Why disease numbers can be misleading

When people hear that some vaccinated children still get sick during an outbreak, it can sound as if the vaccine is ineffective. Anti-vaccine groups often use this point to argue that vaccines are ineffective. However, such claims overlook an important factor: the number of people vaccinated compared to those who are not. Let’s look at an example to understand why this matters.

During a pertussis outbreak at a school, 11 children fell ill:

At first glance, it might look surprising—there are more sick children in the vaccinated group (7 versus 4). But because almost every student at that school had been vaccinated, those raw numbers hide a crucial truth.

The school had 1,000 students.

Let’s look at what percentage of each group became sick.

This table tells a very different story. Out of every 1,000 students vaccinated, fewer than 1 in 100 get sick. Among those without vaccination, 4 out of 5 get sick – 80%. The difference is dramatic.  This means the pertussis vaccine reduced the risk of illness by about 99 percent in this group. Even though a few vaccinated students fell ill, the vaccine prevented almost all potential cases. Without vaccination, nearly every student in the school might have become infected.

Why do some vaccinated people still get sick?

No vaccine is perfect. The acellular pertussis (aP) vaccine used in most countries, including the United States and Europe, provides strong protection—about 97–98% in the first year after the last dose—but this protection slowly decreases over time. After five years, about 70–80% of children remain protected, and booster doses are recommended to maintain immunity.

This “waning immunity” is one reason why we sometimes see pertussis in vaccinated individuals. But these cases are usually milder, shorter, and much less likely to lead to hospitalization compared to unvaccinated cases.

The importance of high vaccination rates

Large vaccination coverage acts like a shield for the entire community, a concept called herd protection. When most people are protected, germs cannot spread easily, so even the few who are unprotected, such as infants too young for vaccination, remain safer.

When vaccination rates drop, however, that shield weakens, and outbreaks become more frequent and more severe. This has been seen in several countries where vaccination rates fell due to misinformation. In those cases, hospitals reported more infants in intensive care and even deaths from pertussis.

How data can be misused

Numbers can easily be twisted to spread fear. Saying, “Seven vaccinated children caught pertussis while only four unvaccinated did,” sounds worrying if you ignore how many children were in each group. But when we see the full picture, the data strongly support vaccination.

This misunderstanding is a classic example of the base rate fallacy, judging something by raw counts without considering the size of each group. It’s the same as saying “more people die in car accidents who wear seatbelts than those who don’t,” while forgetting that almost everyone wears seatbelts.

Real-world effectiveness

Around the world, data consistently confirm that pertussis vaccines work. Many studies across countries show vaccine effectiveness between 74% and 97%, depending on age, time since the last dose, and booster schedules. The U.S. Centers for Disease Control and Prevention (CDC) notes that DTaP protects:

In other words, vaccinated children are hundreds of times less likely to catch or spread the disease.

What does this mean for you?

Vaccines are not magic shields, but they are one of the most effective tools modern medicine has given us. In the school example, the vaccine protected nearly every child, turning what could have been a major outbreak into a tiny cluster of mild cases.

It’s worth remembering that while no vaccine can guarantee zero illness, they drastically reduce your risk and help keep communities safe. When people stay up to date with their boosters, especially for diseases like pertussis that can resurge with waning immunity, the whole population benefits.

In short, the pertussis vaccine in this example worked extraordinarily well (about 99% effective), and the apparent contradiction disappears once we look at percentages instead of absolute numbers. It’s a vivid reminder that data can tell very different stories depending on how they’re read, and understanding the full context is the key to making smart health decisions.

By Global Health Press