Pandemic influenza is an epidemic of infectious disease that can spread among human population across the world. Its first appearance was in 1918 (Spanish Influenza H1N1), where it caused an epizootic outbreak in swine causing the most devastating and terrifying pandemic in history. Pandemic influenza H1N1 was then re-emerged again in 2009 (Swine Flu). According to the World Health Organization (WHO) several factors must be present to be considered a pandemic such as: infected droplets released into the air and breathed in by others or close contact with infected pigs.
Swine influenza is caused by Influenza A virus, which belongs to family orthomyxoviridae. They are pleomorphic-enveloped viruses therefore Influenza viruses are named based on the major surface antigens: Hemagglutinins (HA) and Neuraminidase (N). Neuraminidase is an enzyme that is responsible for releasing the progeny virus from infected cells, by cleaving sugar molecules that bind to the virus. By contrast, hemagglutinin is a lectin that mediates binding of the virus to the host cell and entry of the viral genomes into the host cell. Overall, there are at least 16 different HA subtypes named H1 through H16. The first three hemagglutinins, H1, H2, and H3 are the main HA subtypes present in Influenza viruses.
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It is important to understand the emergence of 2009 pandemic influenza, for which one has to recognize its genetic origins. Influenza A viruses are composed of a protein-studded coat that surrounds a genome made up of eight single-stranded RNA segments (PB2, PB1, PA, HA, NP, NA, MP and NS). These RNA segments of pandemic 2009 have resulted from co-mingling of genomes from two influenza viruses- N. American H1N1 and Eurasian swine H1N1. This concept helps to explain how viral genetic re-assortment has resulted in the emergence of the pandemic strain, with a mixture of swine, human and avian influenza viruses.
One would predict that changes in the genetic makeup of this virus requires us to develop new vaccines on an annual basis to attempt to prevent this virus especially in those that are elderly or chronically ill, since for them, influenza can be extremely fatal. According to CDC (Centers for disease control and prevention) epidemiological data indicates that elder people (>65 years) tend to be less affected by H1N1 virus. Perhaps this explains the age-related pre-existing immunity.
Figure 1- shows age groups most affected by the H1N1 influenza reported to CDC in the United States from April 15 to July 24, 2009.As can be seen, the number of reported cases per 100,000 population was highest among individuals five to twenty-four years of age group (26.7 per 100,000).
Recent research on the crystal structure of the HA from the pandemic virus A/California/04/2009 has revealed the similarity of the Sa antigenic site located on the hemagglutinin molecule, in particular, of the 2009 H1N1 virus with the early 1918 human H1 viruses. Exposure to older viruses circulating from 1918 through the 1930’s or 1940’s are likely elicited 2D1-like antibodies that can cross-react with the 2009 viral protein HA. As a result individuals exposed to these viruses, now over the age of 65 have some protection from swine flu by antibodies originally elicited more than 50 years ago. Further research has shown that 2D1- antibody can bind to identical Sa antigenic site in the 1918 and 2009 pandemic and cross-neutralize both 1918 and 2009 pandemic viruses in vivo. Perhaps this provides a strong explanation for the age-related pre-existing immunity and the low levels of mortality rate in elder people, in the current H1N1 pandemic.
Today in this 21st century, new advanced vaccines and antiviral drugs have been introduced to prevent this virus especially in those who are unable to produce immunity against Influenza viruses. Compare to the third world where only small proportion of people were fortunate to have vaccines as a safety precaution. Even then, 3% of the world’s population (1.8 billion people) at the timedied of the disease.
Nowadays scientists and many health organizations such as CDC or WHO are intensely looking in to whether or not the same virus will strike again. They have the knowledge and the discipline to get the world ready. Recently, CDC recommended a seasonal flu vaccination (trivalent seasonal influenza vaccine) as the most important step in protection against influenza viruses. This seasonal influenza vaccination has been recommended for every individual from the age of 6months onwards as not only it protects against H3N2 virus but also Influenza B and H1N1 Influenza A virus. This proves that seasonal influenza vaccine can still be used to protect against various viruses including 2009 H1N1 Influenza and H3N2.
Recent research on neutralising influenza antibodies also proves that seasonal influenza vaccine can lead to increase in the production of neutralizing antibodies up to 80%. But vaccination with plasmid DNA encoding H1N1 influenza (HA) and boosting with seasonal vaccine can stimulate a greater than 50 fold increase in neutralizing antibody than that produced by one does of seasonal vaccine alone or DNA alone.
The future impact of the 2009 H1N1 influenza virus is not yet possible to predict as to which strain will cause the next pandemic or epidemic. On the one hand, the 2009 H1N1 influenza virus is still expected to continue to circulate as a seasonal influenza strain. Due to this, more people will now develop immunity to this current version of the virus. Despite of that, the World Health Organization (WHO) has warned many countries including Africa and Asia to prepare for more devastating flu epidemics in the future. This concern has been promoted due to the possible mixture of swine flu viruses with seasonal strains during the winter months and can transform in impulsive ways. The best possible way for an individual to protect against 2009 H1N1 influenza virus and other influenza viruses is through defensive measures such as covering mouth when sneezing or coughing, good hygiene and vaccinations against H1N1 influenza virus when possible.
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