B11: Vaccines and Drugs
Vaccination, antibiotics and painkillers
Vaccines And Drugs
📋 Key Concepts
Drugs are substances that affect the body. Some drugs treat disease (like antibiotics), some relieve symptoms (like painkillers), and some prevent disease (like vaccines).
| Type | Purpose | Example |
|---|---|---|
| Vaccines | Prevent disease by stimulating immunity | MMR vaccine, flu jab |
| Antibiotics | Kill bacteria that cause infection | Penicillin, amoxicillin |
| Painkillers | Relieve symptoms but don't cure the disease | Paracetamol, ibuprofen |
| Antivirals | Stop viruses replicating (less common) | Aciclovir for herpes |
📝 Vaccination
Vaccination involves injecting a small amount of dead or inactive pathogen into the body. This triggers the immune system to produce antibodies and memory cells, so the body can respond quickly if the real pathogen ever enters.
How Vaccination Works
- The vaccine contains a dead or inactive form of the pathogen (or its toxins)
- The white blood cells recognise the pathogen as foreign
- Lymphocytes produce antibodies specific to that pathogen
- Memory cells are produced and remain in the blood
- If the real pathogen enters later, the immune system responds much faster and stronger
- The pathogen is destroyed before it can cause illness
Vaccine → Immune response → Antibodies + Memory cells → Future protection
The first exposure is slow. The second exposure is fast and strong.
The first exposure is slow. The second exposure is fast and strong.
Herd Immunity
Herd immunity means that if a high enough percentage of the population is vaccinated, the spread of the disease is reduced so much that even unvaccinated people are protected because the disease cannot easily spread.
Example 1
Question: Explain how a vaccine provides immunity to a disease. (3 marks)
Answer: A vaccine contains a dead or inactive pathogen which is injected into the body. White blood cells (lymphocytes) produce antibodies specific to that pathogen. Memory cells are also produced and remain in the blood. If the live pathogen enters later, the memory cells recognise it and produce antibodies quickly, destroying the pathogen before it causes disease.
Example 2
Question: Explain what is meant by herd immunity. (2 marks)
Answer: Herd immunity is when a large proportion of the population is vaccinated, so the disease cannot spread easily. This protects unvaccinated people because there are fewer people carrying the disease to pass it on.
📝 Antibiotics
Antibiotics are medicines that kill bacteria inside the body. They do NOT work against viruses because viruses live inside body cells, so it is difficult to kill the virus without also killing the body's cells.
Discovery of Penicillin
Alexander Fleming discovered penicillin in 1928. He noticed that mould (Penicillium notatum) on a petri dish had killed the bacteria around it. This led to the development of the first antibiotic.
Antibiotic Resistance
Antibiotic resistance is a major problem. Bacteria can evolve to become resistant to antibiotics through natural selection:
- A random mutation in a bacterium may make it resistant to an antibiotic
- When the antibiotic is used, non-resistant bacteria are killed
- The resistant bacteria survive and reproduce
- The population of resistant bacteria increases
- The antibiotic no longer works against this strain (e.g. MRSA)
Antibiotic resistance is an example of natural selection (evolution).
Mutation → Variation → Selection pressure (antibiotic) → Resistant bacteria survive → Reproduce → Resistance spreads
Mutation → Variation → Selection pressure (antibiotic) → Resistant bacteria survive → Reproduce → Resistance spreads
Reducing Antibiotic Resistance
- Only use antibiotics when they are really needed (not for viral infections)
- Always complete the full course of antibiotics (don't stop early even if you feel better)
- Reduce the use of antibiotics in farming
- Develop new antibiotics to replace those that no longer work
Example 3
Question: Explain why antibiotics cannot be used to treat viral infections like the common cold. (2 marks)
Answer: Antibiotics kill bacteria by targeting their cell walls or internal processes. Viruses live inside the body's own cells, so it is difficult to kill the virus without also damaging the body's cells. Antibiotics have no effect on viruses because viruses have a different structure to bacteria.
Example 4
Question: Explain how antibiotic-resistant bacteria like MRSA have developed. (3 marks)
Answer: A random mutation made some bacteria resistant to the antibiotic. When the antibiotic was used, it killed the non-resistant bacteria (selection pressure), but the resistant bacteria survived. The resistant bacteria then reproduced, passing on the resistance gene. Over time, the population of resistant bacteria increased, meaning the antibiotic no longer works against this strain.
Example 5
Question: A doctor tells a patient to complete their full 7-day course of antibiotics. Explain why it is important not to stop taking the antibiotics after 3 days even if the patient feels better. (3 marks)
Answer: Stopping the course early means not all the bacteria are killed. The surviving bacteria are the ones most resistant to the antibiotic. These resistant bacteria can then reproduce, creating a population of antibiotic-resistant bacteria. This makes future infections harder to treat because the antibiotic will be less effective.
📝 Painkillers and Other Drugs
Painkillers (like paracetamol and ibuprofen) relieve the symptoms of a disease but do NOT kill the pathogen. They reduce pain and fever but do not cure the infection.
Other drugs that treat symptoms:
- Antiseptics - chemicals that destroy pathogens outside the body (on surfaces or skin)
- Disinfectants - strong chemicals that destroy pathogens on surfaces (not on skin)
- Antiviral drugs - can stop some viruses from replicating (but are much harder to develop than antibiotics)
📝 Drug Development and Testing
New drugs must be thoroughly tested before they can be given to patients. This is to check they are safe and effective, and to find the correct dose.
Stages of Drug Development
- Preclinical testing - tested on cells and tissues in the lab, then on animals (to check for toxicity and effectiveness)
- Clinical trials - Phase 1 - tested on a small number of healthy volunteers (to check for side effects and safe dosage)
- Clinical trials - Phase 2 - tested on a larger number of patients who have the disease (to check if it actually works)
- Clinical trials - Phase 3 - tested on a very large number of patients (to confirm effectiveness and monitor side effects)
Double-Blind Trials
In a double-blind trial, neither the patients nor the doctors know who has the real drug and who has the placebo. This eliminates bias in the results.
- Placebo - a fake treatment that looks like the real drug but has no active ingredient
- Blind trial - the patient does not know if they have the real drug or placebo
- Double-blind trial - neither the patient nor the doctor knows
The Thalidomide Tragedy
Thalidomide was developed in the 1950s as a sleeping pill. It was then given to pregnant women to treat morning sickness. However, it had not been properly tested on pregnant women, and it caused severe birth defects (babies were born with shortened limbs). This led to much stricter drug testing regulations.
Example 6
Question: Explain why new drugs are tested using double-blind trials. (3 marks)
Answer: In a double-blind trial, neither the patients nor the doctors know who is receiving the real drug and who is receiving the placebo. This eliminates bias because the doctors cannot influence the results by subconsciously favouring one group, and patients cannot be influenced by knowing they are taking the real drug (the placebo effect). This means any difference in outcomes is due to the drug itself, not psychological factors.
❓ Practice Questions
Q1: Explain how vaccination provides immunity to a disease. (3 marks)
Q2: Explain why antibiotics cannot be used to treat viral diseases. (2 marks)
Q3: Describe how antibiotic-resistant bacteria develop. (3 marks)
Q4: Suggest two ways to reduce the problem of antibiotic resistance. (2 marks)
Q5: Explain why new drugs must be tested using double-blind trials. (3 marks)
Q6: What is the difference between a painkiller and an antibiotic? (2 marks)
✅ Answers
- A vaccine contains a dead or inactive pathogen. When injected, white blood cells produce antibodies specific to the pathogen and memory cells are formed. If the live pathogen enters later, memory cells produce antibodies rapidly, destroying the pathogen before illness develops.
- Antibiotics target bacterial structures (like cell walls). Viruses live inside body cells, so antibiotics cannot reach them without damaging the body's own cells. Viruses also have different structures to bacteria, so antibiotics have nothing to target.
- A random mutation in a bacterium makes it resistant to an antibiotic. When the antibiotic is used, non-resistant bacteria are killed but resistant ones survive. The resistant bacteria reproduce, passing on the resistance gene. Over time the resistant population increases.
- Only use antibiotics when necessary (not for viral infections); always complete the full course of antibiotics; reduce use of antibiotics in farming; develop new antibiotics (any two).
- In a double-blind trial, neither patients nor doctors know who receives the real drug or the placebo. This eliminates bias - doctors cannot subconsciously influence results, and the placebo effect is controlled. Any difference in outcomes must be due to the drug itself.
- Antibiotics kill bacteria that cause infection (they cure the disease). Painkillers only relieve symptoms like pain and fever - they do not kill the pathogen or cure the infection.
🎯 Exam Tips
- Always say "dead or inactive pathogen" when defining a vaccine - not "weakened"
- Antibiotics kill BACTERIA only - they do NOT work on viruses (very common exam question)
- Antibiotic resistance is an example of natural selection - mention mutation, selection pressure, survival and reproduction
- For drug trials, remember the order: preclinical (cells/animals) → clinical (healthy volunteers → patients)
- "Double-blind" means neither patient NOR doctor knows - explain why this removes bias
- The thalidomide tragedy is why drug testing is now so strict
- Painkillers relieve symptoms; antibiotics cure the infection - know the difference
🔢 Maths Skills
Mathematical Skills
Interpret vaccination coverage and disease data from graphs and tables. Calculate the percentage of a population that needs to be vaccinated for herd immunity. Interpret antibiotic resistance data showing the proportion of resistant vs non-resistant bacteria.
Maths Example
In a population of 60,000, 80% are vaccinated against measles. How many people are vaccinated, and is this likely enough for herd immunity (typically 95% needed)?
Vaccinated = 0.80 × 60,000 = 48,000. No — 95% × 60,000 = 57,000 would be needed, so 9,000 more vaccinations are required.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Vaccines give you the disease Correct: Vaccines contain dead or inactive pathogens that cannot cause the disease — they only trigger the immune system to produce antibodies and memory cells
2. Wrong: Antibiotics kill viruses Correct: Antibiotics only work against bacteria — they target bacterial structures like cell walls; viruses live inside body cells and have different structures, so antibiotics are ineffective
✍️ 6-Mark Question
Extended Answer
6 marks: Explain how vaccination works and how antibiotic resistance develops.
Vaccination involves injecting a small amount of dead or inactive pathogen into the body. The white blood cells recognise the pathogen as foreign and lymphocytes produce antibodies specific to its antigens. Memory cells are also produced and remain in the blood. If the live pathogen enters later, the memory cells rapidly produce large quantities of the correct antibodies, destroying the pathogen before it causes illness. Antibiotic resistance develops through natural selection. A random mutation in a bacterium may make it resistant to an antibiotic. When the antibiotic is used, it kills non-resistant bacteria but the resistant bacteria survive. The resistant bacteria then reproduce, passing on the resistance gene to their offspring. Over time, the population of resistant bacteria increases, and the antibiotic becomes ineffective against this strain — for example, MRSA is resistant to several antibiotics.
Mark scheme: 1 mark for vaccine contains dead/inactive pathogen; 1 mark for antibody and memory cell production; 1 mark for secondary response; 1 mark for mutation causing resistance; 1 mark for selection pressure (antibiotic kills non-resistant); 1 mark for reproduction of resistant strain
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A country introduced a new vaccination programme. The table shows disease cases before and after:
| Year | Vaccination coverage (%) | Disease cases |
|---|---|---|
| 2018 | 45 | 12,000 |
| 2019 | 62 | 8,500 |
| 2020 | 78 | 4,200 |
| 2021 | 89 | 1,800 |
| 2022 | 92 | 600 |
(a) Describe the trend in the data.
(b) Explain why disease cases drop sharply between 89% and 92% coverage but dropped more slowly at lower coverage.
Answers: (a) As vaccination coverage increases from 45% to 92%, disease cases decrease from 12,000 to 600 — there is a negative correlation. (b) At higher vaccination rates, herd immunity begins to take effect — the disease cannot spread easily because most people are immune, so even unvaccinated people are protected. The effect is non-linear: near the herd immunity threshold, small increases in coverage cause large drops in cases because the pathogen effectively runs out of hosts to infect.
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