Vaccines

Protecting one body is the smaller half of the story. The larger half is what happens when enough bodies are protected at once. A disease that spreads from person to person needs a supply of people it can still infect. Cut that supply below a certain threshold and the chain of transmission breaks — an infected person, on average, passes it to fewer than one other, and the outbreak fades instead of growing. This is herd immunity, and it means a vaccine protects not only the person who receives it but the people around them.

The threshold isn't fixed; it depends on how contagious the disease is. Measles is astonishingly transmissible — a single case can infect a dozen or more susceptible people — so roughly ninety-five percent of a community needs immunity to hold the line. For less contagious diseases the number is lower. The demanding thing about the measles figure is how little slack it leaves. Drop from ninety-five to ninety percent coverage in a town and you haven't shaved off a sliver of protection; you've reopened the door the whole community was relying on being shut.

Modern vaccine hesitancy has many roots, but one event sits at the centre of the story more than any other. In 1998, the British medical journal The Lancet published a paper by a gastroenterologist named Andrew Wakefield, based on just twelve children, suggesting a link between the combined measles, mumps and rubella vaccine and autism. The study was small, and its authors were cautious in their wording. The press coverage that followed was neither. A frightening idea had been handed a headline, and it travelled.

What came next is unusually well documented. Investigative reporting and formal inquiry established that the research was not merely weak but fraudulent: data had been altered, children had been selected in ways the paper concealed, and Wakefield had undisclosed financial interests tied to litigation against vaccine manufacturers. In 2010 The Lancet fully retracted the paper, and Wakefield was struck off the British medical register. Study after study, involving millions of children, has since found no link whatsoever between the MMR vaccine and autism. On the science, the matter is closed.

Step back from the mechanism and the math, and vaccines turn out to ask something unusual of the people who receive them. Most medicine responds to a problem you can feel: you are sick, you take something, you get better, and the connection is obvious. A vaccine works in the opposite direction. It is given to a healthy person to prevent something that then, if it works, never happens. The reward is an absence — an illness you'll never know you avoided. Success erases its own evidence.

This is the strange predicament immunisation creates for itself. The more thoroughly a vaccine works across a population, the more the disease disappears from living memory, and the more the vaccine's remaining, tiny risks come to feel large by comparison. Nobody now remembers a ward of iron lungs or a child blinded by measles. What people can see is the needle in front of them and the rare side effect they've read about. When the threat is invisible and the intervention is visible, the intervention starts to look like the thing worth fearing. Hesitancy tends to bloom precisely where vaccines have succeeded most.

Jenner's scratch on a boy's arm in 1796 worked before anyone could explain why, and the two centuries since have supplied the explanation in exhausting detail: the body's memory, the community's threshold, the retracted paper, the strangers who ride on everyone else's protection. The mechanism is not in doubt. Smallpox is genuinely gone. The children who once filled measles wards mostly don't exist anymore, which is exactly the problem — their absence is the vaccine's greatest achievement and its quietest liability.