Immunization Issues

Understanding Vaccine Research Terms

Updated: Octubre 22, 2007

Vaccine research concerns various scientific disciplines, such as epidemiology, genetics, immunology, and statistics. The terms used in vaccine research are sometimes technical and hard to understand for the lay reader. Below is a list of common terms used in the scientific literature related to vaccines and immunizations.

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Absolute risk: The incidence of a disease in a population. It indicates the magnitude of the risk in a group of people with a certain exposure to a disease. For example, more than 95% of susceptible household contacts to a case of chickenpox will develop chickenpox.

Acellular vaccine: Vaccines composed of antigens of a pathogen such as the acellular pertussis vaccine.

Adjuvant: Substances that, when mixed with an antigen, increase the immune response, such as aluminum salts.

Adverse event: Something untoward that occurs at the time of or immediately following an event, such as an immunization. Adverse events may or may not be caused by the event.

Antibody: A molecule produced by some cells in the body that binds to a specific antigen during an immune response. An antibody can bind to a toxin, a virus or a bacterium, preventing them from doing harm.

Antigen: A chemical substance that induces an immune responsiveness after a latent period (days to weeks). For example, there is a protein antigen on the surface of the influenza virus called hemagglutinin which induces protective antibody to be made after infection or vaccination.

Attack rate (or case rate): The increasing number of new cases during an epidemic—usually expressed as percentage.

Attributable risk: The amount or proportion of disease incidence (or disease risk) that can be attributed to a specific exposure.


Bias: A systematic error in the design, conduct or analysis of a study that results in a mistaken estimate of that which is being measured. This research use of this word is in contrast to the non research use of the word to imply a conscious attempt to distort findings.


Case-control study: A study that compares individuals affected by a disease with a comparable group of people who do not have that disease, to seek possible causes or associations.

Case-fatality rate: The percentage of persons diagnosed as having a disease who die as a result of that disease within a particular period of time.

Clinical Trial: A research study to determine whether a vaccine works. Clinical trials answer questions about the safety and effectiveness of vaccines (and other type of drugs and treatments).

Cohort study: A study of a group of people, or cohort, followed over time to see how some disease develops.

Combination vaccine: A vaccine against more than one pathogen, that is given in a single dose, such as the measles, mumps and rubella vaccine.

Community immunity (or herd immunity): The indirect protection against a disease or infectious agent that unimmunized persons obtain by being surrounded by immunized persons.

Confidence interval: In a measurement of a result in a sample of a larger population, the confidence interval estimates the range within which the real value probably lies.

Confounders: Other factors or explanations that may affect a result or conclusion.

Conjugate vaccine: a vaccine where the antigen(s) is linked to another molecule, enhancing the immune response. Conjugation of a fragment of the Haemophilus influenzae type b capsule to a protein makes it possible to immunize young children who cannot respond to the fragment of the capsule alone.

Control group: A group of animals or people used in research as an untreated standard for comparison, to see how they differ from an experimental group—such as people receiving a vaccine.

Correlation: The extent to which two or more variables in an association are related. For example, the extent by which disease incidence changes in response to immunization rates.

Cost-benefit: In vaccine economics, cost-benefit analyses include all the costs and benefits that might arise from an immunization program up to a particular time.

Cost-effectiveness: In vaccine economics, cost effectiveness analyses take into account the costs and savings that result from an immunization program using a predefined unit of health outcome—for example, lives saved or cases of the disease avoided.


Disease carrier: A person or animal that harbors a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection.

Double-blinded study: see placebo.


Effectiveness: The extent to which a vaccine achieves its intended purpose—for example, a vaccine protects against a disease, such as pneumonia.

Efficacy: The extent to which a vaccine produces a specific outcome, such as the prevention of influenza.

Efficiency: A measure of effectiveness. Efficiency refers to the production of the desired effects or results with minimum waste of time, money, effort, or skill. For example, an efficient vaccine delivery system would allow immunizing a population without spending excess money or efforts.

Endemic: The constant presence of a disease in a community or region.

Environmental factors: External factors such as overcrowding or poor sanitation that contribute to the development of disease. These interact with genetic factors.

Epidemic: The occurrence in a community or region of cases of an illness in excess of what is normally expected, such as the recent polio epidemics in countries that had not seen a case in decades.

Epidemiology: The study of how disease is distributed in a population and of the factors that influence or determine this distribution. Epidemiology helps to identify the causes and risk factors of a disease in a community.

Etiology: The cause(s) of disease.

Experimental group: The treated group in a research study. For example, the group that receives the vaccine that is being tested, as opposed to the control group which receives a placebo or no treatment.


False negative: The failure to find a result or effect when there is one. For example, a false negative could be the false indication that a vaccine is not working when it really is working.

False positive: Finding a result or effect when it does not exist. For example, a false positive could be the false indication that a vaccine is working when it really is not working.


Genetics: The branch of science concerned with the biology of inheritance and its consequences.

Genetic factors: Inherited factors that contribute to the development of disease (or immunity to the disease).

Genetic predisposition: An inherited susceptibility to disease, which may be activated under certain conditions.


Health outcome: A measure of whether or not a patient benefits from a vaccination—for example, a health outcome may be expressed in the number of cases of measles prevented or lives saved through measles immunizations.


Immunity: The resistance against a disease, often measured by the presence of antibodies against the pathogen which causes the disease.

Inactivated vaccine: A vaccine made from pathogens that have been killed, such as the inactivated polio vaccine.

Incidence: The number of new cases of a disease during a specified period in a specified population. Incidence is usually expressed as cases per 1,000 or per 100,000 of the population per year. For example, between 1990-2002, the incidence of acute hepatitis B declined 67%, from 8.5 per 100,000 population (21,102 total cases reported) to 2.8 per 100,000 population (8,064 total cases reported).

Incubation period: The time interval between exposure to a pathogen and the first symptoms of the disease.

Intussusception: A serious and potentially life-threatening condition that occurs when one portion of the intestine telescopes into another portion, causing intestinal obstruction and, if untreated, a portion of the intestine can die. The most common site is where the small intestine joins the large intestine. The disease is most common among young infants. While at times the cause is a tumor or enlarged lymph nodes, it mostly occurs spontaneously. It has only rarely been associated with rotavirus infections but one rotavirus vaccine, Rotashield®, was shown to cause intussusception at a rate of about one in 10,000 infants, usually within 3-14 dsays after reciept of the first dose.


Live attenuated vaccine: The virulence of the pathogen has been reduced but the pathogen retains the ability to induce immunity. For example, live attenuated measles virus vaccines.


Meta-analysis: An analysis that looks at the results combining several studies.

Morbidity: The proportion that becomes sick.

Mortality: The proportion that dies.


Observational study: A study that simply observes and describes, offering clues but not a positive determination of cause and effect.

Odds ratio: A way of comparing whether the probability of a certain event is the same for two groups—for example, is the probability of developing a disease the same for vaccinated children as for unvaccinated children. The odds of acquiring a disease are calculated as the number of cases divided by the number of non-cases. An odds ratio of 1 implies that both groups are equally likely to develop the disease; an odds ratio greater than one implies that the vaccinated group is more likely to develop the disease; an odds ratio less than one implies that the vaccinated group is less likely to develop the disease. Odds ratios are usually calculated as a range. For example an odds ratio that goes from less than one to more than one implies no association but a ratio that goes from more than one to a higher number implies an association.

Outbreak: A cluster of cases of a disease.


Pandemic: A disease affecting the population of an extensive region, such as a world-wide outbreak. Pathogen: An infectious agent, such as a bacterium or a virus that causes a disease.

Person-years: The product of the number of years times the number of members of a population who have been affected by a certain condition. For example, the number of years of paralysis a group of polio survivors experienced.

Phase I, II, III and IV: Clinical trials are conducted in phases. Phases I, II and III describe the regulatory steps in vaccine development before the vaccine is licensed. Phase IV refers to studies after the vaccine is licensed by the FDA (for detailed information about each of these phases, visit the CBER site).

Placebo: An inert compound identical in appearance to material being tested in experimental research, which may or may not be known to the investigator and/or patient, administered to distinguish between drug action and suggestive effect of the material under study. For example, in a case-control study, a control group might receive saltwater—the placebo—instead of the actual vaccine. If neither the person giving the vaccine nor the person receiving the vaccine knows whether it is vaccine or placebo the study is said to be “double blinded”.

Prevalence: The total number of cases of a disease existing in a given population at a specific period of time (period prevalence) or at a particular moment in time (point prevalence). For example, the prevalence of Hepatitis A infection is higher among Native Americans than among other ethnic groups in the United States.

P value: The probability that an observed result or effect could have occurred by chance if there had actually been no real effect. See “statistical significance”.


Randomized trials: In clinical trials, when the selection of individuals into the experimental and control groups is done by some random method that reduces the chances of bias.

Range: A measure of spread—for example, the spread between the highest and lowest values in a distribution.

Rate: The proportion of a disease in a group during a particular time period, with a numerator and a denominator. For example, the rate of a disease can be “12 cases per 100,000 population per year”.

Recombinant vaccine: Vaccine produced by the introduction of a piece of genetic material into another cell which then produces the antigen. For example the hepatitis B surface antigen is manufactured in yeast cells for preparation of vaccine.

Relative risk (or risk ratio): A comparison of two morbidity or mortality rates by calculating the ratio of one to the other. Relative risk (RR) is equal to the risk in those exposed divided by the risk in those not exposed. If RR is equal to 1, then the risk in exposed persons is the same in non exposed persons (therefore, the disease is not related with the alleged exposure); if RR is greater than 1, then the risk is greater in the exposed person (thus a causal association may exist); if RR is less than 1, then the risk is less in the exposed person (that means a “protective effect” exists). For example, this last case can be observed in people who are given an effective vaccine—“exposed” to the vaccine.

Resistance: The natural or acquired ability to resist the effects of an antagonistic agent, such as a pathogen.


Sample: A part of a population that is used to represent the whole population.

Selection bias: A non random selection of study subjects that could have influenced the results.

Statistical significance: In an experiment or a clinical trial, statistical significance means that there is only a small probability that the same result could have been found by chance alone. For example, p<0.05 means that the chance of the outcome having occurred by chance was less than 5% (but also means that the outcome could have been found by chance 5% of the time).

Surrogate Measures: These are outcomes—other than the number of cases of disease prevented—that are used to estimate effectiveness. For example, a rise in antibody after immunization to a level expected to protect against disease.

Susceptible: The lack of resistance against a particular infection or disease; the individual is capable of contracting the disease when exposed to the pathogen (such as a virus or bacterium).


Toxoid vaccine: Clostridium tetani and Corynebacterium diphtheriae are bacteria that cause disease because they make very potent toxins. Tetanus and diphtheria vaccines contain inactivated toxins. These toxoids are no longer toxic but they induce antibodies that inactivate the toxin.


Virulence: The ability of a virus or bacterium to invade and cause illness; the degree of disease severity the pathogen is able to cause—usually expressed in case-fatality rates.


Wild type: As it occurs in nature (as opposed to vaccine-type virus—the weakened or attenuated type in a vaccine).


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  • Cohen V, and Cope L (2001). News & Numbers (2nd ed.). Ames, IA: Iowa State University Press.
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  • Stedman’s Online Medical Dictionary.