22. Prevention of infectious diseases: vaccination, chemoprophylaxis

From greek.doctor
  • Vaccination prevents 2-3 million deaths every year
    • An additional 1.5 million deaths could be avoided with further vaccination
  • Immunogenicity
    • = the ability of a vaccine to provoke an immune response
    • Can be measured by measuring circulating antibodies
  • Protectivity
    • = how well the vaccine protects against the disease
    • Must be measured with long-term observational studies
  • Reactogenicity
    • = how many adverse reactions the vaccine produces
  • Complications of vaccines
    • = serious side effects
    • Very rare
    • Most commonly severe allergic reactions
      • Staff is trained to handle it
    • The Hempt-vaccine against rabies was used until ’89, but was stopped because it caused demyelination
  • Accidents related to vaccination
    • Misdosage
    • Inappropriate administration technique
    • Using expired vaccines
    • Not following the correct schedule
    • Using contaminated vaccines
  • “Ideal” vaccine
    • 100% efficiency for all ages
    • Only needs a single administration
    • Does not cause side effects
    • Stable under various environmental conditions
    • Easily administrated (preferably oral)
    • Low price
    • Obviously doesn’t exist

Types of vaccines

  • Live, attenuated vaccine
    • The pathogen is attenuated, meaning that it is manipulated to become much less pathogenic
    • The attenuated pathogen has limited ability to reproduce in humans
    • Advantages
      • The closest vaccine to a natural infection
      • Provokes both humoral and cellular immune response
      • Long-term effective protection with only one or two doses
    • Disadvantages
      • Can’t be given to immune compromised patients
      • Need to be continuously refrigerated
      • Small risk of the pathogen turning virulent
      • Currently very hard to make for bacteria
    • Examples
      • MMR
      • Chickenpox (varicella)
      • BCG (Tuberculosis)
      • OPV (oral polio vaccine, Sabin)
  • Inactivated vaccine (= killed vaccine)
    • The pathogen is killed with chemicals, heat or radiation
    • Advantages
      • No risk of the pathogen turning virulent (as it is dead)
      • No refrigeration needed (as it is dead)
    • Disadvantages
      • Stimulates weaker immune response
      • Several doses and booster shots are necessary, requiring regular access to healthcare
    • Examples
      • IPV (Salk polio vaccine)
      • HAV (Hep A)
      • Rabies
      • Influenza
  • Toxoid vaccine
    • Can be made for bacteria who produce toxins
    • Toxin is extracted from the bacterium and inactivated with formalin, so that it is no longer pathogenic -> now called a toxoid
    • Used when the illness is caused by the toxin and not the bacterium itself
    • Advantage
      • Rarely causes side effects
      • Stable, doesn’t require refrigeration
      • No risk of acquiring the disease
    • Disadvantage
      • Several doses may be needed
      • Stimulates weak immune response
      • Do not give herd immunity
    • Examples
      • Diphtheria
      • Tetanus
  • Subunit vaccine
    • Instead of giving the entire microbe as a vaccine, certain subunits (antigens) are given instead
    • Examples
      • Hepatitis B (HBsAg)
      • Acellular pertussis (aP)
  • Conjugate vaccine
    • For bacteria protected by a polysaccharide capsule
    • Polysaccharides are weak antigens, meaning that they stimulate weak immune responses
    • The polysaccharide antigen of a bacterium is conjugated (attached to) a highly immunogenic protein antigen
    • This stimulates a strong immune response against both the polysaccharide antigen and the protein antigen
    • Advantages
      • Give a strong immune response with long-term protection
    • Disadvantages
      • Multiple shots or booster shots may be necessary
    • Example
      • Haemophilus influenzae type b
      • Meningococcus (Neisseria meningitidis)
  • RNA vaccine
    • The Comirnaty (by BioNTech and Pfizer) and Spikevax (by Moderna) were the first widely available RNA vaccines, but the RNA vaccine technology has been worked on for decades
    • These vaccines deliver non-replicating RNA to human host cells at the injection site. The RNA encodes the spike (S) protein from the SARS-CoV-2 virus. The host cells express this RNA, producing the spike protein, against which the immune system mounts both humoral and cellular immune responses
    • The RNA vaccines are safe and effective
  • DNA vaccine
    • Only in experimental stages
    • Genes for antigens are inserted into bacteria, which produce plasmids (circular DNA) of these antigen genes
    • Plasmids are inserted into the body. Cells of the body will take up the plasmids and start synthesizing the antigen from the plasmid
    • The immune system recognizes the antigen produced as foreign and triggers immune response
  • Recombinant vector vaccines
    • Only in experimental stages
    • Genes for antigens are inserted into bacteria, which synthesize large amounts of the antigen
    • Antigen is then purified and given as a vaccine

Who gets vaccines:

  • Age-specific compulsory vaccines
    • BCG
    • Haemophilus influenzas type b
    • DTaP (diphtheria, tetanus, acellular pertussis)
    • IPV (Salk polio)
    • MMR (measles, mumps, rubella)
    • Hepatitis B
    • Pneumococcus
    • HPV
  • Compulsory vaccines for at-risk groups
    • Tetanus
    • Rabies
    • Hepatitis A
  • Non-compulsory vaccines for at-risk groups
    • Influenza
  • Job-related vaccines
    • Rabies
    • Meningococcus
    • Tick-borne encephalitis
  • Travel-related vaccines
    • Yellow fever
    • Cholera
    • Typhoid fever
    • Japanese encephalitis

Chemoprophylaxis

  • Definition: Administering drugs for the purpose of preventing disease or infection
  • Can be given
    • Preexposure – before the exposure to the pathogen
    • Postexposure – after the exposure to the pathogen but before symptoms develop
  • Examples
    • Giving antiretroviral drugs to people at high risk for HIV right after being exposed to HIV
      • Like health care workers who come in contact with HIV
    • Giving antimalaria drugs to people before, during and after travelling to malaria-prone areas
    • Giving antibiotics to people who travel to areas where Traveller’s diarrhoea (ETEC) is common
    • Giving antibiotics to people who have been exposed to bacillus anthracis
    • Giving antibiotics to people who have been exposed to Neisseria meningitidis