Vaccines are celebrated for their part in fighting disease. But a growing group of people seem to believe that they endanger our health, instead of protecting it. The Internet is full of stories about allergic reactions, the onset of disabilities, and even death following vaccination appointments. And, it's true vaccines can have side effects. Let's look at how they work, and how dangerous they really are.

Your immune system is a complex army of billions of cells: soldiers, intelligence cells, and weapons factories. Every day, you're attacked countless times, but your immune soldiers alone usually deal with that, so you don't even notice. If an infection becomes serious though, our intelligence cells gather intel about the attackers and activate our weapons factories. You know the weapon: antibodies. They're like targeted missiles produced specifically to combat the invader. Unfortunately, this process takes several days to complete. That gives intruders a lot of time to do damage.

Contrary to popular wisdom, what doesn't kill you doesn't make you stronger. Our bodies really don't want to fight serious wars over and over, so our immune system came up with an ingenious way to get stronger and stronger over time. If we fight an enemy that is dangerous enough to trigger our heavy weapons, our immune system automatically creates memory cells. Memory cells remain in our body for years, in a deep sleep. They do nothing but remember. When an enemy attacks for a second time, the slumbering memory cells awaken, and order coordinated attacks and the production of antibodies. This is so fast and effective that many infections you beat once will never make you sick again. You might even be immune against them forever, which is also why little kids are constantly ill; they don't have enough memory cells yet. And this beautiful natural mechanism is what we build on when we use vaccines.

As great as memory cells are, obtaining them through an infection is unpleasant and sometimes dangerous. Vaccines are a way of tricking our bodies into making memory cells and becoming immune to a disease. They pretend to be a dangerous infection. One way of doing this is to inject invaders that can't do harm, for example, by killing them, or by ripping them into pieces. Our immune systems deal with these kinds of vaccines pretty easily. Sometimes, it's necessary to make our immune system work harder though, to produce even more memory cells. 

Live vaccines are the real deal. An enemy that can punch back is a bigger challenge than a dead one. But this also sounds like a sort of horrible idea. What if the germs win? To avoid that, we breed a sort of weak cousin of the real germ in the lab, just powerful enough to annoy the immune system and create enough memory cells. 

Okay, so these are the basic principles of vaccine use. They provoke a natural reaction in our bodies that makes us become immune against very dangerous diseases. Some, like the flu virus, mutate so often that we need a new vaccine every year, but most vaccines protect us for years, or even a lifetime. 

But there's a catch. Like everything in life, vaccines have another side: side effects. What are they, and what happens if your child develops one?

It's complicated to directly compare the side effects of vaccines with the effects of diseases. For example, hundreds of millions of people are vaccinated against measles in the West, but there were only 83,000 cases in Europe in 2018. So, with numbers that different, even mild side effects can seem scary compared to the bad effects of a disease that we don't see as much anymore. Before the measles vaccine became available in 1963, virtually every single child on Earth contracted measles at some point. An estimated 135 million cases in the 1950s, every single year.

But, are measles really that dangerous in the year 2019? With our advanced healthcare and new technologies, are they worth the risk of vaccine side effects?

Let's do a thought experiment based on real numbers. Imagine a developed country in a parallel world. It has good healthcare, but people stop vaccinating. In this scenario, let's say, 10 million children caught measles. What happens? 9,800,000, or 98%, will get a high fever and a very unpleasant rash. Up to 800,000 of them, or 8%, will suffer from dangerous diarrhea. 700,000, or 7%, will suffer from an ear infection, which can lead to permanent hearing loss. 600,000 kids, or 6%, will suffer from pneumonia – the most dangerous effect of measles. It alone will kill 12,000 children. Up to 10,000 children, or 0.1%, will get encephalitis. 2,500 kids, or 0.025%, will contract SSPE, a disease where the measles virus lingers in their brain and kills them a few years later. Taken together, around 2.5 million children will suffer from somewhat serious effects from measles. And, about 20,000 children will be killed by measles.