Lecture 19 - Viral epidemiology Flashcards
Describe HIV distribution.
HIV distribution
HIV has different subtypes, Subtype B is the most common in Europe and America. However, in Africa subtype B is very rare and there are is much more diversity. This suggests that HIV originated in Africa and was spread to America in the 1960s. In former soviet union countries subtype A is found which within Africa is found in certain areas suggesting a different transmission event?????
Type z is very common in southern Africa and parts of India implicating a different event.
Through looking at this kind of data can look at how a virus spread over time.
Discuss the epidemiology of West Nile Virus
Migration Patterns: Studies have suggested that WNV strains found in North America are closely related to strains circulating in the Middle East, Europe, and Africa. It’s believed that migratory birds play a significant role in the long-distance movement of WNV, introducing new strains to different regions.
Evolutionary Dynamics: Genetic analyses of WNV strains have provided insights into the evolutionary history and spread of the virus. By examining the genetic diversity of WNV isolates from different regions and time periods, researchers can infer patterns of viral migration and adaptation.
Discuss the epidemiology of Zika
Emergence in Africa: Zika virus was first identified in Uganda in 1947 in a monkey and later in humans in 1952. It circulated primarily in Africa and Asia with sporadic outbreaks documented.
Spread to Asia: Zika virus spread to Asia, where it caused outbreaks in various countries throughout the latter half of the 20th century and early 21st century. During this time, it remained relatively understudied and considered a mild illness.
Introduction to the Americas: The most significant shift in Zika virus epidemiology occurred with its introduction to the Americas. The virus was first detected outside of Africa and Asia in 2007 in the Yap Island of Micronesia. This marked the beginning of a series of outbreaks in the Pacific Islands.
Outbreak in the Americas: The most notable and widely publicized outbreak of Zika virus occurred in Brazil in 2015. The virus rapidly spread throughout South and Central America and the Caribbean, causing widespread concern due to its association with congenital Zika syndrome, including microcephaly in newborns.
Global Spread: Zika virus continued to spread to other regions, including the United States, where local transmission was reported in Florida and Texas. Cases were also reported in Southeast Asia, the Pacific Islands, and other parts of the world, facilitated by international travel and the presence of competent mosquito vectors.
Genetic Diversity: Genetic studies of Zika virus have revealed multiple lineages and genetic diversity within the virus. This diversity has implications for understanding the virus’s origins, transmission patterns, and potential for further spread.
Vector Dynamics: The spread of Zika virus is closely tied to the distribution and abundance of its primary mosquito vectors, Aedes aegypti and Aedes albopictus. Changes in environmental conditions, urbanization, and globalization have influenced the distribution and behavior of these vectors, impacting the transmission dynamics of Zika virus.
Public Health Response: The emergence of Zika virus as a global health threat prompted significant public health responses, including vector control measures, surveillance efforts, and research into vaccines and therapeutics.
What is meant by R-value.
R value for reproduction number.
For example if the R value is 7, 1 case leads to about 7 others on average such as in small pox
Its calculated by trying to estimate the amount of people an individual infects and find an average between individuals. Or using incidence data over time.
Pandemic influenza - R=2.5
Ebola - R = 1.5-2.8
How spread is also influenced by method of infection and amount of contact needed. For example in Ebola to infect someone you need to be in close contact (forms clusters in families etc.) whereas with influenza it can be transmitted to people at a large distance (doesn’t occur in clusters wide spread) so although they have similar R values they have very different distributions.
What equations are associated with epidemiology and herd immunity.
- Ps = proportion susceptible
- Pst = proportion susceptible at time t
- Rt = R0 x Pst
- Pr = 1 - proportion susceptible
- Prt = proportion resistant at time t
- HET = Herd Immunity threshold = Pr needed for R=1
- HET = (R0-1)/R0
- VE = vaccine efficacy = % vaccinated who are resistant
Proportion to vaccinate = HET/VE
What complicates herd immunity and infection rates in a population.
Reasons things are often not so simple:
Divided populations: When populations are divided, such as in the case of two islands, the proportion susceptible may decrease in the exposed population but remain the same in the unaffected group. A simple overall average may not reflect this difference.
Geographical separation: Geographical separation can affect the transmission dynamics of the disease, leading to different levels of susceptibility and resistance in different regions.
Burnout of the infection: If the proportion susceptible experiences a large drop due to natural infection or vaccination, the infection may burn out, meaning it will no longer spread efficiently within the population.
Superspreaders: Some individuals, known as superspreaders, have a disproportionately high number of secondary infections compared to the average. They can spread the disease within multiple groups and infect more people, complicating the dynamics of disease transmission and herd immunity.
