Pertussis (Whooping Cough) — Why vaccination matters

Let’s talk about how a Pertussis outbreak can happen.

First, Herd Immunity is a real thing.

Second, aschool may report zero non-medical exemptions, implying that 100% of students are vaccinated. However, sometimes students aren’t *fully* vaccinated. That means that they have some vaccines, and haven’t filed a non-medical exemption document, but the school may let them attend school even if they’re not up to date, with the expectation they will get it soon. Some schools are very strict on this, others, less so. The vaccines students are missing tend to be those that require boosters, particularly when they’re older.

The pertussis vaccine is the “P” in DTaP. FIVE doses are recommended to be fully protected when a child enters school. Doses as two, four, and six months of age should be followed by a booster at 15–18 months of age. A fifth dose should be given at four to six years of age, which is about the time the child starts school. That fifth dose is often missing. The family intends to vaccinate, but for whatever reason, has not made it into a doctor yet to get it.

And then, yet another pertussis booster is recommended at age 11, and required for entry into 7th grade. This is the ‘p’ in Tdap. Again, students tend to lag behind in getting this dose.

That’s a lot of doses, right? We’ll explain why. But first, Outbreaks.

What defines an “outbreak?” It is anything above the endemic rate — the normally occurring rate. A few cases of measles constitute an outbreak because, until recently, measles in the US was rare. Pertussis has become increasingly common in the US, so a true outbreak is likely more than one case, but could be as little as 10, depending on the environment. Ten cases in one school may be an outbreak. Ten cases in a city may not be, particularly if they’re not clustered.

Now, back to the doses. Not all vaccines are created equal. So saying one is less effective doesn’t mean ALL are less effective. Measles vaccine with 2 doses is 98% effective! For those children who get the pertussis vaccine ON SCHEDULE — ALL FIVE doses, efficacy is usually about 90%. So, one of every 10 vaccinated kids will not be protected, but NINE out of every 10 will. That is, if they get the doses on schedule, as indicated above.

However, pertussis is a vaccine that has waning immunity. When you get the first three doses of DTaP, protection is fairly good two years out, but starts to wane after that. Five years after being vaccinated, protection drops to 70%, though those other 30% are partially protected. That means those 30% may get ill, but are less likely to be severely ill and less likely to transmit.

This is why a DTap booster is recommended at age 15–18 months, and again at 4–6 years. And yes, again, Tdap is recommended at age 11. That booster at age 11 is about 70% effective. However, immunity continues to wane, dropping to 30–40% after 4 years. Yes, that stinks. But do you take 70% or 0% protection?

Seventy percent protection in adolescence doesn’t mean it’s not worth getting. It means we need another booster as an adult. No vaccine is perfect, it need not be all or nothing.

Now, how does this impact herd Immunity?

The concept of herd immunity is that if 90% or more of the population IS protected, the virus or bacterium has nowhere to go except into the 10% who are not protected due to vaccine immunity waning. But for those 10%, if they ARE vaccinated and due get sick, their disease is less likely to be severe, and most importantly, they are less likely to transmit. Also, note that the level of herd immunity required depends on the disease.

Herd Immunity is how we eradicated smallpox. The virus had nowhere to go because all the potential hosts (humans) were no longer susceptible due to vaccination or natural infection (if they survived). The Pertussis vaccine isn’t perfect, which is all the more reason why herd immunity is essential — it protects those who can’t be vaccinated, either because they are too young or have other health complications. The more people who are vaccinated against pertussis, the less opportunity the bacterium has to travel from person to person.

Do we wish the vaccine was 100% effective? Absolutely. But biology doesn’t work that way. So we go with 70–80–90% efficacy and rely on 100% vaccination to prevent the disease from spreading.

Getting vaccinated for all diseases for which the CDC recommends in the US is essential. Following the schedule and not being “delinquent” on vaccines is essential. Spacing them out puts you at risk.

Finally, it’s essential to understand that each vaccine is different. Each pathogen is different. Measles vaccine is 98% effective. Mumps vaccine is 88% effective. Flu vaccine varies every year. Chickenpox vaccine requires 2 doses to be effective — but from personal experience I will tell you that the “natural disease” after one dose is so mild as to barely be noticeable, so while not perfect, one dose has a huge protective effect against chicken pox.

I saw a seven-week old infant, too young to be vaccinated, nearly die from pertussis. The baby was in NICU for more than seven months with a collapsed lung and survived with complications. I’d take an even 70% chance that could have been prevented.


Dr. Rohr-Allegrini is an epidemiologist and tropical disease scientist currently working to prevent diseases through immunizations.

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store