Two glycoproteins called "hemagglutinin" and "neuraminidase" protrude from the membrane surface of influenza viruses.  Hemagglutinin is responsible for the first step of infection, the binding of the viras to the host cell (the cell which the virus invades and in which proliferates).  It does not bind to any part of the cell; it "snipes" a sugar chain sialic acid having a specific structure on the cell surface.  The influenza virus causes infection by first binding to this sugar chain called sialic acid.

Antibodies that are produced in the body by vaccines prevent viral infection of host cells by binding to this hemagglutinin on the viral surface.  The main component of currently used inactivated vaccines is hemagglutinin.

Neuraminidase is a substance that releases the virus from the cell by cutting off the sialic acid when the virus itself leaves the infected cell.  Several anti-influenza agents prevent the spread of viral infection from one cell to the next by inhibiting neuraminidase.  Neuraminidase is currently the focus of attention as a target for anti-virus agents.

Influenza viruses are classified into three types: type A, type B, and type C.  The types with repeated prevalence among humans and are considered a problem are types A and B.  Type A in particular frequently repeats mutation and often becomes a popular topic.  Current vaccines are typically made from a blend of three kinds of influenza virus vaccines by predicting the type that may prevail that year based on prevalence prediction investigation of type A and B influenza viruses (for example, the vaccine for 2004 was a vaccine containing hemagglutinin proteins of three kinds of viruses: type A Soviet, type A Hong Kong and type B).

Vaccines are made as described above, but their protective effect is not perfect.  It is said, however, that vaccination lessens symptoms such as high fever and reduces hospitalization or death from complications.  Elderly people and people with pre-existing diseases (such as respiratory disease, chronic cardiac failure, diabetes, and renal failure) are especially vulnerable to influenza becoming severe, and vaccination is desirable for them.

There is debate as to why there isn't a 100% protective effect.  Several reasons exist for this, e.g., action on airway mucosa immunity that is thought to play a large role in protection against viral infection is weak because the vaccination is subcutaneous and even if the type of virus for a given year is correctly predicted and a person is vaccinated accordingly, the vaccine may react with a previously infected or vaccinated virus, thus weakening immunostimulation against the actual virus.

There are currently advances in researches for new vaccines such as intranasal administration of attenuated live vaccines to intensify prophylactic effect development of adjuvants (auxiliary agents to intensify the effect of vaccines) and artificial membrane vaccines.