The job of the immune system is to protect the body from disease, but it can’t always do it alone. What it lacks in might, however, it can make up in memory: the immune system frequently remembers diseases that it has encountered and defeated, and if they come back, the immune system springs into action with the same tactics, so the person may never even notice. New illnesses, however, or familiar ones disguised as new, don’t cause this reaction.
That’s where vaccine comes in. A vaccine is a small amount of disease-causing microbe—not usually enough to make someone sick, but enough to give the immune system a taste of it and allow a defense to be mounted. This vaccination makes it possible for the immune system to defend against these illnesses from the first. According to the World Health Organization, vaccination prevents as many as three million deaths annually—but 20 percent of children are unvaccinated.
Many unvaccinated children are in the developing world, where doctors, equipment, and especially medications can be difficult to obtain, transport, store, and protect from ill-intentioned people. In all parts of the world, there are often pockets of resistance, parents who see the risks but none of the rewards of getting their child vaccinated. Though wrong, this position is understandable—the vaccination has no immediate beneficial effect and purports to protect against diseases the parent may well know only as names.
The consequences, however, can be quite real. According to experts, tens of thousands of children and adults each year get diseases they wouldn’t have if they had been vaccinated. Also, not everyone can be vaccinated—frail health, allergies, and other factors might make it impossible for a given person to be immunized. What’s more, some people who do get vaccinated get an ineffective one. Vaccines work best in large groups, so if everyone who can tolerate the vaccine gets it, these people will be safer.
Vaccines are most effective when administered on a specific timetable. Many are given in combination shots so that 14 diseases can be dealt with in fewer than 14 shots each round—though many require multiple rounds. After the initial dose is given to an infant, one or two boosters may be necessary over the next few years for maximum benefit. The schedule front-loads vaccinations so kids are protected as early as possible, and also so that the immune system will be particularly receptive to enhance the effectiveness.