Do More Vaccinated or Unvaccinated Kids Get Sick in Outbreaks?

Some anti-vaccine folks continue to claim that vaccines don’t work and that most outbreaks of vaccine-preventable diseases are actually caused by kids who have been vaccinated.

They also push the myth that more vaccinated than unvaccinated kids get sick in most outbreaks.

Vaccinated vs Unvaccinated in an Outbreak

So are outbreaks usually caused by kids who have been vaccinated?

No, of course not.

Do we sometimes see more vaccinated than unvaccinated kids in some of these outbreaks?

Yes, sometimes we do.

Vaccine Epidemiology

Wait, what?

Yes, we sometimes see more vaccinated than unvaccinated kids in an outbreak.

How can that be if vaccines work?

It is actually very easy to understand once you learn a little math and a little more epidemiology.

Basically, it is because while vaccines work, they don’t work 100% of the time, and more importantly, there are way more vaccinated kids around than unvaccinated kids.

The Mathematics of Disease Outbreaks

That means that you need to understand that more than the absolute number of vaccinated and unvaccinated people that got sick in an outbreak, you really want to know the percentages of vaccinated vs unvaccinated kids who got sick.

For example, in a school with 1,000 kids, you might be very surprised if six kids got a vaccine preventable disease, and three of them were vaccinated, leaving three unvaccinated.

Does that really mean that equal amounts of vaccinated and unvaccinated kids got sick?

I guess technically, but in the practical sense, it only would if half of the kids in the school were unvaccinated. Now unless they go to a Waldorf school, it is much more likely that over 90 to 95% of the kids were vaccinated, in which case, a much higher percentage of unvaccinated kids got sick.

Before we use a real world example, some terms to understand include:

• attack rate – how many people will get sick when exposed to a disease
• basic reproductive number or R_o – different for each disease, R_o basically tells you  just how contagious a disease is and ranges from about 1.5 for flu, 8 for chicken pox, and 15 for measles
• vaccine coverage – how many people are vaccinated
• vaccine efficacy – how well a vaccine works

You also need to know some formulas:

• attack rate = new cases/total in group
• vaccine coverage rate = number of people who are fully vaccinated / number of people who are eligible to be vaccinated
• vaccine effectiveness = (attack rate in unvaccinated group – attack rate in vaccinated group) / attack rate in unvaccinated group x 100

Unfortunately, it is often hard to use these formulas in most outbreaks.

Why?

For one thing, it is hard to get accurate information on the vaccination status of all of the people in the outbreak. In addition to those who are confirmed to be vaccinated or unvaccinated, there is often a large number who’s vaccination status is unknown. And even if you know the vaccination status of everyone in the outbreak, it can be even harder to get the vaccine coverage rate or a neighborhood or city.

Outbreaks of Vaccine Preventable Diseases

We know what starts most outbreaks.

And no, it’s not shedding…

For example, with measles, it is typically an unvaccinated person who travels out of the country, returns home after they have been exposed but are still in their incubation period, and then exposes others once they get sick. And the great majority of folks in these measles outbreaks are unvaccinated.

Some examples of these outbreaks include:

• the 2014 Ohio measles outbreak that started with two unvaccinated Amish men getting measles in the Philippines while on a missionary trip and ended up with at least 388 cases before it was over, almost all unvaccinated
• a 2013 North Carolina measles outbreak with 22 cases started after an unvaccinated traveler had returned from India
• an outbreak of measles in New York, in 2013, with at least 58 cases, tarted with an intentionally unvaccinated teen returning from a trip to London
• a 2011 outbreak of measles in Minnesota, when an unvaccinated child traveled out of the country, developed measles, and returned to his undervaccinated community, causing the state’s largest measles outbreak in 20 years

But what about mumps and pertussis?

Those outbreaks are all among vaccinated kids, right?

Nope.

In one of the biggest mumps outbreak, in Arkansas, only 71% of people were up-to-date on their vaccines!

And keep in mind that while we do know that there are issues with waning immunity with some vaccines, you are still much more likely to become infected and get others sick if you are not vaccinated. And you will likely have a much more severe disease.

A 2013 pertussis outbreak in Florida is a good example that even with all the bad press it gets, the DTaP and Tdap vaccines work too. This outbreak was started by an unvaccinated child at a charter school with high rates of unvaccinated kids. About 30% of unvaccinated kids got sick, while there was only one case “in a person who reported having received any vaccination against pertussis.”

In another 2013 pertussis outbreak in Florida, this time in a preschool, although most of the kids were vaccinated, the outbreak started with “a 1-year-old vaccine-exempt preschool student.” And the classroom with the highest attack rate, was “one in which a teacher with a laboratory-confirmed case of pertussis who had not received a Tdap booster vaccination, worked throughout her illness.”

In outbreak after outbreak, we see the same thing, sometimes with deadly consequences – an unvaccinated child or adult triggers an outbreak and then a lot of unvaccinated folks get sick. Unfortunately, others get caught up in these outbreaks too, including those too young to be vaccinated, those who can’t be vaccinated because of true medical exemptions, and those whose vaccines may not have worked as well as we would have liked.

Get educated.

Vaccines are safe. Vaccines are necessary. Vaccines work.

What to Know About Vaccinated vs Unvaccinated in Outbreaks

Most outbreaks are started by someone who is unvaccinated, often after a trip out of the country, and the resulting outbreak will likely get many more unvaccinated than vaccinated folks sick.

More About Vaccinated vs Unvaccinated in Outbreaks

• CDC – Introduction to Epidemiology
• WHO – Basic Epidemiology
• WHO – Epidemiology of the Unimmunized Child
• Vaccine-preventable Diseases, Immunizations, and the Epidemic Intelligence Service
• Seasonal infectious disease epidemiology
• Epidemiology: Solve the Outbreak
• Exponential Outbreaks: The Mathematics of Epidemics
• Why Do So Many Vaccinated People Get the Mumps During an Outbreak?
• Of Maths and Measles
• The Simple Math of Herd Immunity
• Hey antivaccination gang–it’s really simple math
• Vaccine Awareness Week: If Vaccines Work…
• Anti-vaxers and Math Don’t Mix
• Misconceptions about Vaccines
• Why Do So Many Vaccinated People Get the Mumps During an Outbreak?
• Syracuse University mumps outbreak – bad anti-vaccine math
• Measles Outbreak In Ohio Leads Amish To Reconsider Vaccines
• Study – A Measles Outbreak in an Underimmunized Amish Community in Ohio.
• Study – An outbreak of measles in an undervaccinated community.
• CDC – Notes from the Field: Measles Outbreak Associated with a Traveler Returning from India — North Carolina, April–May 2013
• CDC – Notes from the Field: Measles Outbreak Among Members of a Religious Community — Brooklyn, New York, March–June 2013
• CDC – Notes from the Field: Outbreak of Pertussis in a School and Religious Community Averse to Health Care and Vaccinations — Columbia County, Florida, 2013
• Study – Sustained Transmission of Pertussis in Vaccinated, 1–5-Year-Old Children in a Preschool, Florida, USA
• Study – Reduced Severity of Pertussis in Persons With Age-Appropriate Pertussis Vaccination—United States, 2010–2012