6.3.8 Vaccines Against Disease

• Vaccines are used to induce immunity to infectious diseases
• A vaccine contains harmless versions of a pathogen
• There are several different methods by which scientists ensure that vaccines contain harmless pathogens such as:
  • Killing the pathogen
  • Making the pathogen unable to grow or divide (attenuated vaccine)
  • Using fragments of pathogens, which include the necessary antigens (rather than whole cells)
• A vaccine may be administered orally, nasally or via an injection

How vaccines work

• Once in the bloodstream, the antigens contained within the vaccine can trigger an immune response in the following way:
  • Lymphocytes recognise the antigens in the bloodstream
  • The activated lymphocytes produce antibodies specific to the antigen encountered
  • Memory cells and antibodies subsequently remain circulating in the bloodstream

The process of long-term immunity by vaccination

• Future infection by the same pathogen will trigger a response that is much faster and much larger compared to the initial response
• Due to the rapid nature of the response, the pathogen is unable to grow in sufficient numbers to cause disease and the individual is said to be immune

The number of measles antibodies in the blood following vaccination. The secondary response is much faster and a greater number of antibodies are produced.

Other non-vaccine medicines: antibiotics, antivirals and antiseptics

• Antibiotics are chemical substances produced by one microorganism that has a toxic effect on another, pathogenic organism
  • Antibiotics have been an important branch of medicine for decades since their discovery in the 1930s
  • They generally only work against bacterial pathogens (not fungi or viruses)
  • Penicillin is the first and best-known example
• Antivirals are medicines that act against viruses only
  • It is difficult to target viruses without damaging host cells 
  • A risk is that because viruses use host cell mechanisms to replicate themselves, damaging the virus may well inflict collateral damage on the host cell
  • Most antiviral drugs act by slowing the virus's rate of reproduction down
    • Can a virus be killed as it's debateable whether a virus is a living entity in the first place!
  • A well-known example is the anti-flu drug ^TMTamiflu
• Antiseptics are products that kill microorganisms but are not drugs because they would be toxic if taken
  • They can be used externally eg. on the surface of the skin to clean wounds 
  • Examples include ^TMLysol, a common laboratory antiseptic used to wipe surfaces before and after microbiological work, and common household antiseptics like bathroom cleaners

Why vaccinate?

• Vaccination will prevent illness in an individual by providing artificial immunity
• Vaccination involves exposing an individual to the antigens of a pathogen in some form, triggering an immune response which results in the formation of memory cells which can make antibodies against it
• If a vaccinated individual is infected with the pathogen, they can destroy the pathogen before it becomes infectious
• Consequently, vaccines reduce the likelihood that an infected individual will spread the pathogen they have been vaccinated against to others
• If a large proportion of the population is vaccinated, it is unlikely that an unvaccinated individual will become infected with the pathogen
• This is the principle behind the idea of herd immunity
• There are three main scenarios with vaccination:
  
  • There are no vaccinations and the disease spreads quickly
  • Some of the population are vaccinated and the disease spreads to fewer people
  • Most of the population are vaccinated and this prevents the spread

Herd immunity protects the vulnerable that may not be able to have the vaccine

Worldwide vaccination

• The role of the World Health Organisation (WHO) is to monitor global diseases, they will track if a disease is an epidemic (localised outbreak) or a pandemic (global outbreak)
  • For example, The WHO declared the COVID-19 outbreak a pandemic on 11 March 2020
• The importance of vaccines cannot be underestimated:
  • The number of people with measles worldwide is increasing even though there is a vaccine
  • The increase is due to a drop in the vaccination rate globally
  • There was some controversy over the MMR vaccine in 1998 and the number of vaccinations dropped significantly after this
  • Vaccines have reduced the cases of diseases worldwide drastically

Vaccination Statistics Table

Data source: ourworldindata.org/vaccination

Advantages & Disadvantages of Vaccination Table