Vaccines

TL;DR. Vaccines show your immune system a harmless preview of a pathogen so it can build memory before meeting the real thing. Alongside clean water and sanitation, they are the highest-impact medical intervention in history: smallpox eradicated, polio nearly so, and measles deaths down by the large majority since 2000. The 1998 paper linking MMR to autism was found fraudulent, retracted, and its author struck off, and the claim has been tested in studies of millions of children and found false. Vaccines have real side effects, mostly minor and short-lived, and a small number of rare serious ones that are documented, monitored, and far outweighed by the diseases they prevent.

1. How they work

Your adaptive immune system remembers. On first meeting a pathogen it takes 7 to 14 days to mount an effective response, during which you are ill. It then keeps memory B and T cells so a second encounter is met within hours.

A vaccine creates that memory without the illness. It presents an antigen, a recognisable piece of the pathogen, in a form that cannot cause disease.

Adjuvants are substances added to non-live vaccines to provoke a stronger response, because a purified protein on its own is often too bland to alarm the immune system. Aluminium salts are the oldest and most used; newer adjuvants (AS01, MF59) are used in shingles and some flu vaccines and are more reactogenic, which is why the shingles vaccine commonly makes your arm ache.

2. The types

mRNA vaccines deserve their own explanation, because they are new to the public and much misunderstood. They deliver messenger RNA, wrapped in a lipid nanoparticle, encoding the viral spike protein. Your cells read it, make the protein, display it, and the immune system responds. The mRNA is degraded within days and never enters the cell nucleus, where your DNA is. It cannot alter your genome: there is no mechanism by which RNA becomes DNA in a human cell without a reverse transcriptase, which these vaccines do not supply. The technology took decades to develop, resting on Katalin Karikó and Drew Weissman's work on nucleoside modification, which won the 2023 Nobel Prize.

3. Herd immunity

When enough of a population is immune, transmission chains break and the pathogen cannot sustain spread. This protects the people who cannot be vaccinated: newborns, the immunosuppressed, people on chemotherapy, and those with genuine contraindications.

The threshold depends on how contagious the disease is, expressed by $R_0$, the average number of people one case infects in a fully susceptible population:

$$\text{Threshold} = 1 - \frac{1}{R_0}$$

Measles is the most contagious common human disease, which is why it is always the first to return when coverage falls. It requires around 95 percent coverage, leaving very little margin. Several countries that had eliminated measles have lost that status in recent years following coverage declines, and outbreaks with deaths have followed.

4. What vaccines have achieved

• Smallpox: eradicated. Declared in 1980, after a disease that killed an estimated 300 million people in the twentieth century alone. It is the only human disease ever eradicated.
• Polio: over 99 percent reduction since 1988. Wild poliovirus remains endemic in only Afghanistan and Pakistan.
• Measles: deaths reduced by roughly 80 percent since 2000, an estimated 50-plus million deaths prevented.
• Hib meningitis: near elimination in countries with routine vaccination, from a leading cause of childhood meningitis.
• Hepatitis B and liver cancer: Taiwan's universal infant programme produced measurable reductions in childhood liver cancer, the first demonstration of a vaccine preventing a cancer.
• HPV and cervical cancer: this is now dramatic. A 2021 Lancet study of the English programme found cervical cancer rates roughly 87 percent lower in women vaccinated at 12 to 13, and Scottish data published in 2024 found no cases at all among women vaccinated at that age. Cervical cancer is on course to become the first cancer eliminated by vaccination.
• Rotavirus: large falls in infant hospitalisation for gastroenteritis.
• COVID-19 vaccines: modelling published in Lancet Infectious Diseases estimated roughly 14 to 20 million deaths prevented in the first year alone.

A 2024 WHO analysis estimated that vaccination has saved around 154 million lives over 50 years, the majority of them infants.

5. Side effects, honestly

Common and expected, usually within a day or two and lasting 1 to 3 days: sore arm, redness and swelling, mild fever, fatigue, headache, muscle aches. These are the immune system responding, not the vaccine harming you, and their absence does not mean the vaccine has not worked.

Rare and serious ones, which are real and documented:

These are known because the surveillance systems work. VAERS in the US, the Yellow Card scheme in the UK, and EudraVigilance in the EU collect reports; the signals above were detected, quantified, and acted upon, sometimes with products restricted or withdrawn. A system that finds a one-in-a-hundred-thousand event is a system that is working.

Note that VAERS and Yellow Card are open reporting systems. Anyone can submit a report, and a report is not a confirmed causal link. Raw counts from these databases are routinely misrepresented as confirmed vaccine injuries, and the databases themselves say clearly that they cannot be used that way.

6. The MMR and autism claim

This deserves a full and plain account because it caused measurable harm.

1998: Andrew Wakefield and colleagues published a paper in The Lancet describing 12 children and suggesting a possible link between MMR vaccine, bowel inflammation, and autism.

What was subsequently established:

• The data were falsified. Investigative journalism by Brian Deer, and the General Medical Council's inquiry, found the medical records did not match the reported findings for any of the children.
• Wakefield was paid over £400,000 by solicitors acting for parents seeking to sue vaccine manufacturers, undisclosed at publication.
• He had filed a patent for a competing single measles vaccine.
• Children were subjected to invasive procedures including colonoscopies and lumbar punctures without appropriate ethical approval.
• The paper was fully retracted in 2010, and Wakefield was struck off the UK medical register for dishonesty and serious professional misconduct.

What the science shows: the question has been examined in cohorts totalling millions of children. A Danish study of 657,461 children found no association, including in subgroups with higher autism risk. A Cochrane review of 138 studies found no association. Autism prevalence rose after the paper as diagnostic criteria broadened and awareness increased, and did not fall in countries that removed thimerosal or changed schedules.

The cost: MMR coverage in the UK fell from above 90 percent to around 80 percent, and to under 50 percent in some areas. Measles, which had been declared eliminated in the UK in 2017, returned; the UK lost its elimination status in 2019. Children have died. Outbreaks continue.

Autism signs typically become apparent between 12 and 24 months, which is when MMR is given. That coincidence in timing is the whole basis of the perceived link, and it is what a controlled study is designed to disentangle.

7. Common concerns, addressed directly

"Too many vaccines overwhelm a baby's immune system." An infant's immune system responds to thousands of antigens daily from ordinary life. The entire modern childhood schedule contains around 150 antigens in total, compared with over 3,000 in the smallpox vaccine alone. Modern purified vaccines contain fewer antigens than the schedule of 30 years ago while protecting against more diseases.

"Natural immunity is better." Infection-acquired immunity is often stronger and sometimes longer-lasting for some diseases. It is acquired by having the disease, with its risks: measles kills 1 to 3 per 1,000 in wealthy countries and causes immune amnesia, wiping out existing antibody memory to other pathogens for years. Tetanus kills 10 to 20 percent of cases. There is no natural immunity to tetanus, because the toxin dose that causes disease is too small to provoke a response, which is why survivors still need vaccinating.

"Aluminium is toxic." The quantity in a vaccine (0.125 to 0.85 mg) is comparable to what an infant ingests from breast milk or formula over a few weeks. It has been used since the 1930s and studied extensively.

"Thimerosal / mercury." Thimerosal is ethylmercury, cleared from the body in days, not methylmercury, which bioaccumulates (Chapter 92). It was removed from routine childhood vaccines in the US and EU from 2001 as a precaution, and autism rates continued to rise, which is itself informative. Most vaccines now come in single-dose vials requiring no preservative.

"Vaccines contain fetal cells." Some viral vaccines are grown in cell lines originally derived from two elective abortions in the 1960s (WI-38 and MRC-5). The cells have been dividing in culture for 60 years; no fetal tissue is in the vaccine and no ongoing abortions are involved. Several religious authorities, including the Vatican, have addressed this and concluded vaccination is acceptable.

"The COVID vaccines were rushed." The compression came from running trial phases in parallel rather than in sequence, manufacturing at risk before approval, and unprecedented funding and recruitment, not from skipping safety steps. mRNA technology had been in development for approximately 30 years. Trials involved tens of thousands of participants, and post-marketing surveillance has now covered billions of doses.

8. Practical points

• Check your own status. Adults routinely need: tetanus/diphtheria boosters (every 10 years in most schedules), annual influenza if in an eligible group, shingles from 50 or 60 depending on the country, pneumococcal from 65, and COVID boosters per current guidance.
• Pregnancy: whooping cough (pertussis) vaccination in every pregnancy, ideally 16 to 32 weeks, passes antibodies to the baby and is highly effective at preventing infant whooping cough, which can be fatal in newborns. Influenza and COVID vaccines are also recommended. Live vaccines are avoided in pregnancy.
• Travel: check requirements 6 to 8 weeks before departure; some vaccines need multiple doses and yellow fever certification has legal entry requirements.
• HPV vaccination is most effective before exposure, which is why it is given at 12 to 13, and is offered to boys as well as girls in most programmes because HPV causes throat, anal, and penile cancers too.
• After vaccination: paracetamol for discomfort is fine. Routine prophylactic paracetamol before vaccination is generally not advised, because some studies suggest it slightly blunts the antibody response.
• Keep a record. Vaccination histories are frequently lost between countries and providers.

9. The bottom line

• Vaccines teach the immune system in advance, and alongside clean water they are the highest-impact medical intervention there has ever been: an estimated 154 million lives saved in 50 years.
• Measles needs about 95 percent coverage because it is extraordinarily contagious, which is why it is always the first disease to return when uptake falls.
• HPV vaccination has reduced cervical cancer by around 87 percent in vaccinated cohorts, and Scottish data found no cases at all in those vaccinated at 12 to 13.
• The MMR-autism paper was fraudulent, retracted, and its author struck off. The question has since been studied in millions of children with no association found.
• Side effects are real: mostly sore arms and a day of feeling rough, plus a small number of rare serious events that are known precisely because surveillance systems detect them.
• mRNA vaccines cannot alter your DNA. There is no mechanism.

Sources and notes

Vaccine types, schedules, and contraindications follow the UK Green Book (Immunisation against infectious disease), CDC Pink Book, and WHO position papers. Herd immunity thresholds and R0 estimates follow Anderson and May and subsequent measles-specific analyses. Smallpox eradication follows Fenner et al.'s WHO history. Lives saved estimates follow Shattock et al., The Lancet, 2024, for the WHO Expanded Programme on Immunization. HPV and cervical cancer follows Falcaro et al., The Lancet, 2021, for England, and the 2024 Scottish data reporting no cases in those vaccinated at 12 to 13. COVID-19 vaccine deaths averted follows Watson et al., The Lancet Infectious Diseases, 2022. mRNA nucleoside modification follows Kariko and Weissman's work, recognised by the 2023 Nobel Prize. Myocarditis rates after mRNA vaccination and the comparison with infection-associated myocarditis follow Patone et al., Nature Medicine, 2022. VITT follows Greinacher et al., NEJM, 2021. The Wakefield fraud follows Deer's BMJ investigation, 2011, the GMC determination, and the Lancet retraction of 2010. The refutations follow Hviid et al., Annals of Internal Medicine, 2019 (657,461 Danish children), and Taylor, Swerdfeger, and Eslick's meta-analysis. Antigen counts across the schedule follow Offit et al., Pediatrics, 2002. Measles immune amnesia follows Mina et al., Science, 2019.

Open questions. Duration of protection for several newer vaccines, and optimal booster intervals, are still being established. Why vaccine hesitancy persists despite the evidence is a social rather than a scientific question and is not well solved.

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