Tag Archives: vaccination

Herd Immunity — How vaccines avoid thinning the herd

Posted by on September 3, 2013 Comments Off on Herd Immunity — How vaccines avoid thinning the herd

Part of the difficulty in making a case for universal vaccination against dangerous disease is a lack of understanding of probability. Since there are few certainties in life, except its eventual end for individuals, we are constantly calculating probabilities. What is the probability that I will get to the other side of the street before that oncoming car arrives? What is the probability that the wheel stop on my number or that I will draw the card needed to complete my hand? What is the probability that the game I am going to attend will occur or get rained out? And, of course, what is the probability that the suggested intervention will cure my disease rather than kill me or leave me permanently debilitated?

Some probabilities are more difficult to calculate than others: what are the odds that I will die from prostate cancer, and what are the odds that the operation will leave me impotent? how do I calculate the best course when my choices are an operation with an 80 percent success rate that leaves another 15 percent paraplegic and 5 percent dead? How about the same operation with 93 percent success, 5 percent paraplegic, and 2 percent dead?

The more complex the alternatives, particularly when they are being balanced against complex outcomes from non-intervention, the more difficult it is for us to make a rational decision.

That leads to a discussion of vaccination. Let’s consider a disease such as smallpox, which has killed 100s of millions of people. Not everyone got smallpox. Not everyone who got smallpox died from it–estimates are that about 30 percent, or 3 out of every 10 died. Now, consider that not everyone who gets a vaccination gets 100 percent immunity. Some people get partial immunity. Some very small percentage may not produce antibodies in reaction to the vaccine. Some people actually die of the side effects of the vaccination.

As an individual, you might think, well, it’s not great but 70 percent odds of surviving are better than none, and maybe it will miss me altogether, so why should I vaccinate? Of course, smallpox has been eradicated, and we don’t have to make that kind of determination any more since the dangers of side effects from the vaccination exceed the danger of contracting smallpox, particularly in the United States where the last documented case appeared in 1949.

If you were a free individual, not part of a society, not part of the possible transmission stream of a disease, then no one is likely to care too much what you decide? However, if you are reading this, you are part of a society providing this message to you via a societal mechanism. As part of that society, you have obligations to others in the society, including children, yours and others, who once were considered private property but are now considered individuals with rights, albeit limited compared to adult rights.

The concept of “herd immunity” goes directly to the questions posed and to your obligations within the society.

Herd Immunity Concept

Those who are vaccinated provide a barrier to illness for those who are not:

Consider:

Assume you have 5 friends who do not know one another, and that everyone has 5 such friends.

Assume that vaccinations give almost 100% immunity and that the corresponding disease gives almost 100% probability of infection if you come in contact with a person who has it.

Now, if 80 percent of people are immunized against the disease, it is quite possible that one of your friends is not immunized. However, if 80 percent of that person’s friends are immunized, there is now only 1/5 times 1/5 or 1/25 = 4 percent chance of the disease vector reaching you. It may be that you friend’s friend has friends with 100 percent immunity, all five of them immunized, and the further you are socially from the source of infection, the lower your odds of becoming infected–even if you are not immunized. You are protected by herd immunity.

However, consider if only 60 percent of people are immunized, then 2/5 times 2/5 is 4/25 or 16 percent chance of becoming infected. That is 4 times your chance of infection from a secondary friend, as in the first example. The degree of herd immunity is a complicated calculation depending upon the percent immunized and the way the disease is transmitted. Your chance of infection depends on those factors as well as your social distance from the source of the infection.

For an animated look at the concept, click on “Play Animation” in the three scenarios of The History of Vaccines: Herd Immunity.

Here are the thresholds for different diseases as estimated by the Centers for Disease Control (CDC):

Herd Immunity Thresholds CDC

Think about flu shots for a moment:

  1. The vaccine is based upon recent mutations of the virus, so you might catch a virus that is not part of the vaccine.
  2. Not everyone gets 100 percent protection from a vaccine–it may be sufficient to protect against some strains and give partial protection against others
  3. In part, because of herd immunity, not everyone exposed to influenza is infected

So, we have friends and relatives drawing the wrong conclusions (e.g. the vaccine caused me to get influenza), based on an association of factors that are coincidental or subject to an alternative explanation, such as a new strain or partial immunity. For most young people, the flu is an occasional inconvenience rather than life-threatening; however, their failure to vaccinate exposes others whom they could be protecting by a simple annual injection. Let’s spread the word–it might not save those young individuals–just an older person, or an asthmatic standing close to them.

Vaccination–Now and Then

Posted by on August 26, 2013 Comments Off on Vaccination–Now and Then

Smallpox was long one of the scourges of humanity, killing millions throughout history. Early attempts were made to combat it through “variolation,”inoculation with the scabs of the disease. Such efforts began at least a thousand years ago in China. ChineseVaccination

Without images to guide us, it is easy to think of smallpox as chickenpox that kills. Not exactly. Besides the fact that smallpox does not concentrate on the torso, the number of pustules seems much more severe to my eye. Here is a child in Bangladesh in 1973 with smallpox. This is what we no longer fear because of vaccination:

Child_with_Smallpox_Bangladesh

Vaccination is relatively new in human history–the United States of America is older. Just before and after 1800 Edward Jenner, noticing the apparent immunity to smallpox of milk maids, experimented with inoculation using the relatively benign disease cowpox to which they had been exposed. It was not until 1840 that the British government routinely provided the means for inoculation, as the medical establishment had been slow to accept Jenner’s findings.

Nonetheless, the disease that killed an estimated 400,000 Europeans annually at the time of Jenner’s discovery, was still able to kill several hundred million in the 20th century.

The last documented case of smallpox occurred in Somalia in 1977. By 1980 the World Health Organisation (WHO) was able to declare smallpox eradicated. Consequently, routine smallpox vaccination was discontinued in the 1980’s as the statistical danger from the vaccination (14 to 52 per million per the CDC) exceeded the danger of the disease.

I would prefer to be able to present a time series of smallpox cases, but have been unable to locate one this week. Failing that, here is the impact of vaccination in the US on numerous diseases during the 20th Century, worth considering when someone questions the value of vaccination.
Vaccination_US_thru1998

Celebrity and science: the vaccination controversy

Posted by on August 19, 2013 Comments Off on Celebrity and science: the vaccination controversy

Bill Maher is witty and funny–particularly if you are not politically or religiously conservative.

However, the closest he will get to being a virologist is when a video clip of him goes viral.

In 2009 he provoked a controversy by tweeting that anyone who got a flu shot was an idiot. In a blog post on November 15, 2009 he backed off a bit, but defended himself by:

  1. I’m a comedian
  2. I tweeted it, didn’t say it on my show.
  3. Saying there are questionable things about vaccines.
  4. Endorsing a group opposing vaccinations.

It was a non-apology worthy of a Washington official. The truth is that while Bill Maher is neither an authority nor an expert on vaccines, he has influence based upon his celebrity. And, as a result, he can influence many people who should get vaccinated but are undecided, as can any other celebrity. After all, who likes hypodermic needles except for the rare needle freak? We all want some cover for deciding to avoid needles.

We may all be grateful that celebrities are not the go-to experts on health care for most parents; however, a 2011 University of Michigan study found that 1 out of 50 parents rely on celebrities a lot for information, and that 1 out of 4 rely on celebrities some.

MichiganVaccineSurvey2011

One of the sources that Maher cited was the National Vaccine Information Center (NVIC). It is reasonable that individuals who have suffered side effects from vaccines or any other medication might band together to ask that there be full disclosure on the risks as well as every effort to promote safety. Unfortunately, NVIC goes a step forward, suggesting that vaccination is a matter of preference rather than necessity. It is a bit like receiving a full glass of water and obsessing about the emptiness between the top of the water and the lip of the glass.

A key paragraph in their statement of purpose:

This traditional paternalistic medical model is increasingly being rejected by today’s more educated health care consumers and, along with this challenge, is also an historic challenge to the supremacy of the allopathic medical model as the only means of maintaining health and preventing disease. The movement toward a more diversified, multi-dimensional model health care system is a phenomenon occurring not only in the United States but in many technologically advanced countries.

In short, it is a rejection of science in favor of some other belief system for medical care. The United States makes ample allowance for alternative belief systems; however, alternative behaviors are circumscribed. If you wish religion taught in the schools, you must attend a parochial school, not a public one. Similarly, if you want to attend a public school, then a vaccination prerequisite is reasonable, particularly when you have private alternatives, including home schooling available. Even that stretches the limits, because unvaccinated people lower the safety of everyone. Since vaccines are not 100 percent perfect (and what is in this world?) we depend on an adequate percentage of vaccinated people to prevent an epidemic among those who are only partially protected by vaccines against communicable diseases such as polio, diphtheria, and influenza.

This is “herd immunity,” or:

Indirect protection against disease that results from a sufficient number of individuals in a community having immunity to that disease. With enough immune individuals, the transmission of a disease can be reduced, thus limiting the potential for any one individual to be exposed to it. Herd immunity does not apply to diseases, such as tetanus, that are not spread via person-to-person contact.


One of the best and simplest ways to lower healthcare costs and to improve public health is to increase our rates of vaccination. Consider this: do businesses pay for flu vaccinations because they are loving and generous, or because it will lower absenteeism and paid time off?

All That Jab – one more reason to vaccinate

Posted by on August 12, 2013 Comments Off on All That Jab – one more reason to vaccinate

Those who look toward the scientific for explanations know that only UFOs and and political assassinations attract more conspiracy theories than vaccines do.

The challenge of conspiracy theorists is similar to the challenge of mental illness–no amount of evidence contrary to a deeply held view is sufficient or dissuasive. Astute observations are followed by non-sequitur conclusions, or either-or alternatives with no room for gray in between the black and white alternatives.

Nonetheless there is heartening new evidence that influenza vaccines are benign for pregnant women–or as Reuters put it:

Pregnant women who get flu shots are at no greater risk for complications like high blood pressure, urinary tract infection or gestational diabetes, according to a new U.S. study.

The study of Inactivated Influenza Vaccine During Pregnancy and Risks for Adverse Obstetric Events, which will appear in the September issue of Obstetrics & Gynecology found In this large cohort, influenza vaccination during pregnancy was not associated with increased risks for medically attended adverse obstetric events.

Often such studies are handicapped by small sample bias, not enough people studied to draw a strong conclusion, even if statistically significant. Not the case here, as the authors report that their study group was 74,292 vaccinated females matched on age, site, and pregnancy start date with 144,597 unvaccinated females.

One might ask, “Why bother? So, I get the flu while pregnant–one more nuisance.” The same article in Reuters Health addressed that question:

For a pregnant woman, contracting the flu is “really dangerous,” according to Dr. Laura E. Riley, medical director of labor and delivery at Massachusetts General Hospital in Boston.
Pregnant women with the flu are at greater risk of death, respiratory disease requiring hospitalization and premature labor and delivery, Riley told Reuters Health.
The risk-benefit ratio was already clear, she said, but collecting new safety data is always good.

On the positive side, the benefits of vaccination accrue not only to the mother but to the baby:

“Flu shots protect pregnant women, their unborn babies, and even protect the baby after birth,” Kharbanda said.[lead author Dr. Elyse Kharbanda of Health Partners Institute for Education and Research in Minneapolis, Minnesota]
Babies don’t receive vaccines until six months of age, so they are vulnerable to catching the flu in the first six months of life, he said. But previous studies have found that some of the protection passes across the placenta to the baby and can help shield them from flu after birth.
“What mother doesn’t want to do that?” Schaffner said [ Dr. William Schaffner, chair of the department of preventive medicine at Vanderbilt University Medical School in Nashville, Tennessee]. “There should be no hesitation for women getting the vaccine.”

There is no reason for a pregnant woman not to get vaccinated against the flu; there is every reason to avoid possible consequences of not getting vaccinated; and, if not for you, then for the benefit of your baby, who does not need the flu while getting used to living outside the womb.