Why do anti-vaccine folks think that there are so many vaccine reactions?

It is mostly because they think that anything bad that happens after someone is vaccinated, even if it is weeks or months later, must have been caused by the vaccine.

“Differentiation between coincidence and causality is of utmost importance in this respect. This is not always easy, especially when an event is rare and background rates are not available.”

Heininger on A risk-benefit analysis of vaccination

Of course, this discounts that fact that most people have a basic risk, often called the background rate, for developing most of these very same conditions, and they can just coincide with getting vaccinated.

Put more simply, the “reaction” would have happened whether or not they had been vaccinated.

“…when a number of well-controlled studies were conducted during the 1980s, the investigators found, nearly unanimously, that the number of SIDS deaths temporally associated with DTP vaccination was within the range expected to occur by chance. In other words, the SIDS deaths would have occurred even if no vaccinations had been given.”

WHO on Six Common Misconceptions About Immunization

That doesn’t mean that everything automatically gets blamed on coincidence though.

Is It a Vaccine Reaction?

When trying to determine if a child has had a vaccine reaction, experts typically go through a series of questions, looking at the evidence for and against :

• How soon after the vaccine was given did the reaction occur? Was it minutes, hours, days, weeks, months, or years later?
• Is there any evidence that something else could have caused the reaction?
• Is there a known causal relation between the reaction and the vaccine?
• Is the reaction a table injury?
• Is there evidence that the vaccine does not have a causal association with the reaction?
• Do any lab tests support the idea that it was a vaccine reaction?

Why is it important to consider these and other questions?

Because most of us are very good at jumping to conclusions, are quick to place blame, and like to know the reasons for why things happen.

We don’t like to think that things are just caused by coincidence.

Post hoc ergo propter hoc (after this, therefore, because of this).

We are especially good at linking events and often automatically assume that one thing caused another simply because it occurred afterwards.

It is incident to physicians, I am afraid, beyond all other men, to mistake subsequence for consequence.

Dr Samuel Johnson

But we also know that correlation does not imply causation. And because of the great benefits of vaccines, it is important to find strong evidence for a correlation before we blame vaccines for a reaction.

Too often though, the opposite happens. Despite strong evidence against a correlation, parents and some pediatricians still blame vaccines for many things, from SIDS and encephalitis to autism.

Background Rates vs Vaccine Reactions

Although anti-vaccine folks are always calling for vaccinated vs unvaccinated studies to further prove that vaccines are indeed safe, much of that work is already done by looking at the observed rate of possible reactions and comparing them to the background rate of reactions and conditions.

We often know how many people are expected to develop certain conditions, from seizures and type 1 diabetes mellitus to acute transverse myelitis and juvenile and rheumatoid arthritis.

“On the basis of the reviewed data, if a cohort of 10 million individuals was vaccinated in the UK, 21.5 cases of Guillain-Barré syndrome and 5.75 cases of sudden death would be expected to occur within 6 weeks of vaccination as coincident background cases. In female vaccinees in the USA, 86.3 cases of optic neuritis per 10 million population would be expected within 6 weeks of vaccination. 397 per 1 million vaccinated pregnant women would be predicted to have a spontaneous abortion within 1 day of vaccination.”

Black et al on Importance of background rates of disease in assessment of vaccine safety during mass immunisation with pandemic H1N1 influenza vaccines

Intussusception is a good example of this.

This might surprise some folks, but we diagnosed and treated kids with intussusception well before the first rotavirus vaccines were ever introduced. And then, it was only after the risk of intussusception after vaccination exceeded the background rate that experts were able to determine that there was an issue.

Background rates also explain why unvaccinated kids develop autism. It’s not a coincidence.

“Knowledge of the background incidence rates of possible adverse events is a crucial part of assessing possible vaccine safety concerns. It allows for a rapid observed vs expected analysis and helps to distinguish legitimate safety concerns from events that are temporally associated with but not necessarily caused by vaccination.”

Gadroen et al on Baseline incidence of intussusception in early childhood before rotavirus vaccine introduction, the Netherlands, January 2008 to December 2012

Fortunately, studies have never found an increased risk above the background rate for SIDS, non-febrile seizures, and other things that anti-vaccine folks often blame on vaccines. So when these things happen on the same day or one or two days after getting vaccinated, it almost certainly truly is a coincidence. It would have happened even if your child had not been vaccinated, just like we see these things happen in the days before a child was due to get their vaccines.

For example, using background incidence rates in Danish children, one study found that if you vaccinated a million children with a new flu vaccine, you could expect that naturally, after seven days, you would see:

• facial nerve palsy – one case
• seizures – 36 cases
• multiple sclerosis – one case
• type 1 diabetes – three cases
• juvenile and rheumatoid arthritis – three cases

After six weeks, those numbers of course go up. In addition to 4 kids developing MS, 20 develop diabetes, 19 develop arthritis, and 218 have seizures, and there would have been at least two deaths of unknown cause.

Would you blame the flu shot for these things?

What flu shot?

This was a “hypothetical vaccine cohort” that used 30 years of data from the Danish healthcare system to figure out background rates of each condition.

“In addition, the expected number of deaths in Japan following an estimated 15 million doses of H1N1 vaccine administered would be >8000 deaths during the 20 days following vaccination, based on the crude mortality rate.”

McCarthy et al on Mortality Rates and Cause-of-Death Patterns in a Vaccinated Population

Looking at background rates is especially helpful when folks report about vaccine deaths.

Using the Japan example that McCarthy studied, if they had looked at background rates, then all of a sudden, the 107 deaths they found after 15 million doses of H1N1 vaccine were given in 2009 would not have been so alarming. Background rates would have predicted a much, much higher number of deaths to naturally occur in that time period simply based on crude mortality rates.

Again, none of this means that possible vaccine reactions are dismissed as being coincidences. They just aren’t immediately assumed to have been caused by vaccines, because vaccines are necessary and a lot of research has already gone into demonstrating that vaccines are safe and vaccines continue to go through routine safety monitoring to make sure they stay safe.

What to Know About Evaluating Vaccine Reactions

Vaccines are safe and many of the things that folks think are vaccine reactions can be explained by looking at the background rates for these conditions and understanding that they would have happened anyway.

More on Evaluating Vaccine Reactions

• Study – Use of population based background rates of disease to assess vaccine safety in childhood and mass immunisation in Denmark: nationwide population based cohort study
• Study – Probability of coincident vaccination in the 24 or 48 hours preceding sudden infant death syndrome death in Australia.
• CDC – Clinical Immunization Safety Assessment (CISA) Project
• Study – Algorithm to assess causality after individual adverse events following immunizations
• Vaccine Causality Algorithm
• Study – Algorithm for treatment of patients with hypersensitivity reactions after vaccines.
• Study – Assessment of causality of individual adverse events following immunization (AEFI): a WHO tool for global use.
• Why are there so many reports of autism following vaccination? A mathematical assessment
• WHO – Rates of adverse vaccine reactions
• Study – Reanalyses of case-control studies examining the temporal association between sudden infant death syndrome and vaccination.
• WHO – Six common misconceptions about immunization
• Study – Baseline incidence of intussusception in early childhood before rotavirus vaccine introduction, the Netherlands, January 2008 to December 2012
• When can correlation equal causation?
• How Coincidences Shape Views of Vaccines–and Shouldn’t
• Post hoc, ergo propter hoc
• Post hoc fallacy
• Correlation and Causation
• Study – Deaths following influenza vaccination—background mortality or causal connection?
• Study – Importance of background rates of disease in assessment of vaccine safety during mass immunisation with pandemic H1N1 influenza vaccines
• Properly evaluating vaccine mortality – let’s not abuse VAERS
• Study – Deaths following vaccination: What does the evidence show?
• Study – Mortality rates and cause-of-death patterns in a vaccinated population.
• Dr. Anthony Fauci: Risks From Vaccines Are “Almost Nonmeasurable”
• Study – A risk-benefit analysis of vaccination.
• Causation and Hill’s Criteria
• Evidence in Medicine: Correlation and Causation
• Statistics for Skeptics Part 2 – Correlation vs. Causation