Vaccines

The Immune System in Action

• When the vaccine enters the body, a specific type of white blood cell called a lymphocyte detects the foreign antigens.
• The specific lymphocyte that recognises the antigen is activated and divides rapidly by mitosis to create clones.
• These cloned cells produce a massive number of specific antibodies. These Y-shaped proteins have a shape that is exactly complementary to the foreign antigens.
• This initial reaction is called the primary immune response. It is relatively slow, taking roughly 10–17 days for significant antibody levels to appear in the blood.

Destroying the Pathogen

• Once released, antibodies bind to the antigens on the surface of the pathogen. This often causes the pathogens to clump together in a process called agglutination.
• Clumping makes it much easier for another type of white blood cell, called a phagocyte, to engulf and destroy the pathogens.
• Some lymphocytes also produce an antitoxin to neutralise any harmful toxins released by bacteria.

The Role of Memory Cells

• You can forget a password seconds after setting it, but your immune system can remember a microscopic invader for decades.
• After the primary response is over, some of the cloned lymphocytes differentiate into memory cells. These remain in the bloodstream long-term, sometimes for the rest of your life.
• If the real, virulent pathogen enters the body later, these cells recognise the specific antigens immediately, triggering the secondary immune response.
• The secondary response is much faster (often within 1–3 days) and produces a significantly larger quantity of antibodies. The pathogen is destroyed before it can multiply and cause damage, giving the individual active immunity.

Herd Immunity and Protecting the Vulnerable

• Choosing to get vaccinated does not just protect you; it creates an invisible shield around the most vulnerable people in your community.
• If a crowd is throwing a ball, and 90% of the people act as "buffers" who refuse to catch or pass it, the ball cannot reach the remaining 10% of people. This is how a high vaccination rate breaks the chain of infection.
• This concept is called herd immunity. By vaccinating a large proportion of the population, the probability of a pathogen spreading is drastically reduced.
• This provides indirect protection to susceptible individuals who cannot be vaccinated, such as newborn babies, the elderly, or those with weakened immune systems.

Vaccination Thresholds

• The vaccination threshold is the percentage of the population that must be immune to stop the disease from spreading. Highly infectious diseases require higher thresholds; for example, Measles requires around 95%, while Polio requires approximately 80%.
• If vaccination rates fall below this threshold (often due to vaccine safety controversies), herd immunity breaks down. This leaves the population vulnerable to an epidemic.

Calculating Herd Immunity (Higher Tier)

The threshold required for herd immunity depends on how easily the disease spreads, represented by $R_0$ (the reproduction number).

$$\text{Herd Immunity Threshold} = 1 - (1/R_0)$$

Worked Example:

A new strain of a virus has an $R_0$ value of 10. What percentage of the population must be vaccinated to achieve herd immunity?

Step 1: Write down the formula and values.

• $R_0 = 10$
• $\text{Threshold} = 1 - (1/10)$

Step 2: Calculate the decimal.

• $\text{Threshold} = 1 - 0.1 = 0.9$

Step 3: Convert to a percentage.

• $0.9 \times 100 = 90\%$

Tracking Epidemic Outbreaks

If vaccination rates drop, you may need to calculate the percentage increase in disease cases using this formula:

$$\text{Percentage Increase} = \frac{\text{New Cases} - \text{Old Cases}}{\text{Old Cases}} \times 100$$