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Abstract
The influenza virus A, B and C causes disease in humans, birds and animals. The influenza type A causes moderate to severe illness in all age groups of humans while the illness caused by type B is of milder and it is primarily affects children. Among many subtypes of influenza A viruses, currently influenza A(H1N1) and A(H3N2) subtypes are circulating among humans. Influenza is a serious public health problem that causes severe illnesses and deaths for higher risk populations. Influenza virus is characterized by frequent mutations—antigenic drifts (minor antigenic change, both A and B) and antigenic shifts (major antigenic change, only A). The current human pandemic A/H1N1 is an example of antigenic shift. It slowly established circulation globally; subsequently endemic/seasonal viruses in both hemi-spheres are H3N2 and H1N1. The novel influenza A (H1N1) 2009 virus was first identified by US Centers for Disease Control and Prevention (US CDC) on April 17, 2009 in samples from two Californian children. As of August 2010, 18,000 people had died globally due to the pandemic flu. The illness rates were highest in children and young adults (20–40% of the population), the hospitalization rates highest in children below the age of one. The case fatality rates varied tremendously and were estimated to be between 0.0004–1.5% (0.05% in US, 0.025% in UK, lowest in children). The most effective way to prevent the disease or severe outcomes from the illness is vaccination. The trivalent inactivated vaccines (TIV) are of three types: whole virus, split-product, subunit surface-antigen formulations and they are grown in embryonated hen's eggs. Whole-virus vaccines, because of adverse reactions, especially in children, are not currently used. Most influenza vaccines are split-product vaccines, produced from detergent treated, highly purified influenza virus or surface-antigen vaccines containing purified hemagglutinin and neuraminidase.
