Ch3

Question 1

What is vaccine and how does it work?

Answer 1

• killed or weakened version or a subunit of a pathogen
• trick the immune system as if the body was attacked by pathogens –> generation of memory B and T cells –> secondary immune response –> rapid elimination of pathogens

Question 2

What are the characteristics of a successful vaccine?

Answer 2

• safety
  => no risk of causing disease => few side effects
• efficacy
  => stimulates helper T cells –> maximise the activity of immune cells through the release of cytokines –> development of memory B and T cells and eliminate pathogens by antibody and cytotoxic T cells
• pathogenicity
  => only cause mild disease symptoms
  => targeted pathogens only undergo little mutation
  => no latency
  => no attack on self-immune cells

Question 3

How to prepare live attenuated vaccine?

Answer 3

• prepared by weakened pathogens through attenuation

Question 4

How can attenuation be done?

Answer 4

• growing pathogens under abnormal culture conditions –> pathogens mutate to adapt –> lose their pathogenicity
• mutating virulent genes by recombinant DNA technology

Question 5

What are the pros and cons of live attenuated vaccines?

Answer 5

• pros
  => retain their ability to replicate in the host –> act as intracellular Ag –> form peptide-MHC I complex –> increase both humoral and cell-mediated immune response –> lower the chance of having boosters
• cons
  => requires refrigeration –> high production cost –> high selling price
  => pathogens may mutate back to pathogenic form through reversion as they are still active and living pathogens
  => more side-effects as they mimic real pathogens –> too effective

Question 6

What are inactivated vaccines?

Answer 6

• whole pathogens are killed by heat, gamma irradiation or chemicals –> protect epitopes but remove pathogen’s virulence

Question 7

What are the pros and cons of inactivated vaccines?

Answer 7

• pros
  => no possibility of reversion –> safer to use
  => more stable than live vaccines
• cons
  => dead microbes cannot act as intracellular Ag –> Ag not presented on MHC I –> mainly B cell response but no cytotoxic T cell response –> boosters and adjuvants are often required

Question 8

How are protein subunit vaccines produced?

Answer 8

• produced by recombinant DNA technology
• DNA encoding the protein subunit antigen is introduced into E. coli –> expression of large amount of protein Ag –> direct inject protein as vaccine

Question 9

What are the pros and cons of protein subunit vaccines?

Answer 9

• pros
  => contains only the protein Ag –> safer as there is no risk of reversion
• cons
  => no cytotoxic T cell activation as Ab response predominates

Question 10

How do polysaccharide subunit vaccine works?

Answer 10

• stimulate immune response against polysaccharide molecules in bacterial capsule

Question 11

What are the pros and cons of polysaccharide subunit vaccine?

Answer 11

• pros
  => no need to use the whole bacterial pathogen –> safe
• cons
  => only T-independent antigens involved –> B cells response only –> as there is no helper T cells response –> no memory cells

Question 12

How can polysaccharide subunit be improved?

Answer 12

• improved by chemically linking a carbohydrate antigen to a carrier protein –> conjugate vaccine –> act as T-dependent antigen –> activation of both B and helper T cells response –> production of Ab and memory cells

Question 13

What are recombinant vector vaccines?

Answer 13

• transfer a gene encoding a protein Ag of a pathogen into an attenuated viral vector –> form a recombinant vector –> infect host muscle cells –> present protein Ag both intracellularly and extracellularly

Question 14

What are the pros and cons of recombinant vector vaccines?

Answer 14

• pros
  => strong humoral and cell-mediated response
  => low risk of reversion as most of the pathogen genome is eliminated
• cons
  => side effects due to host immune response to the recombinant vector
  
  1. vector may be damaged –> vaccine becomes useless
  2. may be life-threatening as the vector may still cause death

=> low stability as refrigeration is required

Question 15

What are DNA and RNA vaccines?

Answer 15

• plasmid DNA or RNA coding for a protein Ag of a pathogen is injected into the muscle –> host muscle cells take up DNA or RNA and express the protein Ag intracellularly and extracellularly –> present Ag via MHC I and MHCII –> increase humoral and T cells response

Question 16

What are the pros and cons of DNA and RNA vaccines?

Answer 16

• pros
  => induce both humoral and cell-mediated immunity
  => customisable and easy to scale up
• cons
  => integration of plasmid DNA in host genome may induce mutation (as the DNA will get into the host nucleus and randomly insert into the genome)
  => ultra-low temperature is required for the storage of mRNA vaccines

Question 17

How to identify the four types of immunisation and what is the pros and cons for passive immunisation?

Answer 17

• natural active –> infection
• artificial active –> vaccination
• natural passive –> transfer of antibodies from mother to foetus
• artificial passive –> injection of ready-made antibodies from other individuals
• pros –> provide instant protection
• cons –> no memory cells are produced –> short-term protection

Question 18

What are the characteristics of prophylactic vaccines? Give two examples.

Answer 18

• given during infancy to prevent childhood diseases
• booster doses are usually given 2-3 times
• combination vaccines
• examples:
  => DTaP vaccine
  > against diphtheria, tetanus and pertussis (a = acellular)
  > pertussis protein combines with diphtheria and tetanus toxoids
  > 80% vaccinees who received all 3 doses are completely protected
  => MMR vaccine
  > live attenuated measles, mumps and rubella viruses
  > both humoral and cell-mediated immune response with memory B and T cells generated
  > highly effective –> measles, mumps and rubella are rare in developed countries

Question 19

What are the vaccines that are included in HK childhood immunisation programme and what are the diseases prevented respectively?

Answer 19

• Bacille Calmette-Guerin (BCG) –> tuberculosis
• Hepatitis B –> Hepatitis B
• DTaP-IPV (inactivated poliovirus) –> diphtheria, tetanus, pertussis and poliomyelitis
• pneumococcal –> pneumonia
• MMR or MMRV (varicella) –> measles, mumps, rubella and chickenpox
• Human papillomavirus (HPV) –> cervical cancer

Question 20

What are the drawbacks of vaccines?

Answer 20

• may associate with serious side effects
  => preservatives or antibiotics in vaccines –> allergic reactions
  => adverse effects in vaccinees allergic to egg proteins as some vaccines are produced in chicken eggs
  => live attenuate viral vaccines –> infect foetus of vaccinated pregnant woman through placenta –> birth defects
  => oral polio virus vaccines may revert –> paralytic poliomyelitis
  => COVID-19 vaccines may lead to myocarditis in children