PANDEMIC INFLUENZA A (H1N1)

Pandemic influenza A (H1N1)



The term influenza is thought to be derived from the Italian word for influence. Influenza viruses belong to the Orthomyxoviridae family of RNA viruses, which are classified into three groups, known as Types A, B, and C. Type A viruses cause infections in birds, humans, and some other mammals. Type B viruses typically infect only humans, while type C viruses, which are rare, infect both humans and pigs.

Influenza is the virus which is further classified into subtypes based on structural proteins projecting from their surfaces like spikes. One type of spike contains the glycoprotein hemagglutinin (H), an enzyme that helps the virus attach to and penetrate host cells. The other type of spike contains the enzyme neuraminidase (N), which helps the virus enter the host cell and to exit after replication. There are 16 different types of H and 9 different types of N known, creating 144 possible combinations. Subtypes of viruses are identified by their particular combination of H and N proteins. According to the U.S. Centers for Disease Control and Prevention (CDC), human infections have been caused mainly by influenza. A virus containing the H1, H2, and H3 and N1 and N2 proteins, although only some subtypes (that is, H1N1, H1N2, and H3N2) are currently in general circulation among people.

Animals (including humans) can acquire protective immunities against influenza viruses by developing antibodies against these H and N proteins. Such antibodies are also the basis of influenza vaccines. However, mutations in viruses can cause chemical alterations in these proteins, yielding new strains of a virus that existing antibodies do not recognize. As a result, relatively minor variations (antigenic drift) in a virus can leave people vulnerable to getting the flu more than once. Consequently, influenza A and B viruses cause seasonal flu epidemics every year. Because viruses are constantly changing, there is no universally effective vaccine for influenza. Therefore, yearly seasonal flu vaccinations must be updated to match changes in the currently circulating flu viruses. (Jamieson, et. al, 2009)

Abrupt, major mutations (antigenic shift) happen only occasionally, but may produce new strains to which no one has immunity, creating the potential for a pandemic. According to the World Health Organization (WHO), three such pandemics occurred during the 20th century. In 1918 to 1919, an H1N1 virus referred to as “Spanish flu” caused an estimated one third of the world's population to become sick, killing 40 to 50 million people worldwide, many within days of infection. Subsequent pandemics were both milder and kept in check by vaccines developed to fight the viruses. In 1957, an H2N2 virus called “Asian flu” killed an estimated 2 million people, and in 1968 to 1969, an H3N2 virus, the “Hong Kong flu,” killed an estimated 1 million.

Surveillance of outbreaks of the H1N1 2009 pandemic influenza in different regions of the world provides information to draw some tentative conclusions about the likely evolution of the pandemic in the coming months. The countries of the Northern Hemisphere were most likely to suffer because it was estimated that a second wave of pandemic will ...