Impact of demographical change on the severity of an epidemic

Abstract

The thesis consists of two parts. The topic of the two parts are different, the title and the abstract of each part is:

1. The impact of demographical change such as ageing on the severity of an epidemic: a theoretical and numerical study of the final size and the basic reproduction number. Abstract Ageing and the effect of birth restriction change the demography in populations. These changes in demography influence contact patterns in societies, and, since contact patterns are important in disease transmission, influence the severity of infectious disease outbreaks. Here we use a deterministic multi-type SIR model to study theoretically and numerically how demographical changes impact the basic reproduction number and the final size of an infectious disease outbreak. We study either a density dependent contact pattern or a frequency dependent contact pattern. There is no consensus in literature in which multi-type contact pattern should be called frequency dependency. We have defined several new contact patterns in which the number of contacts per unit of time of an individual with a specific group is constant if we scale all group sizes by the same factor. In a model with reciprocal contacts where individuals have a fixed amount of contacts per unit of time with their own group and each young individual has a fixed amount of contacts per unit of time with older individuals, the basic reproduction number and final size increases when the number of young individuals increases. For other contact patterns, including the density dependent contact pattern, we have found theoretical results of a similar character. Numerical results suggest that if the society would have been more aged in Hong Kong in 2009, the number of cases of H1N1-2009 infection would have been smaller. However, due to the dependence of severity of an infection on the characteristics of the infected individuals, this need not imply a decrease in burden on the health care system. In a case study, we have found that birth restriction in Hong Kong may prolong the time before a similar H1N1 strain to the H1N1-2009 can cause an epidemic in Hong Kong.

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2. Effectiveness of interventions targeted at health care workers and residents in reducing the probability and the size of an infectious disease outbreak in a long-term care facility for the elderly Abstract Protecting elderly from an infectious disease infection is important due to the high case fatality ratio among elderly. It is predicted that the fraction elderly will increase in most developed countries, which can lead to an increase of the number of required long-term care facilities for the eldery in the future. Hence, in the future, protection of individuals in long-term care facilities for the elderly from infectious diseases infection becomes even more important in order to reduce the severity of an outbreak of an infectious disease outbreak. Previous studies contradict each other on how effective of interventions targeted at residents and health care workers are in reducing the probability of a major outbreak in a long-term care facility for the elderly. We used a stochastic simulation model and an analytical model to clarify this issue. Interventions can target health care workers and/or residents and may reduce the susceptibility and/or infectiousness of the individuals. In this study we define the level of an intervention of any type by the reduction of susceptibility and/or infectiousness of the targeted individual in percent. We call an intervention of any type weak, if the level of the intervention is less than 40% in comparison with the situation with no interventions. This study shows that weak interventions targeted at health care workers or visitors cannot reduce the probability of a major outbreak in the facility when the infectious disease has a basic reproduction number higher than 1.6 in the general population. These interventions hardly influence the average outbreak size given a major outbreak in the facility, in contrast to residents of the facility. Weak interventions aimed at residents can be effective even when the basic reproduction number exceeds 1,6. We showed that intervention level and the effectiveness of interventions in reducing the size and probability of a major outbreak in the long-term care facility for the elderly are in general not linearly related. Our study also reveals that interventions must start early in the epidemic and should not be stopped when the peak of the force of infection has been reached.