Individuals who have recovered from a particular virus are said to have acquired antibodies spacific to the disease. In the case of the flu virus, the vaccine is believed to be needed every year due to the appearance of as called new strains. The flu virus appears to change over and over again, and the yearly vaccine may protect against several strains as predicted by world health organisation. The cost per year is not known.
Vaccines are reported to contain a small amount of the virus which is described as dead or weakened. The virus is said to have been grown on human or animal cells, sometimes eggs, and may contain other ingredients, mainly thought to be complacencies.