Contact tracing strategies in heterogeneous populations

Contact tracing is a well-established disease control measure that seeks to uncover cases by following chains of infection. This paper examines mathematical models of both single-step and iterative contact tracing schemes and analyses the ability of these procedures to trace core groups and the sensitivity of the intervention to the timescale of tracing. An iterative tracing process is shown to be particularly effective at uncovering high-risk individuals, and thus it provides a powerful public health tool. Further targeting of tracing effort is considered. When the population exhibits like-with-like (assortative) mixing the required effort for eradication can be significantly reduced by preferentially tracing the contacts of high-risk individuals; in populations where individuals have reliable information about their contacts, further gains in efficiency can be realized. Contact tracing is, therefore, potentially an even more potent tool than its present usage suggests.     

The impact of contact structure on infectious disease control: influenza and antiviral agents

Planning adequate public health responses against emerging infectious diseases requires predictive tools to evaluate the impact of candidate intervention strategies. With current interest in pandemic influenza very high, modelling approaches have suggested antiviral treatment combined with targeted prophylaxis as an effective first-line intervention against an emerging influenza pandemic. To investigate how the effectiveness of such interventions depends on contact structure, we simulate the effects in networks with variable degree distributions. The infection attack rate can increase if the number of contacts per person is heterogeneous, implying the existence of high-degree individuals who are potential super-spreaders. The effectiveness of a socially targeted intervention suffers from heterogeneous contact patterns and depends on whether infection is predominantly transmitted to close or casual contacts. Our findings imply that the various contact networks' degree distributions as well as the allocation of contagiousness between close and casual contacts should be examined to identify appropriate strategies of disease control measures.     

Improving Estimates of Social Contact Patterns for Airborne Transmission of Respiratory Pathogens

Data on social contact patterns are widely used to parameterize age-mixing matrices in mathematical models of infectious diseases. Most studies focus on close contacts only (i.e., persons spoken with face-to-face). This focus may be appropriate for studies of droplet and short-range aerosol transmission but neglects casual or shared air contacts, who may be at risk from airborne transmission. Using data from 2 provinces in South Africa, we estimated age mixing patterns relevant for droplet transmission, nonsaturating airborne transmission, and Mycobacterium tuberculosis transmission, an airborne infection where saturation of household contacts occurs. Estimated contact patterns by age did not vary greatly between the infection types, indicating that widespread use of close contact data may not be resulting in major inaccuracies. However, contact in persons >50 years of age was lower when we considered casual contacts, and therefore the contribution of older age groups to airborne transmission may be overestimated.     

Social mixing patterns for transmission models of close contact infections: exploring self-evaluation and diary-based data collection through a web-based interface

Although mixing patterns are crucial in dynamic transmission models of close contact infections, they are largely estimated by intuition. Using a convenience sample (n=73), we tested self-evaluation and prospective diary surveys with a web-based interface, in order to obtain social contact data. The number of recorded contacts was significantly (P<0.01) greater on workdays (18.1) vs. weekend days (12.3) for conversations, and vice versa for touching (5.4 and 7.2 respectively). Mixing was highly assortative with age for both (adults contacting other adults vs. 0- to 5-year-olds, odds ratio 8.9-10.8). Respondents shared a closed environment significantly more often with >20 other adults than with >20 children. The difference in number of contacts per day was non-significant between self-evaluation and diary (P=0.619 for conversations, P=0.125 for touching). We conclude that self-evaluation could yield similar results to diary surveys for general or very recent mixing information. More detailed data could be collected by diary, at little effort to respondents.     

Estimating the role of casual contact from the community in transmission of Bordetella pertussis to young infants

The proportion of infant pertussis cases due to transmission from fcasual contact in the community has not been estimated since before the introduction of pertussis vaccines in the 1950s. This study aimed to estimate the proportion of pertussis transmission due to casual contact using demographic and clinical data from a study of 95 infant pertussis cases and their close contacts enrolled at 14 hospitals in France, Germany, Canada, and the U.S. between February 2003 and September 2004. A complete case analysis was conducted as well as multiple imputation (MI) to account for missing data for participants and close contacts who did not participate. By considering all possible close contacts, the MI analysis estimated 66% of source cases were close contacts, implying the minimum attributable proportion of infant cases due to transmission from casual contact with community members was 34% (95% CI = 24%, 44%). Estimates from the complete case analysis were comparable but less precise. Results were sensitive to changes in the operational definition of a source case, which broadened the range of MI point estimates of transmission from casual community contact to 20%–47%. We conclude that casual contact appears to be responsible for a substantial proportion of pertussis transmission to young infants.     

云南省德宏州新报告艾滋病病毒感染者溯源调查

目的 对新报告HIV感染者的高危接触者进行逐级追踪调查和HIV检测(溯源调查),评价其作为一种艾滋病检测和防控策略的有效性和可行性.方法 对云南省德宏州2008年8-10月新报告的HIV感染者335例及其高危接触者(接触者)进行溯源调查.结果将335例中的309例以及从接触者中发现的148例HIV感染者纳入本次溯源调查.累计报告接触者3395人,其中有联系信息的接触者704人(20.7%);704人中成功追踪调查361人(51.3%);其中67.6%(244/361)为HIV感染情况不详;有83.2%(203/244)接受HIV检测,新检出HIV阳性56例,阳性检出率为27.6%(56/203).HIV感染者的配偶有联系信息的比例和追踪调查率分别为68.8%(280/407)、68.2%(191/280);远高于商业性伴1.2%(24/1978)、16.7%(4/24),非商业非配偶性伴37.3%(202/542)、22.3%(45/202),以及共用针具者34.1%(140/410)、56.4%(79/140).结论 溯源调查新检出的HIV阳性率高;但是对商业性伴和非商业非配偶性伴的追踪调查存在难度;开展溯源调查有助于发现更多的HIV感染者、掌握和控制艾滋病疫情.     

Estimating the sexual mixing patterns in the general population from those in people acquiring gonorrhoea infection: theoretical foundation and empirical findings.

STUDY OBJECTIVES--To describe mathematically the relationship between patterns of sexual mixing in the general population and those of people with gonorrhoea infection, and hence to estimate the sexual mixing matrix for the general population. DESIGN--Integration of data describing sexual behaviour in the general population, with data describing sexual behaviour and mixing among individuals infected with gonorrhoea. Use of these data in a simple mathematical model of the transmission dynamics of gonorrhoea infection. SETTING--The general population of London and a genitourinary medicine (GUM) clinic in west London. PARTICIPANT--These comprised 1520 men and women living in London who were randomly selected for the national survey of sexual attitudes and lifestyles and 2414 heterosexual men and women who presented to the GUM clinic with gonorrhoea. MAIN RESULTS--The relationship between sexual mixing among people with gonorrhoea and sexual mixing in the general population is derived mathematically. An empirical estimate of the sexual mixing matrix for the general population is presented. The results provide tentative evidence that individuals with high rates of acquisition of sexual partners preferentially select other individuals with high rates as partners (assortative mixing). CONCLUSIONS--Reliable estimates of sexual mixing have been shown to be important for understanding the evolution of the epidemics of HIV infection and other sexually transmitted diseases. The possibility of estimating patterns of sexual mixing in the general population from information routinely collected in gonorrhoea contact tracing programmes is demonstrated. Furthermore, the approach we describe could, in principle, be used to estimate the same patterns of mixing, using contact tracing data for other sexually transmitted diseases, thus providing a way of validating our results.     

Predictors of Unprotected Sexual Contacts in HIV-Infected Persons in Switzerland

Predictors of sexual risk behavior with regular and casual partners among HIV-infected heterosexual and gay persons were addressed. Sociodemographic data as well as self- and interviewer-reported data on sexual behavior were obtained from 117 asymptomatic HIV-infected persons enrolled in the Zurich part of the Swiss HIV Cohort Study (SHCS). Participants who reported sexual intercourse at least once (vaginal and/or anal) without condom use in the 6 months prior to interview were considered to have risk behavior. Sexual risk behavior was analyzed separately in contacts with regular and casual partners. In the 6-month preinterview period, 92% (108 of 117) of the HIV-infected study population reported sexual contacts, and 30/117 (26%) had at least one unprotected vaginal and/or anal contact. Among 93 persons using condoms, 25% of the heterosexual and 10% of the homosexual participants reported breakage of the condom. The main predictor for unprotected sexual behavior with regular partners was an elevated number of contacts. Predictors for sexual risk behavior with casual partners were the combination of alcohol and sexual encounters and the change of sexual behavior since the epidemic of AIDS. These findings did not differ between persons with hetero- and homosexual behavior. Considering that these contacts may have passed the virus on to the seronegative population, that the probability of unsafe sex increased with the growing number of contacts, and that the number of condom breakages was remarkable, it is imperative that this group be educated and motivated to take the active role in insisting on safer sex practices in each encounter.     

Condom use due to the risk of AIDS. Trends in the general population of Sweden.

This study examines the use of condoms due to the risk of AIDS in the general population of Sweden during 1986-1989. The study is based on annual mail surveys (1986-1989) of random population samples aged 16-44 years - 4000 individuals in each sample. The response rate was 71%. The study indicates that the use of condoms increased significantly during the first years of the study period, particularly among 18-24-year-olds. Roughly the same tendency was observed regarding the perception of personal risk related to HIV infection. In 1988 a positive connection was established between condom use and personal risk perception. Between 1988 and 1989 (as the public interest in the AIDS issue declined) no further increase in condom use was observed but rather a tendency of reversal towards previous levels. Having several sexual partners was not significantly associated with a more frequent use of condoms, nor was the occurrence of casual sexual contacts. In most of the casual sexual contacts reported condoms were not used.     

Analyzing sensitivity to model form assumptions of infection transmission system models

PURPOSE: Transmission system models make restrictive assumptions that might distort the conclusions of model analyses. We propose methods to progressively relax the following assumptions of classical deterministic compartmental models: 1) that the population has an effectively infinite size 2) that contact is instantaneous with no duration, 3) that mixing in this large population is instantaneously thorough after contact.METHODS: Analyses of contact patterns between high and low risk groups on gonorrhea transmission were performed. Initial models were similar to those analyzed by Hethcote and Yorke with compartments corresponding to sets of individuals. The instantaneous contact assumption in these models was relaxed by using continuous deterministic pairing models in the style of models presented by Dietz and Hadelar. That model makes restrictive assumptions about concurrent contacts, population sizes, and instantaneously random mixing. To relax these assumptions, we simulated our GERMS model of discrete individuals forming pairings and transmitting infection in continuous time.RESULTS: Relaxing the instantaneous contact assumption demonstrated a progressively decreased effect of mixing between high and low risk groups as the duration of contact was increased. The GERMS model simulations were shown to effectively reproduce pairing model behavior given the same restrictive assumptions as the pairing model. Further GERMS model analysis then demonstrated that concurrency assumptions alter the effects of contact rates between risk groups in ways that are dependent upon contact parameters. Finally GERMS models were used to structure mixing into four local areas. This affected the dynamics of reaching equilibrium but not the equilibrium value.CONCLUSIONS: Assessing the effects of assumptions in continuous compartmental models of transmission systems is feasible and important.     

Abdominal obesity and age-related macular degeneration

A prospective survey of social mixing patterns relevant to respiratory disease transmission by large droplets (e.g., influenza) or small droplet nuclei (e.g., tuberculosis) was performed in a South African township in 2010. A total of 571 randomly selected participants recorded the numbers, times, and locations of close contacts (physical/nonphysical) and indoor casual contacts met daily. The median number of physical contacts was 12 (interquartile range (IQR), 7-18), the median number of close contacts was 20 (IQR, 13-29), and the total number of indoor contacts was 30 (IQR, 12-54). Physical and close contacts were most frequent and age-associative in youths aged 5-19 years. Numbers of close contacts were 40% higher than in corresponding populations in industrialized countries (P < 0.001). This may put township communities at higher risk for epidemics of acute respiratory illnesses. Simulations of an acute influenza epidemic predominantly involved adolescents and young adults, indicating that control strategies should be directed toward these age groups. Of all contacts, 86.2% occurred indoors with potential exposure to respiratory droplet nuclei, of which 27.2%, 20.1%, 20.0%, and 8.0% were in transport, own household, crèche/school, and work locations, respectively. Indoor contact time was long in households and short during transport. High numbers of indoor contacts and intergenerational mixing in households and transport may contribute to exceptionally high rates of tuberculosis transmission reported in the community.     

Re-emerging syphilis in the UK: a behavioural analysis of infected individuals

In-depth interviews with 27 individuals infected with syphilis in a recent UK outbreak (out of a total of 58 diagnosed between May 1999 and August 2000 in three city hospitals) were carried out to examine behaviour and attitudes. Most (23/27) participants were homosexual men, seven of whom were HIV positive. Between them, the 23 gay men had 1,494 different contacts in the twelve months prior to their awareness of having syphilis, but only 10% of these contacts could be named. While oral sex (usually unprotected) was the most prevalent behaviour (median = 30 partners per year), only 39% perceived unprotected oral sex as a syphilis risk (c.f. 70% for anal sex). Many gay men (61%) used gamma hydroxybutyrate (GHB) during sex as an aphrodisiac. This syphilis-infected subset of the population had high levels of unprotected and anonymous sex, which brings into question the usefulness of contact tracing to control syphilis outbreaks. The majority of partners were casual oral sex partners. More awareness is urgently needed around syphilis symptoms and risks, and risks of using drugs to reduce sexual inhibitions.     

A Quantitative Benefit-Risk Analysis of Isoniazid for Treatment of Latent Tuberculosis Infection Using Incremental Benefit Framework

BackgroundWe undertook a quantitative benefit-risk analysis of a targeted isoniazid (INH) therapy for latent tuberculosis (TB) infection for different groups of contacts of active TB cases.MethodsWe developed a decision-analytic model to compare the treatment of latent TB infection in subgroups of contacts to no treatment over a 6-year time horizon in a Canadian setting. Contacts were stratified into 32 groups on the basis of five binary variables: type of contact (close or casual), tuberculin skin test (TST) results (positive or negative at 5 mm cutoff), Bacillus Calmette-Guérin vaccination status, place of birth (foreign- or Canadian-born), and age group (cutoff 35 years). Risk of TB reactivation was calculated for each subgroup from a longitudinal registry of contacts, adjusted for several potential confounders and comorbid conditions. We calculated the quality-adjusted life-years gained because of delayed or prevention of active TB via treatment of latent TB infection versus quality-adjusted life-years lost because of the adverse events to INH.ResultsA targeted policy based on adopting INH therapy only in subgroups with positive expected incremental net health benefit resulted in a different treatment decision than the current guidelines in five subgroups comprising 3.9% of the contacts. Namely, the targeted policy comprised no INH therapy in casual contacts with a positive vaccination history even with a positive TST result and INH therapy in foreign-born close contacts younger than 35 years even with a negative TST result.ConclusionsFrom a benefit-risk viewpoint, INH treatment of contacts should be tailored on the basis of risk assessment algorithms that consider a range of factors at the time of screening.     

A Piece of the Public Health Surveillance Puzzle: Social Contacts among School-Aged Children

Objective

To enhance public health surveillance and response for acute respiratory infectious diseases by understanding social contacts among school-aged children

Introduction

Timely and effective public health decision-making for control and prevention of acute respiratory infectious diseases relies on early disease detection, pathogen properties, and information on contact behavior affecting transmission. However, data on contact behavior are currently limited, and when available are commonly obtained from traditional self-reported contact surveys [1, 2]. Information for contacts among school-aged children is especially limited, even though children frequently have higher attack rates than adults, and school-related transmission is commonly predictive of subsequent community-wide outbreaks, especially for pandemic influenza.Within this context, high-quality data are needed about social contacts. Precise contact estimates can be used in mathematical models to understand infectious disease transmission [3] and better target surveillance efforts. Here we report preliminary data from an ongoing 2-year study to collect social contact data on school-aged children and examine the transmission dynamics of an influenza pandemic.

Methods

Our aim is to capture mixing patterns and contact rates of school-aged children in 24 schools and other non-school-related venues. We used a stratified design to ensure coverage of urban, suburban, and rural school districts, as well as climatically different areas (mountains and desert) in Utah. Elementary, middle, and high schools were chosen in each stratum. We defined a self-reported contact as anyone with whom the participant talked to face-to-face, played with, or touched. Contact logs collected subjective information (age, location, and duration) on self-reported contacts during a 2-day period. Objective contact data were collected by using proximity sensors [4] that recorded signals from other sensors within approximately 3–4 feet.Mixing patterns during school and non-school-related activities were summarized for participating school-aged children. We developed contact networks using proximity sensor data, providing visualizations of contact patterns as well as numeric contact measures. Contact networks were characterized with respect to degree distribution, and density. The degree for each person was calculated as the number of unique contacts. The density for a network was calculated as the number of observed contacts divided by the number of possible contacts.

Results

Two elementary schools, four summer camps, and one club participated in the study between May and August, 2012. Data were processed for the two schools and one camp. The mean degrees for the two schools were 28 and 29, with network sizes 109 and 129, respectively. The mean degree from camp was 43, whose network size was 141. The density of contacts was 0.26 and 0.22 for the schools and 0.31 for the camp. The density within classrooms at the two schools ranged from 0.78 to 0.98. School-aged children typically underreported contacts using the contact log compared with objective proximity sensor data; this difference was statistically significant.

Conclusions

The variability in these and other contact network characteristics represent factors that could impact influenza transmission. Quantifying these factors improves our understanding of influenza transmission dynamics, which in turn can be used to adapt surveillance methods and control and prevention strategies. Almost all contact among students in our two elementary schools occurs within the classroom and the contact patterns differ by classroom, due to desk arrangement or other characteristics. Thus, during an elementary school outbreak it may be beneficial to focus on classroom-specific surveillance and control strategies.The study is ongoing and we expect the variability in contact rates and mixing patterns will be even greater for middle and high schools where students switch classrooms and classmates each period. These schools could benefit from alternative surveillance and control strategies that account for the heightened overall mixing of the student body.     

Sexual behaviour in a fishing community on Lake Victoria, Uganda

This study describes the sexual behaviour of men and women in a fishing village on the shores of Lake Victoria in southwest Uganda. The village is near a well known trading town-truckstop on the main trans-Africa highway with a high recorded prevalence of HIV infection. Data were obtained on the daily travel and sexual activities of 26 women and 54 men with particular attention paid to the rate of partner change and the proportion of sexual contacts with people outside the village. During a total of 587 person weeks the men made 1086 trips, mostly returning home the same day. They had a total of 1226 sexual contacts, most of which occurred either in their own village (83%) or a neighbouring fishing village (11%); 17 per cent of sexual contacts were with new partners. Fifteen of the women described themselves as married; 42 per cent of their sexual contacts were casual, paying partners. Of the eleven women who were single, between 80 and 100 per cent of contacts were with paying partners. Most of the women's partners were resident in the village. These data show a very high rate of sexual mixing within the village. Such communities should be targeted in future STD control programs.     

Globalization,Women, and Health in the 21st Century

Sexual partner mixing among men who have sex with men (MSM), based on both gender and partnership status, is an understudied theme in India. Using data from Round 2 of the Integrated Bio-behavioral Survey, this paper reports on gender and partnership status-based sexual mixing and levels of consistent condom use (CCU) among MSM in Maharashtra. A total of 689 MSM were sampled using probability-based sampling. Bivariate and regression analyses were carried out on condom use and partnership mixing. Over half (52%) of all MSM reported having only male partners while about one-third (34.5%) reported having partners of both gender. Over 70% of MSM engaged in sex with a mix of casual, regular, commercial and non-commercial partners. MSM with only male partners reported lower CCU as compared to MSM with partners of both genders (47.3% and 62%, respectively, p = 0.11). CCU levels differed significantly by status of sex partner. Overall, MSM having ‘men only’ as partners and those with partners of mixed status have greater risk behaviour in terms of low CCU. HIV prevention interventions need to focus attention on men in ‘exclusively male’ sex partnerships as well as MSM with a mix of casual, regular and commercial partners.     

Social network diversity and risks of ischemic heart disease and total mortality: findings from the Copenhagen City Heart Study

Measures of various types of social contacts were used as predictors of ischemic heart disease events and total mortality in an age-stratified random sample of 9,573 adults enrolled in the Copenhagen City Heart Study (Copenhagen, Denmark). Baseline examinations were conducted in 1991-1994, and participants were followed until the end of 1997. Contacts with parents, children, family members, and friends were associated with better health. The presence of a spouse or partner was protective for men. Contacts with neighbors showed a trend toward a reversed pattern, and the effects of contacts with work colleagues and children differed by gender. Most types of contacts that occurred at least monthly were just as protective as those occurring more frequently. An index of intimate social contact diversity with family and friends had graded relations with both outcomes. Comparisons of persons reporting three or more types of contacts with those reporting fewer types yielded age- and gender-adjusted hazard ratios of 0.73 (95% confidence interval (CI): 0.64, 0.82) for mortality and 0.75 (95% CI: 0.61, 0.91) for ischemic heart disease. Comparable fully adjusted hazard ratios were 0.83 (95% CI: 0.73, 0.95) and 0.82 (95% CI: 0.67, 1.00). These data suggest that health benefits are derived from the diversity of social roles, especially those involving intimate relationships.     

An outbreak of paratyphoid fever in the UK associated with a fish-and-chip shop

An outbreak of Salmonella paratyphi B infection in the UK associated with a fish-and-chip shop is reported. The source of infection for the first three cases was believed to be a food handler who was infected overseas 6 years earlier. His wife whose faeces and urine were originally culture negative continued to run the shop but subsequently her faeces became positive on one occasion. She was considered to have been the source of two further cases, and secondary household spread of infection from these two cases resulted in one symptomatic and two asymptomatic infections. A second household contact of the proprietor also became a faecal excretor 2 months later. We recommend that food handlers living in households or in intimate contact with cases or carriers of S. paratyphi B should be put off work until all household contacts cease excreting the organism.     

Limitations of national guidelines in the management of an outbreak of tuberculosis

National guidelines offer no clear definition of a close (non-household) contact of tuberculosis. The resulting lack of distinction between close and casual contacts may lead to excessive screening in outbreaks of pulmonary tuberculosis in the United Kingdom. Poor compliance with chemoprophylaxis suggests that priority should be given to the follow up of high risk contacts, who should be more clearly defined. A review of the management of a family outbreak of tuberculosis illustrates the problem. Fifty-nine out of 213 non-household contacts screened for infection had grade 3 and 4 Heaf reactions and none developed clinical tuberculosis within one year. Three of the 59 did not attend for chest radiography and a further 11 patients did not attend the outpatients appointment offered. Twenty-three took chemoprophylaxis for three months, 14 did not complete chemoprophylaxis, six (11%) declined it and two were offered x-ray follow up.     

Factors associated with social contacts in four communities during the 2007-2008 influenza season

Mathematical models of influenza pandemics are sensitive to changes in contact rates between individuals. We conducted population-based telephone surveys in four North Carolina counties to determine the number of social interactions between individuals during the 2007-2008 influenza season. Influenza activity was monitored through sentinel medical practices. Among 3845 adults, the number of social contacts varied with age, was lower on weekends than on weekdays, and further decreased during school holiday periods. Adults with influenza-like illnesses had fewer social contacts. Adults' contacts in the community setting increased during periods of peak influenza activity. Among 290 children, potential contacts (i.e. other people in the same location) were lowest among preschool-age children and decreased on weekends and during school holidays. In adjusted analyses, children's potential social contacts did not change during periods of peak influenza activity. These results should be useful for modelling influenza epidemics and pandemics and in planning mitigation and response strategies.