The Risk Assessment Trap

posted in: Q&A | 0
Photo by Pieter Musterd on Flickr (
Photo by Pieter Musterd on Flickr (

“But what’s the risk that my child will actually catch one of these diseases?”

It’s often one of the first questions parents ask us after we explain the (relatively low) risks of vaccines, and one of the most difficult to answer. It’s very easy to provide data on the outcomes for people who have been infected with a vaccine preventable disease. We can tell you that 1 or 2 in 100 infants who contract pertussis will die, that about 1 in 4 will develop pneumonia. We have these statistics, carefully tracked, for all vaccine preventable diseases, and we can say with a high degree of certainty that there are significantly greater risks to suffering the diseases that we vaccinate for than there are from the vaccines that protect against them.

But the actual risk of contracting the disease remains an unknown factor. Many advocates against vaccination point to the low incidence of most vaccine preventable diseases and note that the risk of falling ill is so low that the real risk of these complications is much lower, and it is true that the absolute risk, if averaged across all people in the United States, is quite low. However, there are many factors that make your child’s individual risk very hard to calculate.

When we calculate the risk of vaccination, we’re looking at the risk of a discrete event. That is, you receive each vaccination a few times in your life and each time there’s a known risk (about 1 in 6 children will have a fever after the MMR vaccine, for example), and that risk stops after you’re finished receiving vaccines. For a child without immunity to a disease, on the other hand, there is a daily risk of falling ill, depending on whether the child comes in contact with a contagious person. This risk is increased if you happen to live in an area with low vaccination rates (people who refuse or delay vaccination tend to cluster together), or if a child attends a school with a large number of exemptions.

Another complicating factor in determining risk, is that the risk of contracting these diseases is so low precisely because most people are vaccinated. If more people elect to skip vaccination, overall vaccination rates decline, and risk of contracting vaccine preventable diseases increase. We have strong evidence that the recent measles outbreak in Southern California was caused by declining vaccination rates, so while we might look at the overall risk of contracting measles as being very low, we know that it is increasing, and if vaccination rates continue to drop, that trend will continue.

Graph showing measles vaccination rate by county

Despite the difficulties in calculation, it makes sense to look at the absolute risk of contracting a disease. ACIP, the Advisory Committee on Immunization Practices, does that assessment for the United States. There are a great many vaccines available that aren’t given as standard practice in the United States because the risk of contracting the disease is sufficiently low and/or the risks or complications with the vaccines are sufficiently high. We no longer give the smallpox vaccine because the risk of contracting the disease is very near zero. In the US we don’t typically give the tuberculosis vaccine because incidence of tuberculosis is very low and the vaccine causes people to test positive for exposure to the disease, which makes controlling outbreaks much more difficult.

ACIP is made up of experts in various fields of public health, and they are advised by a great many other experts who perform the best possible risk assessment for each disease and decide whether or not a vaccine should be provided. If a vaccine is included in the CDC schedule, it means that the best available evidence gathered from studies and research in the field has shown that the risks of contracting the disease are greater than the risk of being vaccinated.

It’s understandable that parents want absolute numbers. When we talk about the safety of vaccines we emphasize the small number of reactions versus the high number of disease complications. Risk of contracting a vaccine preventable disease is an admittedly missing piece of the puzzle in that conversation. We’ve been asked more than once for a side-by-side comparison of the risk of contracting a disease versus the risk of having a vaccine reaction, and we hope this post goes some small way towards explaining why it’s so hard to provide meaningful numbers. What we can tell you is that the risks of vaccine reactions are very low, and that while the risk of contracting most vaccine preventable diseases is also very low, that will only remain true as long as the overwhelming majority of those who can receive vaccines do.