Every fact in the three vaccine lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications and care points: this is general science education, not medical advice. Questions about a pupil's own vaccinations, side effects or health go to a parent, the school nurse or a doctor, and pupils should never be asked to say which vaccines they have had. The lessons describe the immune response in a simplified way (antigens, antibodies and memory cells); real responses involve many more cell types. Dose schedules differ between countries and change over time: the examples quoted come from the US CDC and are given only to show why some vaccines need more than one dose. The classroom experiments are models: the paper shapes and the 'herd immunity' simulations assume perfect vaccines and simple mixing between people, which real life does not have. The herd immunity threshold 1 − 1/R₀ is a textbook approximation; R₀ for measles is often quoted as 'about 15', but published estimates vary.
All sources
[1] Explaining How Vaccines Work
Centers for Disease Control and Prevention (USA) · Reliability: High
Type: National public health agency
Vaccines work by imitating an infection to engage the body's natural defences. The active ingredient in all vaccines is an antigen: weakened or killed bacteria or viruses, pieces of their surface or genetic material, or a bacterial toxin treated to make it non-toxic. Antibodies are proteins made by white blood cells; after an infection a few white blood cells stay 'to keep watch'. Immunity can take weeks to develop. Active immunity takes longer to develop but lasts longer than passive immunity, which is immediate but fades within weeks or months. A single dose gives only partial protection: live-attenuated vaccines (e.g. chickenpox, MMR) usually need two doses; non-live vaccines need three or more doses and boosters. Vaccinated people can still be infected but are far less likely to become seriously ill. Live-attenuated vaccines can be dangerous for people with a weak or suppressed immune system. The response may cause tiredness and discomfort for a day or two.
Vaccines contain weakened or inactive parts of a particular organism (antigen); newer vaccines carry a genetic blueprint (DNA or RNA) that tells the body to make the antigen. The weakened version won't cause the disease but prompts the immune system to respond much as it would to the real pathogen. The body creates antibody-producing memory cells that remain after the pathogen is defeated, so a later response is faster and stronger. Some vaccines need several doses given weeks or months apart. When many people are vaccinated, the pathogen has a hard time circulating, which protects people who cannot be vaccinated. No single vaccine provides 100% protection.
[3] Why vaccination is important and the safest way to protect yourself
NHS (UK National Health Service) · Reliability: High
Type: National health service
Vaccines teach your immune system how to create antibodies that protect you from diseases. All vaccines are thoroughly tested and carefully monitored. Common side effects: the area where the needle goes in looking red, swollen and a bit sore for 2 to 3 days, or feeling a bit unwell or having a high temperature for 1 or 2 days. If enough people are vaccinated, it is harder for a disease to spread to people who cannot have vaccines. Smallpox, polio and tetanus used to kill or disable millions of people.
An antigen is a molecule that stimulates a response in the immune system. Antibodies are proteins produced by plasma cells after stimulation by an antigen. A memory cell is an antigen-specific B or T lymphocyte; on re-exposure a step is skipped and immune defences are produced more rapidly (secondary response). Vaccination with non-infectious antigens generates a mild primary response and immune memory. Antibodies in a pregnant person's blood move across the placenta into the fetus.
The adaptive response takes days or even weeks to become established. Memory B cells that become plasma cells produce tens to hundreds-fold greater antibody amounts than in the primary response. Certain vaccine courses involve one or more booster vaccinations to mimic repeat exposures.
Smallpox killed at least 1 in 3 people infected and hundreds of millions over thousands of years. In May 1796 Edward Jenner inoculated 8-year-old James Phipps with matter from a cowpox sore on the hand of Sarah Nelmes: the world's first vaccine. The Intensified Smallpox Eradication Programme began in 1967, and in 1980 WHO declared smallpox officially eradicated.
Herd immunity occurs when an infectious disease can no longer spread in a community because enough people are immune. It indirectly protects people who cannot have a vaccine, such as some people with weakened immune systems and very young babies. The level needed depends on how easily a disease spreads; for a highly transmissible disease such as measles, 95% or more of people must be vaccinated.
History of Vaccines, The College of Physicians of Philadelphia · Reliability: Medium-high
Type: Medical college's educational site
The herd immunity threshold is calculated as 1 − (1/R₀). Measles has an R₀ of about 15 (one person infects about 15 others), giving 1 − 1/15 ≈ 0.933, or about 93% immunity needed. Vaccination targets quoted: measles 95%, polio 80%.