Why Were Some Countries Hit Harder by the H1N1 Flu Pandemic?

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A study explains the important risk factors of the mortality burden during the H1N1 flu pandemic through a literature search and mathematical models.

In 2009, the world was hit by the H1N1 flu pandemic. The virus was initially described in April of 2009 where the first death was confirmed in Mexico. Although the pandemic was contained by November of 2009, researchers are still working on seeking answers on the global impact that H1N1 had, and how to potentially improve the future responses of pandemics. Many studies around the world have aimed to analyze the mortality and risk factors that are associated with H1N1. Numerous studies have really focused on age, pregnancy, healthcare, climate, and treatment regarding H1N1. Some other studies have also looked at pollution, pandemic preparedness, travel and times of outbreaks in regarding to H1N1 and transmission.

Limitations of Previous Studies

Although these studies have immensely increased our knowledge about this flu, they also have some limitations. The first limitation is that these studies are conducted in local areas, which doesn’t reflect a clear picture of what’s going on worldwide. The second limitation is that the number of factors that are addressed in the study are very low and don’t address many factors at once. The third and final limitation is that the definition or method to calculate mortality of those with H1N1 has a high variation. Very few studies have ever addressed factors in a large number of countries. And no studies to date, have addressed the other limitations.

Kathleen Morales and her colleagues addressed these limitations and published their results in the journal BMC Infectious Diseases. Her team tried to address the limitations of what other studies around the world had overlooked. Their study looks at global H1N1 mortality rates within all countries and all the risk factors. In addition, they looked at how the seasons affected the H1N1 flu pandemic. Lastly, their group used standardized data collection techniques to ensure proper comparability throughout their study. They filled in gaps that were missing and helped identify factors that could explain why the H1N1 flu pandemic was high in some countries and low in others around the world.

Risk Factors for H1N1

Through a literature search, this group identified the risk factors that were worth evaluating for H1N1. Then they used a number of different mathematical models that eventually led them to explain the differences in mortality due to H1N1. They ran 16 models to make sure that their results were reproducible and robust enough to draw strong conclusions from. To be able to conclusively state the role that each factor (age, latitude, other flu in the region, air pollution, etc.) had on the pandemic mortality, this group compared the differences between all mathematical models and took into consideration the effect-size for each one of the factors.

They found that that the following factors, from the most significant difference between countries to the least significant difference, were the factors that explained the H1N1 flu pandemic mortality around the world the best:

  • age of the population,
  • latitude of the country,
  • other flus present in the population during the H1N1 flu pandemic,
  • other flus present before the H1N1 flu pandemic,
  • air pollution, and
  • the presence of other infections

Interestingly, obesity, distribution of vaccines, and travel did not have an effect on the global H1N1 flu pandemic mortality differences.

This study is the first of its kind to take a large number of risk factor into consideration and use a high number of mathematical models to conclusively explain why some countries around the world were affected much more significantly than others by the H1N1 flu pandemic.

Higher Mortality Rate in Countries with High Population of Young People

This study concluded that countries with a high population of young people had significantly higher H1N1 flu pandemic mortality rate during the outbreak in 2009. Furthermore, the cohabitation of different flu viruses that were ongoing during the pandemic and at the immediate time preceding the pandemic was also significantly associated with a greater mortality rate.

By taking into account the limitations of previous data and research done on the H1N1 flu pandemic, this study determined that other assessments of the 2009 H1N1 mortality factors could have led to many false positive results. Moving forward, this study also provides a way to assess future risk factors in other pandemics so that we can decrease the number of false positive information concerning mortality rates and what factors play a role in this.

Written by Ingrid Qemo, BSc

Source: Morales, K.F., Paget, J., and Spreeuwenberg, P. (2017) Possible explanations for why some countries were harder hit by the pandemic influenza virus in 2009 – a global mortality impact modeling study. BMC Infectious Diseases 17:642



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