Principles of Virology, Volume 2. S. Jane Flint

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Principles of Virology, Volume 2 - S. Jane Flint


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in the two groups, showing that vaccination is not a risk factor for the development of this disorder (Chapter 7).

       Mortality, Morbidity, and Case Fatality Ratios

      In contrast, a case fatality ratio is a measure of the number of deaths among clinical cases of the disease, expressed as a percentage. As an example, if 200 people are diagnosed with a respiratory tract infection and 16 of them die, the case fatality ratio would be 16/200, or 8%. In a technical sense, the use of the word “ratio” is incorrect; a case fatality ratio is more a measure of relative risk than a comparison between two numbers.

      R-naught (R0)

      The original host-parasite theory assumed well-mixed, homogeneous host populations in which each individual host has the same probability of becoming infected. Although the general concepts remain valid, additional parameters and constraints have been added to the mathematical models as more has been learned about population diversity and the dynamics of viral infections (Chapter 10). For example, immune-resistant viral mutants with differences in virulence and transmissibility can be selected, and some individuals (called super transmitters) can pass infection to others much more readily than the majority. We also now know that virus populations are more diverse than first imagined, and the constellation of possible host populations affects their evolution in ways not easily captured by mathematical equations. Consequently, although the calculations are useful indications of the thresholds that govern the spread of a virus in a population (that is, they help to determine if a disease is likely to die out[R0 is <1] or become endemic [R0 is >1]), they cannot be used to compare possible outcomes in particular cases or for different diseases.

Virus R 0 a
Measles 12–18
Smallpox 5–7
Polio ∼7
SARS–CoV–2 2–3
Influenza
2009 (H1N1) 1.47
1957, 1968 pandemics 1.8
1918 pandemic 2.4–5.4
Ebola 1.3–1.8b

      aValues from Centers for Disease Control and Prevention website.

      bSource: Chowell G et al. 2004. J Theoret Biol 229:199–126.

      Following the descriptive phase, analytical epidemiological methods are used to test hypotheses using control populations in either retrospectively or prospectively focused studies. Clinical epidemiology focuses on the collection of biospecimens, such as blood, sputum, urine, and feces, to search for viral agents or other pathogens and to help determine the potential route of transmission. Once specimens are collected, nucleic acid sequencing is often performed on the samples to deter mine the nature of the infectious agent, or to define how genetic variants may have spread within a population. Studies may also include serological analyses, in which antibodies in the blood that implicate previous infection are identified.


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