The Vaccine-Preventable Diseases

TL;DR. For most of human history, a large fraction of children died before their fifth birthday, and infectious disease did most of the killing. Measles, whooping cough, diphtheria, tetanus, polio, and smallpox were ordinary features of childhood, and every family expected to lose someone. Vaccination removed them so thoroughly in much of the world that the diseases became abstractions, and the abstraction is itself a problem: people now weigh a vaccine's small, visible risks against a disease they have never seen. This chapter is about what those diseases actually do, how the vaccines work, and why measles is always the first one to come back.

Key takeaways

• Smallpox is the only human disease ever eradicated, declared gone in 1980 after killing an estimated 300 million people in the twentieth century alone.
• Measles is the most contagious of the common human diseases, with an R0 around 12 to 18, which is why it needs about 95 percent coverage and why it reappears first when coverage slips.
• Measles does something distinctive: it erases existing immune memory ("immune amnesia"), leaving children vulnerable to infections they were already immune to, for years afterwards.
• Tetanus is not contagious at all. It comes from soil bacteria entering a wound, so herd immunity does not protect anyone, and everyone needs their own vaccination.
• Vaccination is estimated to have prevented on the order of 150 million deaths over the past 50 years, most of them in children under 5.
• The claim linking MMR to autism originated in a 1998 paper that was retracted, whose author lost his medical licence, and which has been contradicted by studies covering millions of children.

How a vaccine works, in one page

In short: A vaccine supplies the first encounter without the disease, and the type of vaccine determines its strength, its safety, and who can receive it.

Chapter 13 explains adaptive immunity: the first encounter with a pathogen takes 7 to 14 days to mount a response, and afterwards memory cells respond in hours. A vaccine supplies the first encounter without the disease.

Conjugate vaccines deserve a note because they solved a specific problem. Bacteria such as Haemophilus influenzae type b and the pneumococcus are coated in polysaccharide (sugar) capsules, and infants' immune systems respond poorly to pure sugars. Chemically linking the sugar to a protein converts it into a target the infant immune system handles well. That chemistry is why Hib meningitis, once a common cause of childhood death and deafness, has virtually disappeared from countries using the vaccine.

Adjuvants are substances added to non-live vaccines to provoke a stronger innate response so that the adaptive one is better. Aluminium salts have been used for this since the 1930s.

Smallpox: the one that was eradicated

In short: Five specific properties made smallpox eradicable, and most infections lack at least one of them.

Variola virus caused a disease with a rash of deep, pus-filled lesions, a mortality around 30 percent, and permanent scarring or blindness in many survivors. It killed an estimated 300 million people in the twentieth century.

Its defeat is worth understanding because it is the template. Variolation (deliberate infection with material from a mild case) was practised in Asia and Africa for centuries and brought to England by Lady Mary Wortley Montagu in 1721. In 1796 Edward Jenner demonstrated that inoculation with cowpox protected against smallpox, giving us both the practice and the word (vacca, cow). The WHO eradication campaign, intensified from 1967, used ring vaccination: rather than vaccinating everyone, find each case, vaccinate their contacts and their contacts' contacts, and starve the virus of susceptible people.

The last natural case was Ali Maow Maalin in Somalia in 1977. Eradication was certified in 1980.

Smallpox was eradicable because it had no animal reservoir, an obvious rash that made cases findable, no asymptomatic carriage, a stable virus, and a heat-stable effective vaccine. Most infections lack at least one of those, which is why smallpox remains the only one, with rinderpest in cattle the only other eradicated disease of any species.

Measles

In short: The most contagious common human disease, and it erases existing immune memory, leaving children vulnerable to infections they had already survived.

What it is. A virus so contagious that if you are not immune and you enter a room two hours after an infected person left it, you are likely to catch it. It infects around 90 percent of susceptible close contacts.

What it does. Ten to fourteen days after exposure: high fever, cough, runny nose, conjunctivitis, then a characteristic rash spreading from the face downward. Complications are common: ear infection, diarrhoea, and pneumonia (the usual cause of death). About 1 in 1,000 develops encephalitis, and a similar order of magnitude die in high-income settings, with much higher case fatality where malnutrition and limited healthcare prevail. Subacute sclerosing panencephalitis (SSPE) is a rare, invariably fatal degenerative brain disease appearing 7 to 10 years after infection, and it is more common after infection in infancy.

Immune amnesia is the underappreciated feature. Measles infects and destroys memory lymphocytes, wiping out a portion of a child's accumulated immunity to other pathogens. Studies published in 2019 showed that measles eliminated 11 to 73 percent of the antibody repertoire in unvaccinated children, and population data show elevated mortality from other infections for two to three years after measles. The vaccine prevents this too.

Why it is the sentinel. With R0 around 12 to 18, the herd immunity threshold is roughly 92 to 95 percent. Almost every other vaccine-preventable disease has a lower threshold. So when coverage falls, measles returns first, and returning measles is the reliable early warning of a weakening immunisation programme. Global measles deaths remain around 100,000 a year despite an inexpensive, extremely effective vaccine, and outbreaks have recurred in high-income countries with pockets of low coverage.

Polio

In short: Paradoxically a product of better sanitation, and the two vaccines differ in ways that decide how each is used.

What it is. An enterovirus spread by the faecal-oral route. In over 99 percent of infections it causes nothing or a mild illness. In fewer than 1 in 200, it invades motor neurons in the spinal cord and destroys them, causing permanent flaccid paralysis, most often in the legs. If it hits the neurons controlling breathing, the patient cannot breathe, which is what the iron lung was for.

History. Paradoxically, polio epidemics were a product of improved sanitation. Where water was contaminated, infants met the virus while still protected by maternal antibodies and were immunised by that early infection. Cleaner water delayed exposure into later childhood, when paralysis is more likely, producing the twentieth-century epidemics.

Two vaccines, and the difference matters:

Oral vaccine's transmission-blocking property is what made mass campaigns possible and drove cases down by over 99 percent since 1988. Its reversion risk is why countries switch to injected vaccine as they approach elimination, and why remaining circulating vaccine-derived outbreaks occur in under-immunised populations. Wild poliovirus type 1 now persists only in Afghanistan and Pakistan, where conflict, distrust, and attacks on vaccinators have repeatedly disrupted campaigns.

Whooping cough (pertussis)

A bacterial infection (Bordetella pertussis) causing weeks of paroxysmal coughing fits ending in the characteristic "whoop" as air is dragged back in. Adults get a prolonged unpleasant cough; infants under 6 months get apnoea, pneumonia, seizures, and death. It is sometimes called the 100-day cough.

Two features shape modern policy. The acellular vaccine used since the 1990s (safer and less reactogenic than the old whole-cell one) produces less durable protection, so immunity wanes over several years, which is part of why pertussis has resurged in countries using it. And because infants are most vulnerable before they can be fully vaccinated, protection is provided by vaccinating pregnant women in the third trimester, transferring antibodies across the placenta, which is highly effective at preventing infant disease, and by boosting the adults around the baby.

Diphtheria and tetanus

In short: Tetanus comes from soil rather than from people, so herd immunity protects nobody and everyone needs their own vaccination.

Diphtheria produces a toxin that kills the lining of the throat, forming a grey membrane that can suffocate the patient, and it damages heart and nerves. It killed thousands of children a year in Europe and North America before immunisation. Its resurgence in the former Soviet Union in the 1990s, with over 150,000 cases after health systems collapsed, is a demonstration of what happens when a programme stops.

Tetanus is different from everything else in this chapter and worth understanding precisely. Clostridium tetani spores live in soil and manure everywhere, enter through wounds, and produce a toxin that blocks the inhibitory signals in the nervous system. Muscles contract and cannot relax: lockjaw, arching of the back, and spasms severe enough to fracture bones, with the patient fully conscious. Mortality is high even with intensive care.

Tetanus is not contagious. It is acquired from the environment, so herd immunity offers no protection whatsoever: your neighbours' vaccination status is irrelevant to your risk. This is the cleanest counterexample to the belief that vaccination is only a collective matter. Maternal and neonatal tetanus, from unclean cord care, killed hundreds of thousands of newborns a year and has been enormously reduced by vaccinating pregnant women and improving delivery hygiene.

The rest of the schedule

In short: Rubella vaccination exists to protect pregnancies, and conjugate vaccines exist because infants respond poorly to sugar coatings.

Is any of this deadly?

Historically, overwhelmingly so, and still is where coverage is low. Before vaccines, measles killed an estimated 2.6 million people a year, diphtheria and pertussis killed children by the hundreds of thousands, and polio paralysed tens of thousands of children annually in high-income countries alone.

A 2024 analysis for WHO estimated that vaccination has averted on the order of 154 million deaths over the past 50 years, roughly 146 million of them in children under 5, and that measles vaccination alone accounts for a large share of that total. Whatever the precision of such a number, the direction and magnitude are not in dispute: immunisation is the most consequential public health intervention of the last century after clean water and sanitation.

Is it contagious?

Most of these are extremely contagious, which is why they were universal before vaccination. The exceptions worth remembering: tetanus (environmental, not contagious) and shingles (a reactivation of your own dormant virus, though a person with shingles can give chickenpox to someone who has never had it).

Who gets it now

In short: Two very different causes of under-vaccination: no access, and hesitancy, and they need opposite responses.

Under-vaccinated children, and the pattern has two very different causes.

Access. Most of the world's "zero-dose" children (those who have received no vaccines at all) live in low-income countries, conflict zones, remote areas, and urban slums. The barrier is supply chain, distance, cost, and disrupted health systems.

Hesitancy. In high-income countries, coverage gaps cluster in specific communities and tend to be geographic rather than uniform, which matters because measles transmission depends on local, not national, immunity. A country at 92 percent national coverage can have neighbourhoods at 60 percent, and that is where outbreaks happen.

The MMR and autism claim, stated plainly because vague treatment of it does not help: a 1998 paper in The Lancet by Andrew Wakefield proposed a link between MMR and autism based on 12 children. It was retracted in 2010. The UK General Medical Council found the research dishonest and struck Wakefield off the medical register. Subsequent studies covering millions of children across multiple countries, including a Danish cohort of over 650,000, found no association. Vaccination rates fell in the meantime and measles returned, with deaths that would not otherwise have occurred. It is one of the most damaging episodes of scientific fraud on record.

That said, dismissing all vaccine hesitancy as ignorance is both inaccurate and ineffective. The evidence on what actually improves uptake favours convenience (default appointments, on-site provision, reminders), trusted local messengers, and clinicians who take questions seriously, rather than argument.

Treatment, and what it costs

In short: Most of these diseases have no specific treatment once established, which is the whole argument for prevention.

Most of these diseases have no specific treatment once established, which is the core argument for prevention.

• Measles: supportive care, plus vitamin A, which reduces mortality in children with deficiency.
• Polio: no antiviral. Supportive care and rehabilitation.
• Pertussis: antibiotics reduce transmission and shorten the infectious period; they do little for the cough once the paroxysmal stage has begun.
• Diphtheria: antitoxin plus antibiotics, and antitoxin supply is globally scarce.
• Tetanus: antitoxin, antibiotics, wound care, muscle relaxants, and often weeks of intensive care and ventilation.

Vaccine side effects, honestly stated: sore arm, mild fever, and irritability are common. Febrile seizures occur after MMR at roughly 1 in 3,000 doses, are frightening, and are not associated with long-term harm. Anaphylaxis occurs at roughly 1 per million doses, which is why vaccination sites keep adrenaline and observe patients briefly. Live vaccines are contraindicated in severe immunosuppression and pregnancy. Rotavirus vaccine carries a small excess risk of intussusception, estimated at roughly 1 to 6 per 100,000 first doses, against a disease that hospitalised or killed vastly more.

The right comparison is never "vaccine risk versus zero." It is "vaccine risk versus disease risk," and for every vaccine on a national schedule that comparison is not close.

What the person can do

• Keep the childhood schedule on time. The timing is not arbitrary; it is set by when maternal antibodies wane and when infants are most vulnerable.
• Check your own status as an adult. Tetanus boosters every 10 years, MMR if you were born after routine vaccination began but never completed the course, pertussis in every pregnancy, shingles and pneumococcal vaccines at the recommended ages, and annual influenza vaccine.
• Before travel, check requirements and recommendations early, since some vaccines need weeks.
• Protect infants indirectly. Newborns cannot be vaccinated against pertussis or measles immediately; vaccinating the adults around them is how they are protected.
• Ask, and expect a real answer. A clinician who cannot explain what a vaccine is for and what its risks are is not doing their job.

What's next

• Polio eradication, now a matter of reaching a small number of communities in two countries, plus stopping vaccine-derived outbreaks with the newer, more genetically stable oral vaccine (nOPV2).
• Measles and rubella elimination regionally, which requires sustained 95 percent coverage rather than any new technology.
• Better pertussis vaccines that produce durable, transmission-blocking immunity.
• Malaria, TB, and RSV vaccines, covered in their own chapters, extending the model to diseases that resisted it for a century.
• Thermostable formulations and microarray patches, removing the cold chain and the needle, which are the two largest practical barriers to reaching remote populations.

Sources and notes

Deaths averted by vaccination: Shattock et al., The Lancet, 2024 (approximately 154 million over 50 years, 146 million in children under 5). Smallpox eradication and the last natural case (Ali Maow Maalin, Somalia, 1977; certified 1980): WHO. Measles immune amnesia: Mina et al., Science, 2019, and Petrova et al., Science Immunology, 2019. Measles R0 estimates vary between 12 and 18 across studies. Wakefield: The Lancet 1998, retracted 2010; GMC ruling 2010; Danish cohort, Hviid et al., Annals of Internal Medicine, 2019 (657,461 children). Diphtheria resurgence in the former USSR: Vitek and Wharton, Emerging Infectious Diseases, 1998. Intussusception risk after rotavirus vaccine: post-licensure surveillance studies. Adverse event rates are from national surveillance systems and vary somewhat by vaccine and product.

Open questions. Whether a more durable pertussis vaccine can be produced without the reactogenicity of the whole-cell version is unresolved. How to sustain very high coverage in societies where the diseases are invisible is a social problem without a technical answer.

Next: the diseases you get from what you eat and drink. 👉