Vaccines and Vaccination

History of Immunization

• 15th century China: Variolation, where individuals who recovered from smallpox never suffered from it again, led to deliberately infecting children with smallpox through dried scabs, resulting in local disease with lower mortality.
• 1700s: Variolation spread to England.
• 1796: Edward Jenner discovered that infection with cowpox protected against smallpox, leading to the first vaccine.
• Louis Pasteur developed vaccines for Pasteurella multocida (1879), Anthrax (1881), and Rabies (1885).

Types of Immunization

• Passive Immunization: administration of preformed antibodies specific to a particular antigen, providing immediate but short-lived protection with no immunological memory.
• Active Immunization: administration of antigens, inducing an immune response and immunological memory, and is the most common form of immunization.

Types of Vaccines

• Live-Attenuated Vaccines: contain weakened pathogens, providing long-term immunity.
• Killed/Inactivated Vaccines: contain inactivated pathogens, providing short-term immunity.
• Subunit Vaccines: contain small components of pathogens, providing short-term immunity.
• DIVA Vaccines: allow differentiation between infected and vaccinated animals.
• Nucleic Acid Vaccines: contain DNA or mRNA, generating an immune response.
• Recombinant Organism Vaccines: use a vector to express immunogenic proteins.

Adjuvants

• Substances that enhance or facilitate an immune response to an antigen.

Vaccine Efficacy

• Expressed as a reduction in disease attack rate between unvaccinated and vaccinated individuals.
• Can refer to different outcomes, such as death, severe disease, clinical disease, or infection, with varying efficacies for each outcome.

History of Vaccine Development

• Early vaccines: Live-attenuated, Killed/Inactivated, and Component vaccines.
• Modern vaccines: Molecular-based technologies, Chimeric organisms, Recombinant subunit vaccines, Nucleic acid vaccines, Virus-like particles, and Reverse vaccinology.

Importance of Diagnostic Tests

• Confirm or exclude a diagnosis
• Determine treatment strategies
• Epidemiological surveillance
• Prevention, control, and eradication strategies
• Identification of new pathogens

Phases of Testing

• Pre-analytical: test selection, sampling, storage, transportation
• Analytical: handling and analysis of the specimen
• Post-analytical: report results, interpretation

Test Selection

• Determined by type of pathogen, type of sample, test characteristics, phase of disease, availability, and cost

Sample Considerations

• Samples must be correctly selected, collected, stored, and transported
• Inadequate samples result in inadequate results

Understanding Test Results

• Results can be divided into 4 categories: true positive, false positive, false negative, true negative
• Cut-off value divides results between positive or negative

Diagnostic Sensitivity and Specificity

• Sensitivity: measures the capacity of a test to correctly identify positive individuals (true positives)
• Specificity: measures the capacity of a test to correctly identify negative individuals (true negatives)
• Equations: sensitivity = (TP / TP + FN) × 100, specificity = (TN / TN + FP) × 100

Impact of Cut-off Change

• Increase sensitivity, decrease specificity
• Decrease sensitivity, increase specificity

Test Predictive Value

• Positive Predictive Value (PPV): probability of a positive test being a true positive
• Negative Predictive Value (NPV): probability of a negative test being a true negative
• Equations: PPV = (TP / TP + FP) × 100, NPV = (TN / TN + FN) × 100

Prevalence

• Proportion of a population affected by a disease at a specific time
• Not to be confused with incidence, which refers to the number of new cases in a time interval

Impact of Prevalence on Predictive Values

• PPV and NPV are affected by prevalence, diagnostic sensitivity, and specificity