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Flu also known as Influenza is a contagious viral disease that affects the respiratory system. It is caused by influenza viruses. It is highly infectious unpredictable disease that spreads though secretions of nose and lungs. Flu causes mild to severe illness and sometimes even leads to death. "According to U.S. CDC, in an average year, 5 to 20 percent of the U.S. population gets the flu, more than 200,000 people are hospitalized with seasonal flu-related complications and about 36,000 people die from flu-related causes.  "
Flu vaccination is one of the best ways to protect the community from the seasonal and pandemic flu effects. Pandemic flu is different from seasonal flu,  it is a global disease outbreak that usually occurs when a flu strain new to humans emerges and causes widespread illness. The pandemic flu is very dangerous because of newly originated strain to which humans have little pre-existing immunity and vaccines would probably not be available immediately in early stages of pandemics.2 The pandemic outbreaks have potential impact on society causing high levels of illness, death, economic loss and social disruption.
"Recently in 2009 a novel H1N1 virus emerged which became pandemic. It is estimated that in U.S., approximately 43-89 million persons became ill because of this pandemic H1N1. It also resulted in deaths among children, adults, pregnant and post-partum women.1"
On the other hand seasonal flu form occurs seasonally, usually in winter. Seasonal flu causes significant illness and in some cases death.
Flu vaccination is most effective way to control and prevent influenza virus infections and severe complications. It is especially important for younger children and people who are at high risk of catching infections. Flu vaccines are available as Flu shot of trivalent inactivated or killed virus (TIV) or Live Attenuated Influenza Vaccine (LAIV) as nasal spray2. However, it is impossible to prevent influenza by one time vaccination because Influenza viruses undergo changes from year to year and develop resistance making previously available vaccines ineffective. Therefore scientists make different flu vaccine every year. In addition the immunity developed from having the flu caused by one strain does not always offer protection against new strain. Immunity also declines over time after previous year's vaccination and at a point it may be too low to provide protection after year. Hence to combat with changing influenza viruses, vaccination is done every year. Getting seasonal flu vaccination offers protection that lasts throughout the year preventing infection and its complications.
The World Health Organization organizes meetings twice a year and recommends inclusion of specific virus strains in Influenza vaccine based on results of surveillance, laboratory and clinical studies, and the availability of vaccine virus strains. Then individual countries make their own decision about inclusion of virus strains in vaccines licensed in their country.
In U.S., each year, a panel of experts from agencies such as the FDA and the CDC's Advisory committee on Immunizations Practices (ACIP) studies the available data and decides which three strains of influenza viruses will most likely be active during the next flu season. The selection of vaccine strains for inclusion in seasonal flu vaccine is based on circulating virus strains, how they are spreading, and how well current vaccine strain protects against newly identified strains.  The ACIP makes written recommendations for administration of vaccines to children and adults. These recommendations include age for administration, doses, dosing interval, precautions and contraindications.  The seasonal flu vaccine for 2010-2011 offers protection against H3N2 virus, an influenza B virus and pandemic H1N1 virus that emerged in 2009.
Vaccine Shortage Issue:
There are many issues related to flu vaccines. Among many vaccine shortage is the most noticed every year. A close examination reveals that the shortage for vaccine is not one cause but several. Some of them include high risk of contamination in vaccine production, unpredictable consumer demand, and low profits along with lack of liability protection from costly lawsuits made many manufacturers out of flu vaccine business. 
Most of the companies stopped production of flu vaccine because the demand varies from year to year, as it is always unpredictable and once flu season passes away the remaining stock is useless because a new vaccine is required to deal with changing strains of virus  . According to 2003 report by Institute of Medicine, a unit of National Academy of Sciences, the companies producing vaccines dropped from 30 to 5 in year 2004.6 The companies producing injectable influenza vaccine dropped to two (Chiron & Aventis Pasteur) in year 2005.5 Production of flu virus vaccine is very complicated and expensive, but the government keeps price of flu vaccine low. Among the two firms Chiron was forced to shut down its UK plant because of quality control reasons. This resulted in shortage. Another problem is that each year, the U.S. Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee meets in spring identifies the flu virus strains to be protected by flu vaccine manufactured for that fall-winter flu season.5 This time gap between committee's decision and following flu season allows manufactures to make vaccines but the flu strains can undergo changes by the time vaccines are manufactured for flu season5. Thus it is difficult to determine for what strains of flu virus vaccine should be made which may end up with vaccine shortage for specific flu season.
"In 2003 committee voted to include the Panama flu virus in vaccine for the 2003-04 flu seasons, excluding Fujian flu mutant virus, a more virulent but less prevalent at that time. But the winter's flu outbreak consisted almost entirely of the Fujian strain. CDC without noticing it recommended widespread vaccination which did not protect the public against the prevalent flu strain. The CDC later admitted the vaccine had "no or low effectiveness against ILI (influenza-like illness).5"
Latest development in flu vaccine research:
The outbreak of pandemic flu has motivated increase in flu research The recent advancement in field of flu vaccine research is development of Universal flu vaccines. Scientist Dr. Sarah Gilbert and team at Oxford Jenner Institute developed universal flu vaccine that target proteins inside flu that are common across all strains and tested on humans infected with flu  . Gilbert used 11 healthy volunteers for her study and vaccinated and then infected them along with 11 non-vaccinated volunteers. Upon careful monitoring there has been dramatic increase in T-cell count in vaccinated subjects which play important role in producing immune response, protecting against viral infections.
Researchers of University of Adelaide, Dr. Darren Miller and his colleagues, have trialled a universal synthetic flu vaccine in mice which is another step closer to development of a universal flu vaccine. It is derived synthetically which does not require annual reformulation which would be advantageous to control and prevent flu. Dr. Miller used specific peptides derived from noses of mice to trigger an immune response to a tiny region of flu virus that is present in all influenza A and B viruses, which effectively neutralizes the virus.  " The studies have shown that test vaccine provided mice with 100% protection against a laboratory strain of H3N2 and 20% protection against a highly pathogenic bird flu virus.8 This positive response provides scope for further laboratory and clinical testing.
Economics of Flu vaccination:
Economic studies indicate that flu vaccination reduces healthcare, societal, and individual costs and also productive losses associated with influenza illness.  "A study of a larger population comparing persons aged 50--64 years with those aged â‰¥65 years estimated the cost-effectiveness of influenza vaccination to be $28,000 per QALY saved (in 2000 dollars) in persons aged 50--64 years compared with $980 per QALY saved among persons aged â‰¥65 years (393).Two studies in the United States indicated that vaccination can reduce both direct medical costs and indirect costs from work absenteeism and reduced productivity (79,394)."
Latest flu pandemics and their effect on the Regulatory world:
The pandemic flu is unpredictable, spreads rapidly world wide affecting large proportions of the human population. There have been three influenza pandemics of which the recent one was the 2009 flu pandemic. This occurred first in Mexico, March 2009 caused by pandemic H1N1/09 virus also referred to as swine flu. It is subtype of Influenza A virus. On 10th of August 2010, the Director General of WHO announced that H1N1 pandemic virus has moved into post-pandemic period.  CDC estimated that nearly a 1 million cases of 2009 H1N1 pandemic flu had occurred in United States. The pandemic H1N1 flu cases doubled in many countries from mid-June 2009 to early July 2009. According to WHO statistics, 18,000 deaths were reported because of H1N1.
This outbreak resulted in extraordinary illness throughout the world with increased demand for vaccination against the swine flu virus in a short time. The pandemics put intense burden on the regulatory authorities as vaccines have to be made available globally to meet the increased demands. Regulatory agencies adopted procedures for accelerated approval of vaccines against swine flu. In U.S., FDA expanded its capacity to expedite development, evaluation and licensing of additional flu vaccines and manufacturing facilities to meet pandemic preparedness needs  . CBER has issued guidelines encouraging vaccine manufacturers to explore cell-culture and recombinant techniques, and to incorporate biological integrators, such as immune response, into their product-development designs.  The outbreak of pandemic flu also has increased collaboration among foreign regulatory agencies to share vaccine safety information and experiences and mount a coordinated response to the emergency.