information regarding pertussis vaccines. EPS also serves as a platform for conducting critical and timely evaluations of pertussis prevention and control strategies, including targeting of vaccinations and antimicrobial prophylaxis.

An assessment of measles vaccine effectiveness, Australia, 2006-2012.

Vaccine effectiveness analysis serves as a critical evaluation for immunization programmes and vaccination coverage. It also contributes to maintaining public confidence with the vaccine providers. This study estimated measles vaccine effectiveness at the population level using Australian national notifications data between 2006 and 2012. Notification data were obtained from the National Notifiable Diseases Surveillance System. Vaccination status was classified according to whether a case had received zero, one or two doses of measles-containing vaccine. Cases aged less than 1year and those with unknown vaccination status were excluded. All children with disease onset between 1January2006 and 31December2012 who were born after 1996 were included. Cases were matched to controls extracted from the Australian Childhood Immunization Register according to date of birth and jurisdiction of residence. Vaccine effectiveness was estimated by conditional logistic regression. Sensitivity analyses were conducted to test data robustness. Vaccine effectiveness was estimated at 96.7% (95% confidence interval [CI]: 94.5-98.0%) for one dose and 99.7% (95% CI: 99.2-99.9%) for two doses of measles vaccine. For at least one dose, effectiveness was estimated at 98.7% (95% CI: 97.9-99.2%). Sensitivity analyses did not significantly alter the base estimates. Vaccine effectiveness estimates suggested that the measles vaccine was protective at the population level between 2006 and 2012. However, vaccination coverage gaps may have contributed to recent measles outbreaks and may represent a serious barrier for Australia to maintain measles elimination status.

The contribution of PCR testing to influenza and pertussis notifications in Australia.

Influenza and pertussis are the two most common vaccine-preventable infections notified in Australia. We assessed the role of polymerase chain reaction (PCR) diagnosis in influenza and pertussis cases notified to the Australian National Notifiable Diseases Surveillance System (NNDSS). There were a total of 2 10 786 notified influenza cases (2001-2013) and 2 55 866 notified pertussis cases (1991-2013). After 1 January 2007, the majority of influenza and pertussis notifications were PCR-based (80·5% and 59·6%, respectively). Before 31 December 2006, PCR-based notifications were limited (29·1% and 11·7%, respectively). By 2013, PCR-based notifications had largely replaced all other diagnostic methods, with the exception of serology-based notifications in pertussis cases in adults aged ⩾ 25 years.

Dynamics of Pertussis Transmission in the United States.

Past patterns of infectious disease transmission set the stage on which modern epidemiologic dynamics are played out. Here, we present a comprehensive account of pertussis (whooping cough) transmission in the United States during the early vaccine era. We analyzed recently digitized weekly incidence records from Morbidity and Mortality Weekly Reports from 1938 to 1955, when the whole-cell pertussis vaccine was rolled out, and related them to contemporary patterns of transmission and resurgence documented in monthly incidence data from the National Notifiable Diseases Surveillance System. We found that, during the early vaccine era, pertussis epidemics in US states could be categorized as 1) annual, 2) initially annual and later multiennial, or 3) multiennial. States with predominantly annual cycles tended to have higher per capita birth rates, more household crowding, more children per family, and lower rates of school attendance than the states with multiennial cycles. Additionally, states that exhibited annual epidemics during 1938-1955 have had the highest recent (2001-2010) incidence, while those states that transitioned from annual cycles to multiennial cycles have had relatively low recent incidence. Our study provides an extensive picture of pertussis epidemiology in the United States dating back to the onset of vaccination, a back-story that could aid epidemiologists in understanding contemporary transmission patterns.

New Ways to Disseminate the US National Notifiable Disease Provisional Data

The purpose of this study was to identify ideas for an enhanced dissemination of the US National Notifiable Diseases Surveillance System (NNDSS) provisional data. The author conducted a search of all US State Health Department websites looking for on-line data display tables and tools for either reportable or notifiable diseases. In addition, the scope of the search of websites was expanded to include notifiable diseases of several countries, organizations and institutions. As a result of this study the author proposes enhancement of the current US notifiable disease data display from a static format to a more interactive dashboard design.

A Change in Vaccine Efficacy and Duration of Protection Explains Recent Rises in Pertussis Incidence in the United States

Over the past ten years the incidence of pertussis in the United States (U.S.) has risen steadily, with 2012 seeing the highest case number since 1955. There has also been a shift over the same time period in the age group reporting the largest number of cases (aside from infants), from adolescents to 7–11 year olds. We use epidemiological modelling and a large case incidence dataset to explain the upsurge. We investigate several hypotheses for the upsurge in pertussis cases by fitting a suite of dynamic epidemiological models to incidence data from the National Notifiable Disease Surveillance System (NNDSS) between 1990–2009, as well as incidence data from a variety of sources from 1950–1989. We find that: the best-fitting model is one in which vaccine efficacy and duration of protection of the acellular pertussis (aP) vaccine is lower than that of the whole-cell (wP) vaccine, (efficacy of the first three doses 80% [95% CI: 78%, 82%] versus 90% [95% CI: 87%, 94%]), increasing the rate at which disease is reported to NNDSS is not sufficient to explain the upsurge and 3) 2010–2012 disease incidence is predicted well. In this study, we use all available U.S. surveillance data to: 1) fit a set of mathematical models and determine which best explains these data and 2) determine the epidemiological and vaccine-related parameter values of this model. We find evidence of a difference in efficacy and duration of protection between the two vaccine types, wP and aP (aP efficacy and duration lower than wP). Future refinement of the model presented here will allow for an exploration of alternative vaccination strategies such as different age-spacings, further booster doses, and cocooning.

Public Health Impact and Cost-Effectiveness of Hepatitis A Vaccination in the United States: A Disease Transmission Dynamic Modeling Approach

ObjectiveTo assess the population-level impact and cost-effectiveness of hepatitis A vaccination programs in the United States. MethodsWe developed an age-structured population model of hepatitis A transmission dynamics to evaluate two policies of administering a two-dose hepatitis A vaccine to children aged 12 to 18 months: 1) universal routine vaccination as recommended by the Advisory Committee on Immunization Practices in 2006 and 2) Advisory Committee on Immunization Practices’s previous regional policy of routine vaccination of children living in states with high hepatitis A incidence. Inputs were obtained from the published literature, public sources, and clinical trial data. The model was fitted to hepatitis A seroprevalence (National Health and Nutrition Examination Survey II and III) and reported incidence from the National Notifiable Diseases Surveillance System (1980–1995). We used a societal perspective and projected costs (in 2013 US $), quality-adjusted life-years, incremental cost-effectiveness ratio, and other outcomes over the period 2006 to 2106. ResultsOn average, universal routine hepatitis A vaccination prevented 259,776 additional infections, 167,094 outpatient visits, 4781 hospitalizations, and 228 deaths annually. Compared with the regional vaccination policy, universal routine hepatitis A vaccination was cost saving. In scenario analysis, universal vaccination prevented 94,957 infections, 46,179 outpatient visits, 1286 hospitalizations, and 15 deaths annually and had an incremental cost-effectiveness ratio of $21,223/quality-adjusted life-year when herd protection was ignored. ConclusionsOur model predicted that universal childhood hepatitis A vaccination led to significant reductions in hepatitis A mortality and morbidity. Consequently, universal vaccination was cost saving compared with a regional vaccination policy. Herd protection effects of hepatitis A vaccination programs had a significant impact on hepatitis A mortality, morbidity, and cost-effectiveness ratios.

Completeness of reporting of race and ethnicity data in the nationally notifiable diseases surveillance system, United States, 2006-2010.

During 1994-1997, approximately 70% and 60% of the cases of conditions reported to the National Notifiable Diseases Surveillance System included persons of known race and ethnicity, respectively. A major goal of the Healthy People 2020 initiative is to eliminate health disparities. To describe trends in the completeness of race and ethnicity in case reports of the National Notifiable Diseases Surveillance System during 2006-2010. The National Notifiable Diseases Surveillance System is a public health surveillance system that aggregates case reports of infectious diseases and conditions that are designated nationally notifiable and are collected by US states and territories. The Centers for Disease Control and Prevention (Atlanta, Georgia) maintains this surveillance system in collaboration with the Council of State and Territorial Epidemiologists. We used Cochran-Armitage Trend Test (SAS, version 9.2) to test the hypothesis that the percentage of case reports with the completeness of race and ethnicity data increased or decreased linearly during 2006-2010. Completeness of race and ethnicity variables. The 32 conditions reviewed included 1 030 804 case records. Seventy percent of records included a known value for race, and 49% of records included ethnicity during 2006-2010. During 2006-2010, race was known in 70% or more of records in 24 of 32 conditions and in 23 of 51 jurisdictions. During 2006-2010, the systemwide reporting of race remained at the same level of completeness (70%) but the reporting of ethnicity increased slightly from 48% in 2006 to 53% in 2010. In comparison with race, the proportions of records coded to ethnicity were less among all conditions. Significant change has occurred in the completeness of reporting of ethnicity but not race during 2006-2010. However, the reporting of ethnicity still lags substantially behind the reporting of race. Jurisdictions that identify conditions with lower rates of completeness of race and ethnicity can assess the net benefits of efforts to improve the completeness of race and ethnicity data.

Trends in disease and complications of hepatitis A virus infection in the United States, 1999-2011: a new concern for adults.

In recent years, few US adults have had exposure and resultant immunity to hepatitis A virus (HAV). Further, persons with liver disease have an increased risk of adverse consequences if they are infected with HAV. This study used 1999-2011 National Notifiable Diseases Surveillance System and Multiple Cause of Death data to assess trends in the incidence of HAV infection, HAV-related hospitalization, and HAV-related mortality. During 1999-2011, the incidence of HAV infection declined from 6.0 cases/100 000 to 0.4 cases/100 000. Similar declines were seen by sex and age, but persons aged ≥80 years had the highest incidence of HAV infection in 2011 (0.8 cases/100 000). HAV-related hospitalizations increased from 7.3% in 1999 to 24.5% in 2011. The mean age of hospitalized cases increased from 36.0 years in 1999 to 45.1 years in 2011. While HAV-related mortality declined, the mean age at death among decedents with HAV infection increased from 48.0 years in 1999 to 76.2 years in 2011. The median age range of decedents who had HAV infection and a liver-related condition was 51.0 to 68.0 years. Although vaccine-preventable, HAV-related hospitalizations increased greatly, mostly among adults, and liver-related conditions were frequently reported among HAV-infected individuals who died. Public health efforts should focus on the need to assess protection from hepatitis A among adults, including those with liver disease.

An overview of disease surveillance and notification system in Nigeria and the roles of clinicians in disease outbreak prevention and control.

While outbreaks of infectious diseases have long presented a public health challenge, especially in developing countries like Nigeria; within recent years, the frequency of such outbreaks has risen tremendously. Furthermore, with the recent outbreaks of emerging and re-emerging infectious diseases such as Ebola virus disease and other epidemic prone diseases in Nigeria demanding immediate public health action, there is a need to strengthen the existing notifiable disease surveillance and notification system with increased clinicians’ involvement in timely reporting of notifiable diseases to designated public health authorities for prompt public health action. Hence, this paper provides the opportunity to increase awareness among clinicians on the importance of immediate reporting of notifiable diseases and intensify engagement of clinicians in disease notification activities by describing various notifiable diseases in Nigeria using their surveillance case definition, outlines the reporting channel for notifying these diseases and highlights the roles of clinicians in the current disease surveillance and notification network for early disease outbreak detection and public health response in Nigeria.

Evaluation of the Notifiable Diseases Surveillance System in Beitbridge District, Zimbabwe 2015

Back ground: Notifiable Disease Surveillance system serves as an early warning system for public health emergencies. Since January 2013 to August 2014, Beitbridge never submitted T2 forms to the province. Four suspected cases of rabies were reported through the generic report. The electronic District Health Information System 2, T2 forms had not been updated. This discrepancy may imply under reporting of Notifiable Diseases. The study was conducted to evaluate the NDSS in Beitbridge district. Methods: Descriptive cross-sectional study was conducted. Health workers in sampled health facilities were interviewed using questionnaires. Checklists were used to assess resource availability. Epi InfoTM was used to calculate frequencies and proportions. Results: From 11 facilities, 53 respondents were interviewed of which the 59% were females. For Knowledge, 57% recalled at least 9 Notifiable diseases, 11% knew the T1 form required to notify. Respondents willing to participate in the NDSS were 87%. Responsibility to notify was placed other health workers other than themselves by 55% of the respondents. All facilities did not have completed T1 forms. T1 forms were available in 1/11 health facilities. Three outbreaks were reported using the Weekly Disease Surveillance System (WDSS). NDSS information was used for planning and mobilizing resources for indoor residual spraying. It costs an average $12.15 to notify a single case, against $1.50 if it was electronic. Conclusion: NDSS is acceptable, simple, flexible, unstable, not sensitive and useful. Reasons for under reporting were lack of forms, lack of induction and poor knowledge on the NDSS. The cost of operating the NDSS could be reduced if the system is electronic. T1 forms and guidelines for completing the forms should be distributed to all health facilities. On the job training of health workers through tutorials, supervision is recommended.

CARACTERÍSTICAS EPIDEMIOLÓGICAS, CLÍNICAS E ESPACIAIS DE CASOS NOTIFICADOS DE TUBERCULOSE EM ÁREA HIPERENDÊMICA DO NORDESTE DO BRASIL

Backgound and Objectives: The tuberculosis (TB) shows itself as a serious health public problem in Brasil and the world, having unequal distribution in population upon particular biological, social, cultural and economic characteristics. Within this context, this paper has in mind describing clinical epidemiological and spatial characteristics of TB cases notified in citizens of Timon, Maranhão, Brasil, in 2011. Methodology: Epidemiological descriptive and retrospective study based in collected data in Information System for Notifiable Diseases and Health Surveillance System of the Municipal Health of the city of Timon. The analysis of cases was through descriptive statistics and risk analysis. Results: 74 new cases of TB were notified in 2011, 55 being new. 62,1% in man, with prevalence of coefficient of 60, 12 cases/100.000hab. and relative risks of 1,74 for male. The major number of cases occurred in individuals between 20 and 39 years old (56,7%), with less than 3 years of study (32,4%) being the clinical pulmonary kind the most common (79,7%). Serologic testing for HIv was positive in 5,4%. The fatality rate for TB was of 2,7% being 25% bigger in patients with TB-HIV infection. The neighborhood of Parque Piauí concentrated the bigger amount of cases (19,9). Conclusion: This work enabled general characteristics of the disease in the city, as much as like it’s spatial distribution per neighborhood, to potentiate the actions aimed at combating the disease in the city. KEYWORDS: Tuberculosis. Epidemiology. Distribution.

Estimated prevalence of Hepatitis C Virus infection in Canada, 2011.

Prevalence estimates contribute to our understanding of the magnitude of a particular health condition and in planning appropriate public health interventions. To estimate the prevalence of chronic Hepatitis C virus (HCV) infection, anti-HCV-positive status (anti-HCV) and the proportion of undiagnosed HCV infections in Canada. A combination of back-calculation and workbook methods was used. The back-calculation method estimated prevalent chronic HCV infection and the proportion undiagnosed using the Canadian Cancer Registry's data on hepatocellular carcinoma reported between 1992 and 2008 and the Canadian Notifiable Disease Surveillance System's data on Hepatitis C virus (HCV) cases reported between 1991 and 2009 in a Markov multi-state disease progression model with parameters adjusted to Canada. The workbook method divided the total population of Canada into population subsets and developed estimates of population size and anti-HCV prevalence for each. Sub-population size estimates were multiplied by anti-HCV prevalence measures to calculate the prevalence of anti-HCV by sub-population. A measure of spontaneous clearance was used to estimate the number of persons with chronic HCV from estimates of the number of anti-HCV-positive persons. The back-calculation method estimated the prevalence of chronic HCV infection at 0.64% and the proportion of undiagnosed chronic HCV infection at 44% in 2011. The workbook method estimated the anti-HCV prevalence at 0.96% (plausibility range: 0.61% to 1.34%) and chronic HCV infection at 0.71% (0.45 - 0.99%). By combining mid-point estimates from both methods, it is estimated that between 0.64% to 0.71% of the overall Canadian population was living with chronic HCV infection in 2011 and 44% of these individuals were undiagnosed.

Hepatitis C surveillance in Canada.

Hepatitis C is a highly transmissible virus that can lead to chronic liver disease. It continues to be a major public health concern in Canada. To describe surveillance trends of reported cases of Hepatitis C virus (HCV) in Canada by age and sex from 1991-2012. Cases of HCV reported to the Canadian Notifiable Disease Surveillance System were compiled at the national level. As most reported cases are not differentiated by acute or chronic HCV infection status, presented results are based on total HCV cases. Time trends are provided from 1991-2012, with a more detailed examination of age and sex patterns from 2005-2012. The rate of reported HCV infection increased sharply from 1991 (the year it was first notifiable) until 1998, when the highest overall rate of 66.9 per 100,000 was observed. From that time until 2012, rates of reported cases decreased in both sexes, but remained consistently higher among males than females. In 2012, the overall rate of reported HCV infection was 29.3 per 100,000. In younger age groups, rates among females were marginally higher, while males in older age groups (30 and above) exhibited substantially higher rates. This surveillance summary presents the longer-term trends in reported cases and corresponding rates of HCV in Canada using national surveillance data. Canada continues to experience a downward trend in HCV rates; however, the burden of infection will continue to increase as chronically infected individuals develop severe illness.

Estimating incidence of sexually transmissible infections in Australia

The reported numbers of notifiable sexually transmitted infections in Australia are subject to a myriad of biases and limitations, including demographic, clinical, behavioural, and temporal factors. As such, trends in reported diagnosis rates may not reflect trends in true underlying incidence, thereby confounding any direct interpretation of raw notification data for public health research. The development of new methods to determine the true incidence and prevalence of an infection is a critical step for furthering the understanding of disease in the population. Here we devise a statistical model for chlamydia testing, and apply it to routinely available data to estimate the true incidence and prevalence of chlamydia infection within the Australian population. 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Dhar, P. Horner, and 7 coauthors. How much do delayed healthcare seeking, delayed care provision, and diversion from primary care contribute to the transmission of STIs? Sexually Transmitted Infections, 83(5):400&ndash;405, 2007. doi:10.1136/sti.2006.024554 J. S. Hocking, M. S. C. Lim, J. Vidanapathirana, T. R. H. Read, and M. Hellard. Chlamydia testing in general practice&ndash;-a survey of Victorian general practitioners. Sexual Health, 3(4):241&ndash;244, Dec 2006. doi:10.1071/SH06042 R. L. Cook, S. L. Hutchison, L. \T1\O stergaard, R. S. Braithwaite, and R. B. Ness. Systematic review: noninvasive testing for chlamydia trachomatis and neisseria gonorrhoeae. Annals of Internal Medicine, 142(11):914&ndash;925, Jun 2005. doi:10.7326/0003-4819-142-11-200506070-00010 S. A. Morre, A. J. C. van den Brule, L. Rozendaal, A. J. P. Boeke, F. J. Voorhorst, S. de Blok, and C. J. L. M. Meijer. The natural course of asymptomatic Chlamydia trachomatis infections: 45% clearance and no development of clinical pid after one-year follow-up. International Journal of STD and AIDS, 13(1):12&ndash;18, 2002. doi:10.1258/095646202762226092 Australian Bureau of Statistics. Australian Demographic Statistics. Australian Bureau of Statistics, Canberra, Australia, 2013. http://www.abs.gov.au/ausstats/abs@.nsf/mf/3101.0 R Core Team. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, Austria, 2013. http://www.R-project.org