VACCINATION Strictly speaking, vaccination refers to inoculation (from Latin in oculus, into a bud) with vaccinia virus against smallpox. Nowadays the word is broadly used synonymously with procedures for immunization against all infectious disease. The original use of the word was confined to vaccination against smallpox. This was the first method of preventing a lethal disease by immunizing humans. It was introduced by Edward Jenner (1749–1823) and described by him in An Inquiry into the Cause and Effects of the Variolae Vaccinae (1798). Jenner’s discovery led directly to the worldwide eradication of smallpox.

VACCINE Immunobiological substance used for active immunization by introducing into the body a live modified, attenuated, or killed inactivated infectious organism or its toxin. The vaccine is capable of stimulating an immune response by the host, who is thus rendered resistant to infection. The word vaccine was originally applied to the serum from a cow infected with vaccinia virus (cowpox; from Latin vacca, “cow”); it is now used of all immunizing agents.

VACCINE EFFICACY (Syn: protective efficacy) Mathematically, this is defined as the pro- portion of persons in the placebo group of a vaccine trial who would not have become ill if they had received the vaccine; alternatively, it is the percentage reduction of cases among vaccinated individuals.

VALIDATION The process of establishing that a method is sound.

VALIDITY Relative absence of bias or systematic error. Contrast with precision, the rela- tive lack of random error.12,20,31,97 As a principle, internal validity must take precedence over precision. In the health, life, and social sciences, the term is often accompanied by a qualifying word or phrase. See also accuracy; reliability; repeatability; validity, study; confounding bias; selection bias.

VALIDITY, ANALYTICAL The ability of a test to correctly identify a property or charac- teristic in a specimen. This term encompasses both “analytical sensitivity” and “analyti- cal specificity.”

VALIDITY, CLINICAL The ability of a test to correctly identify a person who does or does not have the disease of interest.62,63,134,321 This term encompasses both “clinical sensitiv- ity” and “clinical specificity.”

VALIDITY, INTERNAL See validity, study.

VALIDITY, EXTERNAL See validity, study.

VALIDITY, MEASUREMENT An expression of the degree to which a measurement measures what it purports to measure. See also measurement, terminology of. Several varieties are distinguished, including construct validity, content validity, and criterion validity (concurrent and predictive validity).

  1. Construct validity: The extent to which the measurement corresponds to theoretical
    concepts (constructs) concerning the phenomenon under study. For example, if, on theoretical grounds, the phenomenon should change with age, a measurement with construct validity would reflect such a change.
  2. Content validity: The extent to which the measurement incorporates the domain of the phenomenon under study. For example, a measurement of functional health status should embrace activities of daily living: occupational, family, and social functioning, etc.
  3. Criterion validity: The extent to which the measurement correlates with an external criterion of the phenomenon under study. Two aspects of criterion validity can be distinguished:
    a. Concurrent validity: The measurement and the criterion refer to the same point
    in time. An example is a visual inspection of a wound for evidence of infection validated against bacteriological examination of a specimen taken at the same time.
    b. Predictive validity: The measurement’s validity is expressed in terms of its ability to predict the criterion. An example is an academic aptitude test that is validated against subsequent academic performance.

VALIDITY, STUDY The degree to which the inferences drawn from a study are warranted when account is taken of the study methods and the characteristics of the participants in the study.

Two fundamental types of study validity must be distinguished:

  1. internal validity The degree to which a study is free from bias or systematic error.
    Relative lack of bias. Contrast with precision, the relative lack of random error. The soundness of the study design, conduct, and analysis in answering the question that it posed for the study participants. Internal validity depends on methods and on substantive knowledge. More specifically, it depends on methods used to select the study subjects, collect information, and conduct analyses. For instance, the index and comparison groups must be selected and compared in such a manner that the observed differences between their effect on the outcome variables under study may, apart from sampling error, be attributed only to the hypothesized effect under investigation. Internal validity also depends on subject-matter knowledge; e.g., the application of equipotent doses of drugs in some clinical trials, the choice of valid windows of exposure in longitudinal studies, or the planning of valid intervals and procedures for outcome detection. Internal validity is a prerequisite for external validity.
  2. external validity (Syn: generalizability) The degree to which results of a study may apply, be relevant, or be generalized to populations or groups that did not participate in the study. A study is externally valid, or generalizable, if it allows unbiased inferences regarding some other specific target population beyond the subjects in the study. Hence it concerns the capacity to make fair inferences to an external population that did not participate in the study. In etiological research, such inferences to an external population are not merely statistical in nature: they must be rigorously based on all available knowledge that is relevant for the study hypotheses.

Conclusions about the external and internal validity of a study both require wisdom to apply judgment strictly based on knowledge of the subject matter and of methodology; methodological knowledge may be slightly more important in judging internal validity, whereas substantive knowledge on the subject may be somewhat more relevant in judging external validity; however, these nuances must not obscure the need to integrate the two types of knowledge. 20,93,181

These epidemiological definitions of internal and external validity do not correspond exactly to some definitions found in the sociological literature. See also critical appraisal; evidence-based medicine; feasibility; generalizability.


  1. In the health, life, and social sciences, what we believe in, what we hold dear about the way we live. Values influence the behavior of persons, groups, communities, cultures.. They are strong influences on the health of individuals and populations (scientists are here included).85,104,128,184–187
  2. Concepts used to explain how and why things matter. Values are involved wherever we distinguish between things good and bad, better or worse. Values are pervasive in human activities, including epidemiological research and public health.36
  3. In statistics, the magnitudes of measurements, statistics, or parameters.

VARIABLE Any quantity that varies. Any attribute, phenomenon, or event that can have different values.

VARIABLE, ANTECEDENT A variable that causally precedes the association or out- come under study. See also explanatory variable.

VARIABLE, CONFOUNDING See confounding variable.

VARIABLE,CONTROL Independentvariableotherthanthe“hypotheticalcausalvariable” that has a potential effect on the outcome variable and is subject to control by analysis.

VARIABLE, DEPENDENT See dependent variable.

VARIABLE, DISTORTER A confounding variable that diminishes, masks, or reverses the association under study.

VARIABLE, EXPERIENTIAL See independent variable.

VARIABLE, INDEPENDENT See independent variable.

VARIABLE, INTERVENING See intermediate variable.

VARIABLE, MANIFESTATIONAL See dependent variable.

VARIABLE, PASSENGER See passenger variable.

VARIABLE, UNCONTROLLED A (potentially) confounding variable that has not been brought under control by design or analysis. See confounding variable; residual confounding.

VARIANCE A measure of the variation shown by a set of observations, defined by the sum of the squares of deviation from the mean, divided by the number of degrees of freedom in the set of observations.

VARIATE See random variable.


  1. In infectious disease epidemiology, an insect or any living carrier that transports an infectious agent from an infected individual or its wastes to a susceptible individual or its food or immediate surroundings. The organism may or may not pass through a developmental cycle within the vector.
  2. In statistics, an ordered set (list) of numbers representing the values of an ordered set of variables (which is a vector of variables).

VECTOR-BORNE INFECTION Several classes of vector-borne infections are recognized, each with epidemiological features determined by the interaction between the infectious agent and the human host on the one hand and the vector on the other. Therefore environmental factors, such as climatic and seasonal variations, influence the epidemiological pattern by virtue of their effects on the vector and its habits.

The terms used to describe specific features of vector-borne infections are as follows:

  1. biological transmission: Transmission of the infectious agent to susceptible host by the bite of a blood-feeding (arthropod) vector, as in malaria, or by other inoculation, as in Schistosoma infection.
  2. extrinsic incubation period: Time necessary after acquisition of infection by the
    (arthropod) vector for the infectious agent to multiply or develop sufficiently that it can be transmitted by the vector to a vertebrate host.
  3. hibernation: A possible mechanism by which the infected vector survives adverse
    cold weather by becoming dormant.
  4. In apparent infection: Response to infection without developing overt signs of illness. If this is accompanied by viremia or bacteremia in a high proportion of infected animals or persons, the receptor species is well suited as an epidemiologically important host in the transmission cycle.
  5. mechanical transmission: Transport of the infectious agent between hosts by arthropod vectors with contaminated mouthparts, antennae, or limbs. There is no multiplication of the infectious agent in the vector.
  6. overwintering: Persistence of the infectious microorganism in the vector for extended periods, such as the cooler winter months, during which the vector has no opportunity to be reinfected or to infect a vertebrate host. Overwintering is an important concept in the epidemiology of vector-borne diseases, since the annual recrudescence of viral activity after periods (winter, dry season) adverse to continual transmission depends on a mechanism for local survival of an infectious microorganism or its reintroduction from outside the endemic area. To some extent, the risk of a summertime epidemic may be determined by the relative success of microorganism survival in the local winter reservoir. Since overwinter survival may in turn depend upon the level of activity of the microorganism during the preceding summer and autumn, outbreaks sometimes occur for 2 or more successive years.
  7. transovarial infection (Syn: transovarial transmission): Transmission of the infectious microorganism from the affected female arthropod to her progeny.

VECTOR SPACE The entire collection of possible values for a vector (ordered list) of variables.

VEHICLE OF INFECTION TRANSMISSION The mode of transmission of an infectious agent from its reservoir to a susceptible host. This can be person-to-person, via food, vector-borne, etc.

VENN DIAGRAM A pictorial presentation of the extent to which two or more quantities or concepts overlap.

VERBAL “AUTOPSY” A procedure for gathering information that may make it possible to determine the cause of death in situations where the deceased has not been medi- cally attended. It is based on the assumption that most common and important causes of death have distinct symptom complexes that can be recognized, remembered, and reported by lay respondents. It is promoted as a useful way of enhancing the quality of mortality statistics in developing countries.388

VERIFICATION The process aimed at converting speculative ideas and hypotheses into facts. It usually results either in the rejection of false hypotheses by deduction (see deductive logic) or the acceptance and consensus by induction (see inductive logic). A most illustrative case history is William Harvey’s confirmation of his hypothesis of the circulation of the blood, now universally recognized. In its broadest sense, verifica- tion is a summary of a process of causal inference.

VERIFICATION BIAS In studies evaluating screening and diagnostic tests, this bias occurs when some patients with negative test results are not evaluated with the “gold standard” test. Sensitivity can be overestimated and specificity underestimated; how- ever, the final bias depends on the variables associated with receiving or not receiving the gold standard. When the gold standard is invasive, clinicians will tend to perform it on patients whose experimental test results increase the probability of disease and to refrain from performing it in those whose experimental test results decrease the prob- ability of disease.14,31,327 See also quadas; workup bias.

VERTICAL TRANSMISSION (Syn: intergenerational transmission) The transmission of infection from one generation to the next, especially of HIV infection from mother to infant prenatally, during delivery, or in the postnatal period via breast milk.

VIOLENCE Harm caused by the use of force. Harm takes the form of traumatic injury or death. Epidemiologically, two main varieties, unintentional and intentional violence, can be distinguished; the former occurs mainly in traffic and industry, the latter mainly in warfare and in domestic settings.

VIRGIN POPULATION A population that has never been exposed to a particular infec- tious agent.

VIRULENCE The degree of pathogenicity; the disease-evoking power of a microorgan- ism in a given host. Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay. When death is the only criterion of severity, this is the case-fatality rate.

VIRUS A microorganism composed of a piece of genetic material (RNA or DNA) sur- rounded by a protein coat. To replicate, a virus must infect a living cell: viruses can reproduce only by entering a host cell and using the translational system of the cell to initiate the synthesis of viral proteins and to undergo replication.

VITAL RECORDS Certificates of birth, death, marriage, and divorce required for legal and demographic purposes. Literally, records “to do with living.”

VITAL STATISTICS Systematically tabulated information concerning births, marriages, divorces, separations, and deaths based on registrations of these vital events.

VON NEUMANN–MORGENSTERN STANDARD GAMBLE Procedure used to assess the risk a seriously ill person is prepared to take when the trade-off is between poten- tially enhanced quality of life and a finite possibility that the treatment regimen will be fatal.389


  1. A position of relative disadvantage; e.g., owing to impaired nutrition, cognition, or social position. The extent to which a person, population, or ecosystem is unable or unlikely to respond to threats. May be used as a synonym for susceptibility See also robustness.
  2. A process of negative adaptation in the face of adversity (Opposite: resilience).

VULNERABLE POPULATION A population at risk of coercion, abuse, exploitation, discrimination, imposition of unjust burdens of risk, and poorer health outcomes by reason of diminished competence or decision-making capacity, lack of power or social standing, fragile health, deprivation, or limited access to basic needs, including public health and medical care. Similar acts may be construed to be coercive in a vulnerable population which would not be so in other, well-situated populations. Includes children, institutionalized persons, the frail, and those with mental disorders as well as those on the lower sectors of societies.37