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AQA BiologyInfection and response

Vaccination, drugs and development

Explain vaccination and evaluate how new medicines are tested.

Start here

The key idea

Vaccines expose the immune system to antigens safely, producing memory cells.New medicines must be tested for toxicity, efficacy and dose.

Vaccination creates memory cellsA second exposure triggers a faster antibody response.
Vaccination creates memory cellsA second exposure triggers a faster antibody response.vaccinepathogen latersmall primary responsefaster, larger response
Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

1

How vaccination works

Vaccination involves introducing small quantities of dead or inactive forms of a pathogen into the body.These carry antigens that stimulate the lymphocytes to produce the correct antibodies, and memory cells are made.If the live pathogen later enters the body, the memory cells respond rapidly to produce antibodies, preventing illness.

2

Herd immunity

If a large proportion of a population is vaccinated, the spread of a pathogen is greatly reduced because there are few people for it to infect.This is called herd immunity and it helps protect even those who are not vaccinated.Vaccination has been used to control diseases that were once common, such as measles and polio.

3

Antibiotics and painkillers

Antibiotics, such as penicillin, kill bacteria inside the body and have greatly reduced deaths from bacterial diseases.They do not work against viruses, because viruses live inside cells where antibiotics cannot reach them without harming the cell.Painkillers and other medicines treat the symptoms of a disease but do not kill the pathogen.

4

Developing new drugs

New drugs are tested for toxicity, efficacy (whether they work) and dose.Preclinical testing is carried out on cells, tissues and live animals, then clinical trials use healthy volunteers and patients, starting with very low doses.Double blind trials, where neither doctor nor patient knows who has the real drug or a placebo, are used to give reliable results, and the findings are peer reviewed before publication.

Key terms

Definitions to learn

Vaccine

A preparation of dead or inactive pathogen that stimulates an immune response and immunity.

Herd immunity

Protection of a population because enough people are vaccinated to stop a pathogen spreading.

Antibiotic

A medicine that kills bacteria inside the body but does not affect viruses.

Placebo

A dummy treatment with no active drug, used as a control in a clinical trial.

Double blind trial

A trial in which neither the patient nor the doctor knows who receives the drug or placebo.

Efficacy

How effective a drug is at producing the intended effect.

Worked example

Explain how vaccination can reduce the spread of a disease in a population.

1

Vaccinated people develop memory cells.

2

They respond faster if infected.

3

Fewer people transmit the pathogen, reducing spread.

Final answer

Vaccination reduces transmission because more people become immune.

Exam habit

In clinical trial questions, state each stage and its purpose: pre-clinical → Phase I (safety) → Phase II (efficacy) → Phase III (large sample).Always mention double-blind trials and placebos when comparing groups.

Watch out

Clinical trials do not start with large patient groups. Testing progresses in stages.

Examiner tips

How to score full marks

  • 1Antibiotics kill BACTERIA only — never say they kill or cure viruses; this is a very common exam mistake.
  • 2In vaccination answers always include 'dead/inactive pathogen', 'antigens', 'antibodies' AND 'memory cells' for full marks.
  • 3For drug trials, link placebos and double blind design to making the results VALID/RELIABLE by removing bias.
Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1What is a placebo?
Mark scheme
  1. 1.Define its purpose in a trial.
A treatment with no active drug, used for comparison.
2Why are some trials double blind?
Mark scheme
  1. 1.Consider bias from patients and researchers.
Neither patients nor researchers know who receives the drug, reducing bias.
3Why must antibiotics not be overused?
Mark scheme
  1. 1.Link use to selection pressure.
Overuse selects resistant bacteria, making infections harder to treat.
4State what is introduced into the body during vaccination.[1 mark]
Mark scheme
  1. 1.Recall the form of the pathogen used.
A small quantity of dead or inactive (weakened) pathogen (1).
5Explain why antibiotics cannot be used to treat influenza (flu).[2 marks]
Mark scheme
  1. 1.Consider the type of pathogen and where it lives.
Flu is caused by a virus (1); antibiotics only kill bacteria and do not affect viruses, which live inside the body's cells (1).
6Describe how a vaccine produces immunity to a disease.[4 marks]
Mark scheme
  1. 1.Trace antigens to antibodies to memory cells.
The vaccine contains antigens from a dead/inactive pathogen (1); these stimulate lymphocytes to produce antibodies (1) and memory cells (1), so if the live pathogen enters later, antibodies are produced quickly to destroy it before illness develops (1).
7Explain why placebos and a double blind design are used in clinical trials of a new drug.[4 marks]
Mark scheme
  1. 1.Link each feature to removing bias and giving valid results.
A placebo is a dummy treatment used as a control / to compare against the real drug (1) so any effect of the drug can be identified (1); in a double blind trial neither the patient nor the doctor knows who has the real drug (1), which removes bias from the results / stops expectations affecting the outcome (1).
8A new vaccine is offered to a population, but some parents refuse it. Explain how widespread vaccination protects even unvaccinated children, and evaluate why a high uptake is important.[6 marks]
Mark scheme
  1. 1.Explain herd immunity, then weigh the benefit of high uptake.
If a large proportion of the population is vaccinated, the pathogen cannot spread easily because there are few people it can infect (herd immunity) (1), so unvaccinated children are less likely to come into contact with the pathogen (1). A high uptake is important because if too many refuse, the pathogen can spread among the unvaccinated (1) and may cause an outbreak / epidemic (1); however vaccines may rarely cause side effects, which is why some refuse (1). Overall a high uptake protects the whole population, including those who cannot be vaccinated for medical reasons (1).
9Describe the three stages of testing that a new drug must pass before it can be prescribed to patients.[3 marks]
Mark scheme
  1. 1.Name pre-clinical testing, then the two main phases of clinical trials.
Pre-clinical testing is carried out on cells, tissues and animals to test for toxicity and potential efficacy (1); Phase I clinical trials use small numbers of healthy volunteers at low doses to check safety (1); Phase II/III clinical trials use larger numbers of patients to check efficacy (whether it works), the correct dose and any side effects (1).
10Explain why new drugs are tested on animals before being trialled on humans.[3 marks]
Mark scheme
  1. 1.Link to safety and the information gained.
Animal testing checks whether the drug is toxic / has harmful side effects before it is given to humans (1); it allows the dose to be estimated (1); it provides information about how the drug is absorbed and metabolised in a living organism (1), reducing the risk to human volunteers in later trials.
11Some strains of bacteria have become resistant to antibiotics. Explain, in terms of natural selection, how antibiotic resistance develops in a bacterial population.[5 marks]
Mark scheme
  1. 1.Start with variation from random mutation.
  2. 2.Apply natural selection pressure from antibiotic use.
Random mutations occur in bacteria, creating variation (1); some bacteria may have a mutation that makes them resistant to an antibiotic (1); when the antibiotic is used, non-resistant bacteria are killed (1); resistant bacteria survive and reproduce (1); they pass the resistance allele/gene to offspring (1); over many generations the proportion of resistant bacteria increases (1).
12A doctor prescribes a new antibiotic to a patient with a bacterial lung infection. The patient feels better after five days but is told to complete the full ten-day course. Explain why completing the course is important.[4 marks]
Mark scheme
  1. 1.Think about which bacteria survive a partial course.
  2. 2.Link to natural selection and resistance.
Stopping early means some bacteria that are less susceptible to the antibiotic may survive (1); these surviving bacteria could reproduce and pass on alleles for resistance (1); completing the full course ensures all bacteria are killed (1), reducing the risk of resistant strains developing and spreading (1).
13Aspirin was originally extracted from willow bark. Describe how scientists would have developed aspirin from a traditional plant remedy into a widely prescribed drug.[6 marks]
Mark scheme
  1. 1.Outline the stages from identifying the active compound to clinical trials.
Scientists would identify and extract the active compound from the plant (1); laboratory tests on cells and tissues would check for biological activity (1); pre-clinical animal tests would determine toxicity and the effective dose (1); Phase I clinical trials on healthy volunteers would check safety (1); Phase II/III trials on patients would confirm efficacy and the correct dose (1); results would be peer reviewed before the drug was approved for prescription (1).
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