Efficiency Of Contact Tracing Research Articles

Optimal Contact Tracing and Social Distancing Policies to Suppress A New Infectious Disease

Abstract This paper studies the suppression of an infectious disease in the canonical susceptible-infectious-recovered model. It derives three results. First, if technically feasible, the optimal response to a sufficiently small outbreak is halting transmissions instead of building up immunity through infections. Second, the crucial trade-off is not between health and economic costs, but between the intensity and duration of control measures. A simple formula of observables characterises the optimum. Third, the total cost depends critically on the efficiency of contact tracing, since it allows relaxing costly social distancing without increasing transmissions. A calibration to the COVID-19 pandemic illustrates the theoretical findings.

Downsizing of COVID-19 contact tracing in highly immune populations.

Contact tracing is a key component of successful management of COVID-19. Contacts of infected individuals are asked to quarantine, which can significantly slow down (or prevent) community spread. Contact tracing is particularly effective when infections are detected quickly, when contacts are traced with high probability, when the initial number of cases is low, and when social distancing and border restrictions are in place. However, the magnitude of the individual contribution of these factors in reducing epidemic spread and the impact of population immunity (due to either previous infection or vaccination), in determining contact tracing outputs is not fully understood. We present a delayed differential equation model to investigate how the immunity status and the relaxation of social distancing requirements affect contact tracing practices. We investigate how the minimal contact tracing efficiency required to keep an outbreak under control depends on the contact rate and on the proportion of immune individuals. Additionally, we consider how delays in outbreak detection and increased case importation rates affect the number of contacts to be traced daily. We show that in communities that have reached a certain immunity status, a lower contact tracing efficiency is required to avoid a major outbreak, and delayed outbreak detection and relaxation of border restrictions do not lead to a significantly higher risk of overwhelming contact tracing. We find that investing in testing programs, rather than increasing the contact tracing capacity, has a larger impact in determining whether an outbreak will be controllable. This is because early detection activates contact tracing, which will slow, and eventually reverse exponential growth, while the contact tracing capacity is a threshold that will easily become overwhelmed if exponential growth is not curbed. Finally, we evaluate quarantine effectiveness in relation to the immunity status of the population and for different viral variants. We show that quarantine effectiveness decreases with increasing proportion of immune individuals, and increases in the presence of more transmissible variants. These results suggest that a cost-effective approach is to establish different quarantine rules for immune and nonimmune individuals, where rules should depend on viral transmissibility after vaccination or infection. Altogether, our study provides quantitative information for contact tracing downsizing in vaccinated populations or in populations that have already experienced large community outbreaks, to guide COVID-19 exit strategies.

Contact tracing efficiency, transmission heterogeneity, and accelerating COVID-19 epidemics.

Simultaneously controlling COVID-19 epidemics and limiting economic and societal impacts presents a difficult challenge, especially with limited public health budgets. Testing, contact tracing, and isolating/quarantining is a key strategy that has been used to reduce transmission of SARS-CoV-2, the virus that causes COVID-19 and other pathogens. However, manual contact tracing is a time-consuming process and as case numbers increase a smaller fraction of cases’ contacts can be traced, leading to additional virus spread. Delays between symptom onset and being tested (and receiving results), and a low fraction of symptomatic cases being tested and traced can also reduce the impact of contact tracing on transmission. We examined the relationship between increasing cases and delays and the pathogen reproductive number Rt, and the implications for infection dynamics using deterministic and stochastic compartmental models of SARS-CoV-2. We found that Rt increased sigmoidally with the number of cases due to decreasing contact tracing efficacy. This relationship results in accelerating epidemics because Rt initially increases, rather than declines, as infections increase. Shifting contact tracers from locations with high and low case burdens relative to capacity to locations with intermediate case burdens maximizes their impact in reducing Rt (but minimizing total infections may be more complicated). Contact tracing efficacy decreased sharply with increasing delays between symptom onset and tracing and with lower fraction of symptomatic infections being tested. Finally, testing and tracing reductions in Rt can sometimes greatly delay epidemics due to the highly heterogeneous transmission dynamics of SARS-CoV-2. These results demonstrate the importance of having an expandable or mobile team of contact tracers that can be used to control surges in cases. They also highlight the synergistic value of high capacity, easy access testing and rapid turn-around of testing results, and outreach efforts to encourage symptomatic cases to be tested immediately after symptom onset.

Investigation of the Efficiency of Mask Wearing, Contact Tracing, and Case Isolation During the COVID-19 Outbreak

Background: Case isolation and contact tracing are two essential parts of control measures to prevent the spread of COVID-19. Recent studies have shown that asymptomatic and pre-symptomatic transmissions play an important role in the ongoing SARS-CoV-2 pandemic, which makes outbreak control only through contact tracing and case isolation very difficult. Additional interventions, such as mask wearing, are required. There are various approaches to mask usage during the epidemic and, correspondingly, their effects on disease transmission may differ. We modelled the effects of different interventions on the disease spread and showed that high mask wearing coverage can lead to a better control of the outbreak, even in the absence of a lockdown. Methods: A stochastic model of COVID-19 propagation was implemented in order to estimate the effectiveness of mask wearing in addition to case isolation and contact tracing. It is assumed that after isolation, an infected person cannot transmit the virus anymore. We varied the percentage of mask efficiency and the percentage of people wearing masks and investigated three different approaches towards mask usage: the mask prevents spreading of the infection, and an infected person only starts wearing it after the onset of symptoms; the mask prevents spreading and all people wear masks; all people wear masks, and the mask prevents spreading and protects healthy persons from getting infection. We estimated the effect of mask wearing together with other interventions on the basic reproduction number (R0) and calculated the effective reproduction number after implementation of interventions. We also simulated the possibility of controlling the outbreak. An outbreak was defined as being controlled if it did not exceed a total of 5000 cases over three months and no new cases were found in the following fourth month. Findings: For the 20 initial symptomatic cases, with 40% pre-symptomatic transmission and a R0 = 2·5 , the outbreak could not be controlled even with 100% contact tracing. Wearing of masks only after symptom onset added only a very small improvement on the pandemic situation and cannot be used to control the outbreak. When all people wear masks and it is assumed that a mask prevents spreading and protects healthy individuals, then there is almost 70% probability of outbreak control even without contact tracing. The probability of control increases with an increase in the number of traced contacts. As a result, with 60% efficiency of contact tracing, the outbreak can be controlled with almost 100% probability. When initial number of asymptomatic cases is equal to the number of symptomatic cases, then an almost completely controlled outbreak is achieved when 70% of all people wear masks and 60% of infected individuals (with symptoms) are traced. Similar results can be achieved for larger number of total initial cases. Interpretation: With a large proportion of infectiousness before the onset of symptoms (40%) and the presence of asymptomatic cases, three investigated interventions (isolation of cases, contact tracing, and mask wearing by all people) are sufficient to control the disease spread within three months. A high probability of mask wearing by both infected and healthy people, even in presence of asymptomatic individuals, significantly increased the probability of controlling the outbreak even with only modest effectiveness of contact tracing. This shows that a second wave of COVID-19 could be avoided by tracing contacts and through the use of masks. Funding Statement: This work was supported in part by NHRI (National Health Research Institutes, Taiwan, project number BN-106-PP-08) and MOST (Ministry of Science and Technology, Taiwan, project number MOST 106 2115 M 400 001). Declaration of Interests: The authors declare no comparing interests.

Perspectives on COVID-19 from Singapore: Impact on ESKD Care and Medical Education.

Perspectives on COVID-19 from Singapore: Impact on ESKD Care and Medical Education.

Transmission of Influenza on International Flights, May 2009

Understanding the dynamics of influenza transmission on international flights is necessary for prioritizing public health response to pandemic incursions. A retrospective cohort study to ascertain in-flight transmission of pandemic (H1N1) 2009 and influenza-like illness (ILI) was undertaken for 2 long-haul flights entering Australia during May 2009. Combined results, including survey responses from 319 (43%) of 738 passengers, showed that 13 (2%) had an ILI in flight and an ILI developed in 32 (5%) passengers during the first week post arrival. Passengers were at 3.6% increased risk of contracting pandemic (H1N1) 2009 if they sat in the same row as or within 2 rows of persons who were symptomatic preflight. A closer exposed zone (2 seats in front, 2 seats behind, and 2 seats either side) increased the risk for postflight disease to 7.7%. Efficiency of contact tracing without compromising the effectiveness of the public health intervention might be improved by limiting the exposed zone.

Can the male-to-female ratio of gonorrhoea in one sexually transmissible infection treatment facility be solely used to judge its efficiency?

This facility has for a long time audited its efficacy in contact tracing (case finding) and found results comparable with national guidelines. In addition, we consistently measure control of the disease using three indices. A departure from the norm in one of these (the male-to-female ratio) prompted us to explore whether local case finding, and therefore control, was lacking resulting in the identification of a statistical anomaly. We have learnt a lesson, which may be of use to others who critically evaluate their work. Review of statutory clinic quarterly returns and manually-held contact tracer data, comparison of representative quarters (Wilcoxson sign rank test) and detailed inspection of sampled case-to-case contact tracing efficiency. Evidence was found challenging our belief that male-to-female ratios are at face value an inevitably accurate surrogate for case finding or infection control. In our clinic, we identified recording anomalies giving rise to false concerns that case finding was less efficient than usual. Although the heterosexual male:female ratio for gonorrhoea is one readily available and proxy measure of disease control and tracing efficiency, its sole use should be interpreted with caution. A time lag across quarters between patient and partner attendance and other recording anomalies may mislead. Ratios should therefore be interpreted in the context of partner notification outcomes, which give a more reliable measure of efficiency. The use of the ratio in critical evaluation of a unit's efficiency should be but one part of a package of measures.

Relation between incidence of gonorrhoea in Sheffield and efficiency of contact tracing: a paradox?

An attempt has been made to correlate the yearly incidence of gonorrhoea in Sheffield (1979-85) with two variables to show contact tracing efficiency: infectious patient days (days from the start of contact tracing to the attendance of contacts) and the percentage of source contacts brought to investigation within 30 days. No such correlation has been found. The possible reasons for this, which include the unreliability of incidence figures as a sole criterion of control and the organisation of contact tracing activities, are discussed.