Tag: epidemiology

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 Ro – different for each disease, Ro 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.


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

Reports of measles outbreaks among highly vaccinated populations are from before we started doing a second dose in 1994.
Reports of measles outbreaks among highly vaccinated populations are from before we started doing a second dose of MMR… way back in 1994.

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?


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

Challenging the Concept of Herd Immunity

Before we talked about clusters of unvaccinated children, experts warned about pockets of susceptibles.
Before we talked about clusters of unvaccinated children, experts warned about pockets of susceptibles.
The idea of herd immunity has been around since at least 1923 and became used to describe “the indirect protection afforded to individuals by the presence and  proximity of others who are immune.”

That’s not much different from how the CDC defines herd immunity today:

A situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely. Even individuals not vaccinated (such as newborns and those with chronic illnesses) are offered some protection because the disease has little opportunity to spread within the community.

Also called community immunity, it is often misunderstood by folks in the anti-vaccine movement.

Challenging the Concept of Herd Immunity

That the idea of herd immunity is being challenged is not new.

“Along with the growth of interest in herd immunity,  there has been a  proliferation of views of what it means or even of whether it exists at all.”

Paul E. M. Fine Herd Immunity: History, Theory, Practice

If you get educated about vaccines and understand how herd immunity works, it is easy to refute these challenges, especially the idea that herd immunity isn’t real just because we still have outbreaks of vaccine-preventable diseases among highly vaccinated communities.

Why do we still have outbreaks then?

It is mostly because we live in open communities that don’t mix randomly.

Keep in mind that the best model for herd immunity is a randomly mixing closed community – “one in which the probability of contact within any time interval is the same for every choice of two individuals in the population.”

Again, that doesn’t mean herd immunity doesn’t work.

It just means we can expect to see some outbreaks when someone in a well vaccinated community visits another community with lower vaccination levels and more disease, gets sick, and returns.

“However,  within the population of a community,  there may be pockets of  susceptibles, either because prior epidemics have failed to spread into the group or because they have not accepted immunization.”

John P. Fox Herd Immunity

You must also consider the size of the community when thinking about herd immunity, for example, a family, school, neighborhood, or city, versus the entire state. So you can have herd immunity levels of protection at the state or city level because of high average vaccination levels, but pockets of susceptibles who live in the same neighborhood or go to the same school can mean that you don’t have herd immunity in those places, leading to outbreaks.

“Hib vaccine coverage of less than 70% in the Gambia was sufficient to eliminate Hib disease, with similar findings seen in Navajo populations.”

RA Adegbola Elimination of Hib disease from The Gambia after…

Lastly, there is not one herd immunity level for all diseases. It is a separate threshold for each and every disease, depending on how easily it spreads, how many people are already immune, how long immunity lasts, if there is a vaccine, and the effectiveness of the vaccine, etc. That means that a community can have herd immunity for Hib and polio, but not the flu, and for rubella and measles, but not pertussis.

What happened in The Gambia is a great example of herd immunity. After introducing a three dose primary Hib immunization schedule (no booster dose), rates of Hib meningitis quickly went from 200 per 100,000 to none. A few years later, there were 6 cases of Hib meningitis in mostly vaccinated children (no booster dose) and in the majority of cases, “close contacts had a history of frequent or recent travel to Senegal, a neighboring country with strong kinship links with The Gambia and where vaccination against Hib was not introduced” until the following year.

With a Hib meningitis rate of 3 per 100,000, they are still far below pre-vaccine levels of disease, and their situation doesn’t mean that herd immunity isn’t real, as you will understand once you review these myths about herd immunity.

Myths About Herd Immunity

What are some common myths about herd immunity?

  • that natural immunity is better than getting vaccinated. Not True. Natural immunity often comes with a price. Remember, many vaccine-preventable diseases are life-threatening, even in this age of modern medicine.
  • you can just hide in the herd. Not True. “Freeloaders” can gamble and hope that their intentionally unvaccinated kids won’t get a vaccine-preventable disease, but it won’t always work. There is a risk to “free-riding, in which individuals profit from the protection provided by a well-vaccinated society without contributing to herd immunity themselves.”
  • most adults aren’t immune because they haven’t been vaccinated or don’t get boosters, but since we aren’t seeing that many outbreaks, herd immunity itself must be a myth. Not True. Adults were either born in the pre-vaccine era and likely have natural immunity or were born in the vaccine era and are vaccinated and immune. But again, herd immunity is disease specific, so when we talk about herd immunity for measles, it doesn’t matter if they have immunity against hepatitis A or Hib. And adults get few boosters or catch-up vaccines. Also, some vaccines, like Hib and Prevnar, have indirect effects, protecting adults even though they aren’t vaccinated because vaccinated kids are less likely to become infectious.
  • most vaccines wear off too soon to provide long lasting protection for herd immunity to be real. Not True. While waning immunity is a problem for a few diseases, like pertussis and mumps, and you need boosters for others, like tetanus, vaccine induced immunity is typically long lasting and often life-long.
  • herd immunity wasn’t developed by observing immunized people, it was all about natural immunity. Not True. The first experiments about herd immunity by Topley and Wilson in 1923 involved vaccinated mice. Ok, they weren’t immunized people, but it wasn’t just about natural immunity! And much earlier, in 1840, it was noted that “smallpox would be disturbed, and sometimes arrested, by vaccination, which protected a part of the population.” That’s herd immunity he was talking about.
  • herd immunity is not a scientifically validated concept. Not True. It has been well studied for almost 100 years.
  • if herd immunity was real, diseases would be eradicated once you reached herd immunity levels. Not True. Reaching herd immunity levels simply starts a downward trend in disease incidence. A little more work has to be done at the final stages of eradication, like was done for smallpox and is being done for polio.
  • natural immunity causes much of the decrease in mortality from a disease in the developed world, even before a vaccine is introduced. Not True. While it is certainly true that there was a big drop in mortality in the first half of the 20th century for most conditions because of improvements in sanitation, nutrition, and medical science, it was not a consequence of natural herd immunity. And we continue to see significant levels of mortality and morbidity for many diseases in the modern era, especially for those that can’t yet be prevented by a vaccine, like RSV, West Nile Virus, and malaria, etc.
  • vaccines aren’t 100% effective, so herd immunity can’t really work. Not True. Part of the equation to figure out the herd immunity threshold for a disease takes into account the effectiveness of a particular vaccine.
  • folks with medical exemptions for vaccines put the herd at risk just the same as those who intentionally skip vaccines. Not True. Children and adults with medical exemptions, including immune system problems, those getting treatments for cancer, and other true medical exemptions don’t have a choice about getting vaccinated.

So, like other anti-vaccine myths, none of the herd immunity myths you may have heard are true.

That makes it hard to understand why Dr. Russel Blaylock goes so far as to say “that vaccine-induced herd immunity is mostly myth can be proven quite simply.” Does he just not understand herd immunity? That is certainly a possibility, because “although herd immunity is crucial for the elimination of infectious diseases, its complexity and explicit relationship to health politics cause it to remain under-explained and under-used in vaccine advocacy. ”

He is also really big into pushing the idea that adults have no or little immunity, because when he was in medical school, he was “taught that all of the childhood vaccines lasted a lifetime,” but it has now been discovered that “most of these vaccines lost their effectiveness 2 to 10 years after being given.”

The thing is, Blaylock graduated medical school in 1971, when the only vaccines that were routinely used were smallpox (routine use ended in 1972), DPT, OPV, and MMR (it had just become available as a combined vaccine in 1971). Of these, it was long known that smallpox, diphtheria, and tetanus didn’t “last a lifetime,” and the live vaccines OPV and MMR, except for the mumps component, actually do.

Blaylock, like most anti-vaccine folks who push myths about herd immunity, is plain wrong. And like most anti-vaccine myths, using herd immunity denialism to convince parents that it is okay to skip or delay vaccines puts us all at risk for disease.

What To Know About Herd Immunity Myths

Herd immunity is not junk science or a false theory. Herd immunity is real, it works, and explains how people in a community are protected from a disease when vaccination rates are above a certain threshold.

More About Herd Immunity Myths

Who is Tom Frieden?

CDC Director Tom Frieden in West Africa during the Ebola epidemic.
CDC Director Tom Frieden in West Africa during the Ebola epidemic.

Thomas Frieden, MD, MPH has had a long career in public health, working as Commissioner of the New York City Health Department and most recently as the Director of the Centers for Disease Control and Prevention.

Dr Frieden went to Oberlin College, Columbia University College of Physicians and Surgeons, and did his residency in internal medicine at Yale University.

The field of public health aims to improve the health of as many people as possible as rapidly as possible.

A responsive government can maintain that people are responsible for their own health while also taking public health action that changes default choices to make it easier for people to stay healthy.

Dr. Frieden on The Future of Public Health

During his career, he:

  • worked to reduce rates of cases of multidrug-resistant tuberculosis by 80 percent in New York City
  • assisted the national tuberculosis control program in India
  • directed efforts to reduce smoking, including teen smoking, in New York City
  • led the response to the 2009 H1N1 flu pandemic in the US
  • has pushed for more funding to help control and treat Zika, which he says will likely “become endemic in this hemisphere”

Perhaps most importantly, and despite some criticism, Dr. Frieden led the CDC during the Ebola epidemic in West Africa. An epidemic that spread to the US and other countries and for which the “CDC has undertaken the most intensive outbreak response in the agency’s history.”

Recently, he has also highlighted “two shortcomings of our health system,” that the upward trend in life expectancy that we have seen over the past 50 years (about 9 years), “is neither as  rapid  as  it  should  be  —  we  lag  behind  dozens  of  other  nations – nor is it uniformly experienced by people in the United States.” And that is because “life  expectancy  and  other  key health outcomes vary greatly by race, sex, socioeconomic status, and geographic location.”

And after working to eliminate trans fats from restaurants in New York City and have chain restaurants post calorie information on their menu boards, he has continued to confront many of the more modern era epidemics, like obesity, hypertension, and diabetes.

He resigned from the CDC on January 20, 2017 and was replaced by Anne Schuchat, MD, who became the  became Acting Director.

For More Information on Thomas Frieden

Updated January 22, 2017


This Year’s Flu Season

Breaking News: Flu activity has continued to decrease, but “remains high across much of the United States:” (see below)

Flu is still widespread in 34 states, but influenza-like-illness levels continue to fall.
Flu is still widespread in 34 states, but influenza-like-illness levels continue to fall.

While flu season typically peaks in February, it is very important to understand that there are few things that are typical about the flu.

Since 1982, while we have been twice as likely to see a flu activity peak in February than other winter months, we have been just as likely to get that peak in December, January, or March. That makes it important to get your flu vaccine as soon as you can.

You really never know if it is going to be an early, average, or late flu season.

Flu Facts

There will likely be some surprises this flu season – there always are – but there are some things that you can unfortunately count on.

Among these flu facts include that:

  • there have been over 1,600 pediatric flu deaths since the 2003-04 flu season, including 110 flu deaths last year
  • about 113 kids die of the flu each year – most of them unvaccinated
  • antiviral flu medicines, such as Tamiflu, while recommended to treat high-risk people, including kids under 2 to 5 years of age, have very modest benefits at best (they don’t do all that much, are expensive, don’t taste good, and can have side effects, etc.)
  • a flu vaccine is the best way to decrease your child’s chances of getting the flu

And even in a mild flu season, a lot of kids get sick with the flu.

What about reports that the flu shot will be only 10% effective?

“The majority of the influenza viruses collected from the United States during October 1, 2017 through January 27, 2018 were characterized antigenically and genetically as being similar to the cell-grown reference viruses representing the 2017–18 Northern Hemisphere influenza vaccine viruses.”

CDC Situation Update

Don’t believe them. The flu vaccine works and besides, it has many benefits beyond keeping you from getting the flu.

This Year’s Flu Season

When does flu season start?

In general, flu season starts when you begin to see people around you with signs and symptoms of the flu, including fever, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, and fatigue, etc. To be more accurate, you can also look at reports for flu activity in your area, especially the weekly reports from the CDC. Those flu reports can also help you determine when flu season ends.

It is obvious now that this is an early flu season. And with an H3N2 predominant strain, everyone should understand that is going to be a severe season.

Worst season ever? Probably not. But this season is starting to live up to some of the hype, as influenza-like-illness (ILI) activity is at 7.5% and is approaching the 7.7 peak of the 2009 pandemic and the overall hospitalization rate is higher than the overall hospitalization rate reported during the same week of the 2014-2015 season.

That leaves the next big questions – when will flu season peak and when will it be over?

As of early-March, the CDC is now reporting that “influenza activity decreased in the United States.”

Influenza activity continues to decrease across the United States.
Influenza activity continues to decrease across the United States.

The CDC has also recently reported that:

  • 34 states are still reporting widespread flu activity (down from 45) – Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Idaho, Indiana, Iowa, Kansas, Maine, Maryland, Massachusetts, Michigan, Missouri, Montana, Nebraska, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Dakota, Virginia, Washington, Wisconsin, and Wyoming
  • 12 states are now reporting regional flu activity (up from 2) – Alabama, Alaska, Illinois, Kentucky, Louisiana, Minnesota, Mississippi, Nevada, South Carolina, Tennessee, Texas, and Utah
  • 4 states and the District of Columbia are now reporting local flu activity (up from 3) – Hawaii, Oregon, Vermont, and West Virginia
  • no states are now reporting sporadic flu activity
  • no states still have no flu activity
  • between 151 to 166 million doses of flu vaccine will be available this year, including 130 million doses of thimerosal-free or preservative-free flu shots, so the great majority of flu shots will not contain mercury!
  • FluMist, the nasal spray flu vaccine, will return next year
  • although we don’t know exactly how well the flu vaccine will work this year, we know that it is working better than expected, especially in younger children, and it is good news that “The majority of the influenza viruses collected from the United States during October 1, 2017 through March 3, 2018 were characterized antigenically and genetically as being similar to the cell-grown reference viruses representing the 2017–18 Northern Hemisphere influenza vaccine viruses.”
  • ILI has dropped to 3.7% this week!
  • the overall hospitalization rate is higher than the end-of-season hospitalization rate for 2014-2015; a high severity, H3N2-predominant season.
  • there have already been 119 pediatric flu deaths this year, including 5 new pediatric deaths that were reported this past week, and like in other recent years, most pediatric flu deaths are in kids who are unvaccinated

Have you and your family gotten been vaccinated and protected against the flu yet?

“CDC recommends that everyone 6 months and older get an injectable flu vaccine as soon as possible.”

CDC Influenza Situation Update

If not, this still a great time to get a flu vaccine.

And remember that while this is certainly a bad flu season, it is still comparable to other recent H3N2 seasons, especially the 2012-13 and 2014-15 seasons.

Most importantly, this year’s season seems to finally peaked…

Recent Flu Seasons

Are H3N2 predominant flu seasons really worse than others?

  • 2003-04 flu season – 152 pediatric flu deaths (H3N2-predominant)
  • 2004-05 flu season – 47 pediatric flu deaths
  • 2005-06 flu season – 46 pediatric flu deaths
  • 2006-07 flu season – 77 pediatric flu deaths
  • 2007-08 flu season – 88 pediatric flu deaths (H3N2-predominant)
  • 2008-09 flu season – 137 pediatric flu deaths
  • 2009-10 flu season – 289 pediatric flu deaths (swine flu pandemic)
  • 2010-11 flu season – 123 pediatric flu deaths
  • 2011-12 flu season – 37 pediatric flu deaths
  • 2012-13 flu season – 171 pediatric flu deaths (H3N2-predominant)
  • 2013-14 flu season – 111 pediatric flu deaths
  • 2014-15 flu season – 148 pediatric flu deaths (H3N2-predominant)
  • 2015-16 flu season – 93 pediatric flu deaths
  • 2016-17 flu season – 110 pediatric flu deaths (H3N2-predominant)

In addition to high levels of pediatric flu deaths, the CDC reports that the four flu seasons that were H3N2-predominant in recent years were “the four seasons with the highest flu-associated mortality levels in the past decade.”

For More Information on the 2017-18 Flu Season

Updated March 11, 2018








Efficacy vs Effectiveness of Vaccines

According to the CDC:

Vaccine efficacy and vaccine effectiveness measure the proportionate reduction in cases among vaccinated persons.

But what’s the difference between vaccine efficacy and effectiveness?

Vaccine efficacy is used when a study is carried out under ideal conditions, for example, during a clinical trial.

Vaccine effectiveness is used when a study is carried out under typical field (that is, less than perfectly controlled) conditions.

Postlicensure studies can often help figure out vaccine effectiveness. For example, the study “Varicella Vaccine Effectiveness in the US Vaccination Program: A Review,” that appeared in The Journal of Infectious Diseases in 2008 “reviewed the results of postlicensure studies of varicella vaccine effectiveness and compared these results with those of prelicensure efficacy trials.”

That study of the chicken pox vaccine found that “the estimates of effectiveness are lower than the prelicensure efficacy,” although several studies found the vaccine “100% effective in preventing combined moderate and severe varicella.”

For More Information On Efficacy vs Effectiveness of Vaccines: