Disease Transmission

• Disease spreads when the body contacts a disease-causing pathogen.
• The spread of infection and disease involves three factors: the host, environment, and the pathogen.
• Pathogen features determine disease transmission and spread potential within a population.

Transmission – How Diseases Spread

• Direct Contact:
  • Person-to-Person: Occurs through physical contact like touching, kissing, or sexual intercourse, examples include common colds and STIs.
  • Animal-to-Person: Diseases transmit from animals to humans through bites or scratches, for example, rabies.
• Indirect Contact:
  • Contaminated Objects: Occurs from touching contaminated surfaces like doorknobs.
  • Insect Bites: Diseases spread through insect bites - mosquitoes spreading malaria or ticks spreading Lyme disease.
• Airborne:
  • Respiratory Droplets: Infected person coughs, sneezes, or talks, spreading droplets containing the virus, examples include influenza and COVID-19.
  • Aerosol Particles: Smaller particles remain suspended in the air for longer and travel further, for example, tuberculosis.
• Water & Food:
  • Contaminated Water: Drinking or contacting water contaminated with pathogens spreads diseases like cholera.
  • Contaminated Food: Consuming improperly handled or cooked food leads to foodborne illnesses, such as salmonella or E. coli infection.
• Vectors:
  • Insects or animals carry and transmit pathogens to humans, like mosquitoes, fleas, and ticks.
• Environmental:
  • Some pathogens survive in soil or water and cause infections upon human contact, for example, Legionnaires' disease from contaminated water sources.

Pathogens

• Two main types of pathogens exists that vaccinations target are:
  • Bacteria: single-celled organisms with DNA, some pathogenic, others harmless or beneficial.
  • Viruses: smaller than bacteria, consist of DNA or RNA surrounded by a capsid, and require a host cell's replication mechanism to replicate.
• Influenza viruses (RNA viruses) constantly mutate, necessitating annual vaccination.

Chain of Infection

• Diseases spread through the chain of infection
• Infectious Agent:
  • The process begins with the infectious agent either a virus or bacterium that thrives in specific environments, waiting to spread.
• Reservoir:
  • The reservoir is where the infectious agent resides, which can be a person, an animal, or even a contaminated doorknob.
• Portal of Exit:
  • The pathogen needs a way out, either from respiratory droplets, feces, or blood.
• Mode of Transmission: -Direct contact is one mode, as is airborne transmission.
• Portal of Entry:
  • The agent seeks a new host, and enters via mucous membranes, broken skin, or ingestion.
• Susceptible Host:
  • Someone with a weakened immune system is a susceptible host.
• Breaking the chain, for example with vaccination, is critical for infection control to stop the spread of disease.

How Vaccines Work

• Vaccines harness the body's natural immune system.
• When disease-causing bacteria enter the body, the immune system recognizes them as invaders due to unique surface proteins or sugars.
• This triggers white blood cells or B cells to fight the invaders.
• B cells produce Y-shaped antibodies that identify, bind to, and neutralize foreign organisms and toxins, preventing replication.
• Antibodies attach to the bacteria's surface, killing or disabling them.
• Bodies have billions of white blood cells, each capable of making one shape of antibody (or key).
• Once activated, the right white blood cells produce large amounts of antibodies.
• Some antibodies remain in the blood, and some white blood cells become memory cells or T cells, enabling a faster response if the same bacteria invade again.
• Vaccines contain weakened or dead bacteria or viruses, or proteins/sugars from bacteria surfaces, which mimics an invasion.
• This prompts the immune system to respond without the risks of the disease itself.
• If the body encounters the actual bacteria or virus later, the immune system will quickly eliminate it.

Types of Vaccines

• Inactivated Vaccines:
  • Use the killed version of the germ that causes a disease.
  • Examples include the polio vaccine and the hepatitis A vaccine.
• Live Attenuated Vaccines:
  • Use a weakened (attenuated) form of the germ that causes a disease.
  • Examples include the measles, mumps, and rubella (MMR) vaccine, and the yellow fever vaccine.
• Subunit Vaccines:
  • Use pieces of the germ—like its protein, sugar, or capsid (a casing around the germ).
  • Examples include the HPV vaccine and the hepatitis B vaccine.
• Toxoid Vaccines:
  • Prevent diseases caused by bacteria that produce toxins (poisons) in the body.
  • Use toxins made by the germ that have been weakened so they cannot cause illness.
  • Examples include the diphtheria and tetanus vaccines.
• mRNA Vaccines:
  • Use messenger RNA (mRNA) to instruct cells in the body to produce a protein that triggers an immune response.
  • The COVID-19 vaccines by Pfizer-BioNTech and Moderna are examples.
• Viral Vector Vaccines:
  • Use a modified version of a different virus (the vector) to deliver important instructions to our cells.
  • The Johnson & Johnson COVID-19 vaccine and the Ebola vaccine are examples.

Vaccine Composition

• Understanding the components will enable you to address patient concerns, administer vaccines safely, and mitigate adverse reactions.
• Adjuvants:
  • Enhance the body's immune response to a vaccine by creating a stronger, longer-lasting immunity.
  • Can prolong the presence of the antigen in the body, stimulate a stronger response from the immune system, and enhance the body's ability to recognise the antigen in the future.
  • Adjuvants are especially important in vaccines where a strong immune response is crucial, or where the antigen alone might not be enough to provoke a sufficient defense.
• Preservatives:
  • Added to vaccines to prevent contamination from bacteria or fungi, throughout its shelf-life.
  • This ensures the vaccine remains safe and effective over time, especially in multi-dose vials.
  • Mercury has been removed or reduced to trace amounts in most vaccines, especially those for children.
• Stabilisers:
  • Help protect the vaccine from changes in temperature, light, humidity, and other environmental factors that could degrade its components.
  • Common stabilisers include Gelatine, Sugars, Amino acids, Proteins and Salts.
• Residuals:
  • Small amounts of substances from the manufacturing process that remain in the final product.
  • Can include residues from cell lines used to grow viruses, inactivating agents, and antibiotics. -Amounts present in vaccines are extremely small and rigorously tested.

Herd Immunity

• Herd immunity occurs when a sufficient percentage of a population becomes immune to a disease through infections or vaccination.
• When enough people are resistant to a virus or bacteria, it prevents the disease from spreading freely within the community.
• Increased vaccination rates encourage herd immunity, and the higher the number vaccinated, the fewer hosts are available to the disease.
• The vaccination coverage rate needed to stop transmission of diseases is known as the reproduction number (R0).
• R0 is defined as the average number of people infected through transmissions from a single primary case.
• Diseases with a high R0 need higher vaccine coverage to attain herd immunity, for example, measles requires 95% vaccine coverage.