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Biomedical subjects

J Wallinga

Publications and source records attributed to J Wallinga.

13 recordsLinked to original sources
PubMed2006

[Combating infectious disease using mathematical modelling].

When determining interventions against threatening infectious diseases such as HIV-infection, severe acute respiratory syndrome (SARS), smallpox and pandemic influenza, the use of mathematical models of the spread of infectious diseases is becoming increasingly popular. These models contribute to the structuring of the knowledge already available in various disciplines, to finding epidemiological connnections, to demonstrating lacunas within the pool of knowledge and to the comparison of the expected effects and costs of preventative and intervention measures. The use of models leads to a 'made-to-measure' analysis ofthe effects and costs of preventative and intervention measures which takes account of the specific characteristics of infectious diseases. The integration of knowledge from various disciplines can be supported by more research into the theoretical epidemiology of infectious disease and by better integration of mathematical models into policy development. The resulting and better foundations of this policy that are achieved by means of infectious disease modelling translate into more effective combating of infectious disease.

Cost-Benefit Analysis↗
PubMed2006

Mixing patterns and the spread of close-contact infectious diseases.

Surprisingly little is known regarding the human mixing patterns relevant to the spread of close-contact infections, such as measles, influenza and meningococcal disease. This study aims to estimate the number of partnerships that individuals make, their stability and the degree to which mixing is assortative with respect to age. We defined four levels of putative at-risk events from casual (physical contact without conversation) to intimate (contact of a sexual nature), and asked university student volunteers to record details on those they contacted at these levels on three separate days. We found that intimate contacts are stable over short time periods whereas there was no evidence of repeat casual contacts with the same individuals. The contacts were increasingly assortative as intimacy increased. Such information will aid the development and parameterisation of models of close contact diseases, and may have direct use in outbreak investigations.

Journal Article↗
PubMed2003

Reconstruction of measles dynamics in a vaccinated population.

The evaluation of measles vaccination programmes has been problematic because the change in actual numbers of infections and susceptibles over time cannot be directly observed. In this paper, we present a method for estimating the time series of number of susceptibles and infections, as well as the critical vaccination coverage in a vaccinated population. The proposed method is applied to data on measles outbreaks in The Netherlands. We show that the results are self-consistent and in line with available independent estimates. A potential application of the proposed method lies in detecting the loss of herd immunity and assessing the risk of major outbreaks in vaccinated populations.

Antibodies, Viral↗
PubMed2003

Options for improvement of the Dutch measles vaccination schedule.

We investigated which vaccination schedule gives best protection to the vaccinating population, in case of a measles epidemic in pockets of unvaccinated individuals. We explored the effect of an additional measles vaccination (at 6 or 9 months), advancing the first measles-mumps-rubella (MMR) vaccination from 14 to 11 months, and advancing the second MMR from 9 to 4 years. Measures of protection among vaccinees (percentage of susceptibles, number of reported cases, percentage of lifetime spent susceptible) were estimated with a mathematical model of the impact of antibody level on seroconversion and immunity. Advancing the age of second MMR vaccination prevents considerably more cases among vaccinees than an extra early measles vaccination or advancing the age of first MMR vaccination.

Child, Preschool↗
PubMed2002

Protecting the vaccinating population in the face of a measles epidemic: assessing the impact of adjusted vaccination schedules.

We investigated which vaccination schedule gives best protection to the vaccinating population, in case there is a measles epidemic in an area with low vaccine coverage. We considered combinations of an early measles vaccination (none, at 6 months or at 9 months), a measles-mumps-rubella (MMR) vaccination around the first birthday (at either 11 or 14 months), and MMR vaccination at an older age (at either 4 or 9 years). The different estimates on measures of protection (percentage of susceptibles, number of reported cases in an epidemic year, percentage of lifetime spent susceptible) relied on a mathematical model of decline of maternal antibody levels with age, and the impact of that antibody level on seroconversion and immunity. Model parameters were estimated from a Dutch population-based serological survey on measles antibodies. Different measures of protection favoured different vaccination schedules, but dropping the age of second MMR vaccination prevents considerably more cases than an extra early measles vaccination or dropping the age of first MMR vaccination.

Age Factors↗
PubMed2002

Electron-trapping probability in natural dosemeters as a function of irradiation temperature.

The electron-trapping probability in OSL traps as a function of irradiation temperature is investigated for sedimentary quartz and feldspar. A dependency was found for both minerals; this phenomenon could give rise to errors in dose estimation when the irradiation temperature used in laboratory procedures is different from that in the natural environment. No evidence was found for the existence of shallow trap saturation effects that could give rise to a dose-rate dependency of electron trapping.

Aluminum Silicates↗
PubMed2002

Measurement of the dose in quartz in the presence of feldspar contamination.

To allow the determination of the dose in quartz in the presence of feldspar contamination, the quartz-to-feldspar optically stimulated luminescence (OSL) ratio needs to be maximised. Experiments were carried out to find the optimum procedure to bleach the feldspar by exposure to infrared light, without seriously affecting the quartz OSL signal. A single-aliquot regenerative-dose protocol is proposed which, in our samples, allows accurate measurement of the dose in quartz in the presence of feldspar contamination. The protocol also has potential use for quartz dose determination in mixed-mineral samples.

Aluminum Silicates↗
PubMed2002

Dose estimation using quartz OSL in the non-linear region of the growth curve.

The reliability of equivalent-dose determinations on quartz made with the single-aliquot regenerative-dose (SAR) protocol is examined using a suite of sedimentary samples for which the equivalent dose is expected to be >200 Gy. Experiments on one sample were used to evaluate the precision obtained using the standard measurement procedure with (a) a range of pre-heat temperatures, and (b) a single pre-heat temperature. The precision is compared with that achieved using SAR for a repeated dose point after an initial pre-heat, and for a known dose given before the initial pre-heat. It is concluded that, at least for this sample, the scatter in OSL measurements dominates the spread in the estimates of De.

Luminescent Measurements↗
PubMed2002

Modelling the impact of vaccination strategies.

Many questions in planning for vaccination strategies are not easily answered. In order to assess the expected impact of vaccination on incidence of infection and disease we have to account explicitly for the transmission process of infectious agents, and the interference of vaccination with transmission of infection or disease. This requires, in one way or another, a mathematical description and analysis of the transmission of infection. We shall briefly discuss the basic structure of epidemic models that are used for evaluating vaccination strategies. We shall confront the findings with observations, and we shall review some practical cases where modelling vaccination strategies has provided crucial insights into how vaccination affects patterns of disease incidence.

Denmark↗
PubMed2001

Estimation of measles reproduction ratios and prospects for elimination of measles by vaccination in some Western European countries.

The objective of this study is to estimate the measles reproduction ratio for eight Western European vaccination programmes. Because many plausible age-structured transmission patterns result in a similar description of the observations, it is not possible to estimate a unique value of the reproduction ratio. A method is developed to estimate bounds and confidence intervals for plausible values of the reproduction ratios using maximum likelihood methods. Lower and upper bounds for plausible values of the basic reproduction ratio are estimated to be 7.17 (95% CI 7.14-7.20) and 45.41 (95% CI 9.77-49.57), corresponding to lower and upper bounds on critical vaccine coverage of 86.6% and 98.1%. Of the eight evaluated vaccination programmes, four have vaccine coverage below the lower bound and allow measles to persist, and four have vaccine coverage at the upper bound and may eventually eliminate measles.

Adolescent↗
PubMed2000

[Measles epidemiology in the Netherlands: a exploratory analysis of notification].

OBJECTIVE: Explorative analysis of the effects of vaccination policy on measles incidence. DESIGN: Retrospective study and mathematical modelling. METHOD: Analysis of national and regional case notifications of measles provided by the Inspectorate of Health in the Netherlands over the period from January 1976 (when vaccination was started) through September 1999. Also computer simulations with a mathematical epidemic model of measles were used to calculate the incidence of measles from 1976 onwards. RESULTS: According to the model results, measles should not persist with the current vaccination programme. However, the case notification data showed that measles appeared to persist at a nation-wide level. At a regional level, measles did not persist, not even in regions with low vaccine coverage. A possible cause of the unexpected persistence at the national level is the asynchronous regional course of the 6-year epidemic cycle of measles, where measles infection 'jumps' from one region to the other.

Child↗
PubMed1997

Modelling for precision weed management.

Recently, the need for the development of weed management systems with a reduced dependency on herbicides has increased because of concern about environmental side-effects and cost. The development of such systems requires new strategies based on improvements with respect to (1) prevention, (2) decision making and (3) weed control technology. For the development of improvements in all three aspects, quantitative understanding of weed population dynamics and crop-weed interactions is needed. Models that integrate the available quantitative knowledge can be used to design preventive measures, to develop long-term and short-term strategies for weed management, to assist in decision making to determine if, when, where and how weeds should be controlled and to identify new opportunities for weed control. Ecophysiological simulation models for crop-weed competition simulate growth and production of species in mixtures, based on ecophysiological processes in plants and their response to the environment. Such models help improve insight into the crop-weed system and can be used for purposes such as the development of simple predictive yield-loss models, threshold levels or the design of competitive crop plant types. For strategic weed management decisions, preventive measures and the identification of new opportunities for weed control, quantitative insight into the dynamics and spatial patterns of weed populations is also required. The complexity of the process and the long-term character of weed population dynamics make the use of models necessary. Different modelling approaches have been developed and are described briefly. Opportunities to use the available knowledge and models to improve weed management are discussed. Weeds occur in patches and their sensitivity to herbicides changes strongly with developmental stage, making precision techniques for herbicide application in time and space an option for reducing herbicide use. Limitations related to insight in biological processes as well as the state of technological development are discussed.

Agriculture↗