Preventing and Treating Influenza

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Vaccines

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Every year, WHO convenes two working groups, one for the northern hemisphere in February and one for the southern hemisphere in September, to predict which influenza subtypes are likely to be prevalent in the coming flu seasons so they can develop an appropriate flu vaccine. This task is quite difficult because virus subtype strains can shift spontaneously [see glossary for difference between strain and subtype, and between drift and shift]. When shifts occur the existing vaccine may no longer stimulate the appropriate immune response against the influenza virus strain. Nevertheless, vaccines are the primary tool for preventing influenza.

Flu vaccines are constituted by "split viruses"; in other words, subparticles of the influenza viruses that will stimulate antibody formation by the recipient. Typically, a flu vaccine is trivalent, which means it is designed to counteract three flu strains: two influenza A strains and one influenza B strain. The subtypes constituting the vaccine may be changed from year to year. Typically, one or two of the virus subtypes in the previous season's vaccine are replaced to reflect changes in the circulating influenza virus strains.

To acquire complete protection, the flu vaccine should administered before the seasonal flu season begins. When a person receives a flu vaccine (containing killed flu virus), his or her body produces antibodies in response to the injected virus antigens. Later, when that person is exposed to any of the flu strains that are present in the vaccine, these antibodies will neutralize these viruses and thus preventing infection. In effect, antibodies fasten to the virus's hemagglutinin (HA) structures and thereby prevent the flu virus from attaching to the sialic acid receptors of healthy cells and infecting them.

Vaccine development and marketing requires several months of clinical testing, as well as time for the distribution of the new vaccine. Thus, it is critical that, in the meantime, national and local health departments refine preparedness plans for meeting the challenges of seasonal and pandemic flu and stockpiling adequate antiviral medications (see below) to treat those who become ill.

Some people should not be vaccinated without first consulting a physician. They include:

People who have a severe allergy to chicken eggs.
People who have had a severe reaction to an influenza vaccination in the past.
People who developed Guillain-Barré syndrome (GBS) within 6 weeks of getting an influenza vaccine previously.
Children less than 6 months of age (influenza vaccine is not approved for use in this age group).
People who have a moderate or severe illness with a fever should wait to get vaccinated until their symptoms lessen.

Antiviral Medications

It is very important that people consult their doctors before taking any medication.

Currently, there are two classes of anti-influenza drugs - adamantanes and neuraiminidase inhibitors. Each class possesses two drugs (see below). Of the two classes, adamantanes are less expensive and more readily available than the neuraminidase inhibitors.

Adamantanes

Adamantanes interfere with virus replication. This class of drugs work against influenza A viruses but not influenza B viruses. Further, adamantanes are ineffective in preventing serious influenza-related complications such as bacterial or viral pneumonia. Currently, two specific drugs, amantadine and rimantadine, constitute the adamantane class:

Amantadine is approved for treating and preventing uncomplicated influenza A virus infections in adults and children one year or older.
Rimantadine has approval similar to amantadine, except it is not approved for treating infections in children. By and large, there has been no adequate evaluation of drugs in children less than one year old.

Adamantanes can be toxic to users and have serious side effects. Moreover, the virus can mutate and become resistant to adamantanes. Some influenza A virus strains (including H5N1) are resistant to amantadine and rimantadine so the Centers for Disease Control and Prevention did not recommend using those drugs to fight the flu during the 2008-2009 flu season.

Neuraminidase Inhibitors

The action of neuraminidase inhibitors is complex. Influenza virus surfaces support structures that are constituted by neuraminidase protein that act on the infected cell's wall to release new virus particles to the outside of the infected cell. The released viruses infect other cells and thus the infection spreads. By using drugs that inhibit the neuraminidase protein, virus particles are prevented from being released, and as a result the infectious process stops. However, people infected with the flu virus need to take the drug early in the illness in order to maximize the drug's effectiveness before the virus has had the time to infect many cells. The neuraminidase inhibitors class of drugs include zanamavir (Relenza) and oseltamivir (Tamiflu).

Zanamavir is approved for preventing both influenza A and B virus infections in people five years of age and older and for treating uncomplicated influenza virus infections in people seven years of age and older who have not had symptoms for more than two days.
Oseltamivir is approved for treating and preventing uncomplicated influenza virus infections in people one year of age or older who have not had symptoms for more than two days.

Unlike adamantanes, neuraminidase inhibitors act on both influenza A and influenza B viruses, have very little toxicity, and are far less likely to result in drug-resistant variants or strains.

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