Pandemic Flu: Terms and Definitions

Antibody

A protein substance produced by the immune system in response to a specific antigen that will identify and neutralize foreign material like bacteria and viruses, thus forming the basis of immunity. Each antibody recognizes a specific antigen unique to its target.

Antigen

A substance that when introduced into the body stimulates the production of an antibody.

Antigenic Drift

A mechanism for variation by viruses that involves the accumulation of mutations within the antibody-binding sites so that the resulting viruses cannot be inhibited well by antibodies against previous strains making it easier for them to spread throughout a partially immune population. Antigenic drift occurs in both influenza A and influenza B viruses.

Antigenic Shift

A sudden shift in the antigenicity of a virus resulting from the recombination of the genomes of two viral strains. Antigenic shift is seen only with influenza A viruses. It results usually from the replacement of the hemagglutinin (the viral attachment protein that also mediates the entry of the virus into the cell) with a novel subtype that has not been present in human influenza viruses for a long time. The source of these new genes is the large reservoir of influenza viruses in waterfowl. The consequences of the introduction of a new hemagglutinin into human viruses is usually a pandemic, or a worldwide epidemic.

Bacteria

Single-celled microorganisms which can exist either as independent (free-living) organisms or as parasites (dependent upon another organism for life).

Endemic

The constant presence of a disease or infectious agent within a given geographic area or population group; may also refer to the usual prevalence of a given disease within such area or group; A disease that occurs continuously or in expected cycles in a population, with a certain number of cases expected for a given period.

Epidemic

The occurrence in a community or region of cases of an illness, specific health-related behavior, or other health-related events clearly in excess of normal expectancy. The community or region and the period in which the cases occur are specified precisely. The number of cases indicating the presence of an epidemic varies according to the agent, size, and type of population exposed; previous experience or lack of exposure to the disease; and time and place of occurrence. Epidemicity is thus relative to usual frequency of the disease in the same area, among the specified population, at the same season of the year. A single case of a communicable disease long absent from a population or first invasion by a disease not previously recognized in that area requires immediate reporting and full field investigation; two cases of such a disease associated in time and place may be sufficient evidence to be considered an epidemic. The word may be used also to describe outbreaks of disease in animal or plant populations.

Epidemiology

The study of the distribution and determinants of health-related states and events in a population.

Immune

Protected from or resistant to a disease or infection by a pathogenic organism as a result of the development of antibodies or cell mediated immunity.

Immunity

The state of being immune to or protected from a disease.

Immunity, acquired

Immunity resulting from the development of active or passive immunity, as opposed to natural or innate immunity.

Immunity, active

Immunity resulting from the development within the body of antibodies or sensitized T-lymphocytes that neutralize or destroy the infective agent.

Immunity, herd

Immune protection through vaccination of a portion of the population, which may reduce the spread of a disease by limiting the number of potential hosts for the pathogen.

Immunization

The process of creating immunity to a specific disease in an individual.

Infection

The state or condition in which the body (or part of the body) is invaded by an infectious agent (e.g., a bacterium, fungus or virus), which multiplies and produces an injurious effect (active infection).

Isolation

Restricting the activities of people who are infected with a communicable disease to prevent the transmission to those who have not been infected. (Isolate = sick)

Outbreak

An epidemic limited to localized increase in the incidence of a disease, e.g., in a village, town, or closed institution; upsurge is sometimes used as a euphemism for outbreak; The sudden increase in the incidence of a disease or condition in a specific area.

Pandemic

An epidemic occurring over a very wide area, crossing international boundaries and usually affecting a large number of people.

Toxin

One of a number of poisons produced by certain plants, animals, and bacteria.

The term "toxin" is frequently used to refer specifically to a particular protein produced by some higher plants, animals and pathogenic (disease-causing) bacteria. A toxin typically has a high molecular weight (as compared to a simple chemical poison), is antigenic (elicits an antibody response), and is highly poisonous to living creatures.

Virus

A microorganism smaller than a bacteria, which cannot grow or reproduce apart from a living cell. A virus invades living cells and uses their chemical machinery to keep itself alive and to replicate itself. It may reproduce with fidelity or with errors (mutations)-this ability to mutate is responsible for the ability of some viruses to change slightly in each infected person, making treatment more difficult. Viruses cause many common human infections, and are also responsible for a bevy of rare diseases.

Viruses may contain either DNA or RNA as their genetic material. Herpes simplex virus and the hepatitis- B virus are DNA viruses. RNA viruses include HIV and the hepatitis C virus.

Researchers have grouped viruses together into several major families, based on their shape, behavior, and other characteristics. These include the herpesviruses, adenoviruses, papovaviruses (papilloma viruses), hepadnaviruses, poxviruses, and parvoviruses among the DNA viruses.

On the RNA virus side, major families include the picornaviruses (including the rhinoviruses), calciviruses, paramyxoviruses, orthomyxoviruses, rhabdoviruses, filoviruses, bornaviruses, and retroviruses.

There are dozens of smaller virus families within these major classifications. Many viruses are host-specific, causing disease in humans or specific animals only.

Vaccination

Inoculation with any vaccine or toxoid to establish resistance to a specific infectious disease.

Vaccine

A suspension of infectious agents or some part of them, given for the purpose of establishing resistance to an infectious disease.

Vaccine, live attenuated

Live vaccines are derived from “wild,” or disease-causing, virus or bacteria. These wild viruses or bacteria are attenuated, or weakened, in a laboratory, usually by repeated culturing. For example, the measles vaccine used today was isolated from a child with measles disease in 1954. Almost 10 years of serial passage on tissue culture media was required to transform the wild virus into vaccine virus.

In order to produce an immune response, live attenuated vaccines must replicate (grow) in the vaccinated person.

A relatively small dose of virus or bacteria is given, which replicates in the body and creates enough virus to stimulate an immune response. Anything that either damages the live organism in the vial (e.g., heat, light), or interferes with replication of the organism in the body (circulating antibody) can cause the vaccine to be ineffective. Although live attenuated vaccines replicate, they usually do not cause disease, such as may occur with the natural (“wild”) organism. When a live attenuated vaccine does cause “disease,” it is usually much milder than the natural disease, and is referred to as an adverse reaction.

Vaccine, inactive

These vaccines are produced by growing the bacteria or virus in culture media, then inactivating it with heat and/or chemicals (usually formalin). In the case of fractional vaccines, the organism is further treated to purify only those components to be included in the vaccine (e.g., the polysaccharide capsule of pneumococcus).

Inactivated vaccines are not alive and cannot replicate.

The entire dose of antigen is administered in the injection. These vaccines cannot cause disease from infection, even in an immunodeficient person.

Unlike live antigens, inactivated antigens are usually not affected by circulating antibody. Inactivated vaccines may be given when antibody is present in the blood (e.g., in infancy, or following receipt of antibody-containing blood products).

Inactivated vaccines always require multiple doses. In general, the first dose does not produce protective immunity, but only “primes” the immune system. A protective immune response develops after the second or third dose. In contrast to live vaccines, in which the immune response closely resembles natural infection, the immune response to an inactivated vaccine is mostly humoral. Little or no cellular immunity results.

Antibody titers against inactivated antigens fall diminish with time. As a result, some inactivated vaccines may require periodic supplemental doses to increase, or “boost,” antibody titers.