This post was co-written with Professor Danny Oppenheimer, my friend and costudent during graduate school.
As the world rolls out COVID-19 vaccines, making sure people are properly informed about the risks of side effects is a matter of serious public health concern. All medications occasionally produce side effects, even common medications like aspirin or birth control, so it is not surprising that COVID-19 vaccines do as well. Mostly the side effects are mild, such as soreness at the injection site or mild fatigue. However, for fewer than one in a million people, the Johnson & Johnson vaccine can cause potentially dangerous blood clots.
If you’ve ever seen a pharmaceutical advertisement, you’ll recognize phrases such as “side effects may include…” or “some people may experience…”, but they typically don’t say how likely those side effects are or how many people will experience adverse effects. That’s a problem. People are notoriously bad at understanding the risks of medication side effects. In a study that we published back in 2006 in the journal, Clinical Therapeutics, we documented that when given vague verbal or text-based descriptors like the common ones listed above, people estimate the risk of side effects to be ten times greater than reality. This leads to fear of medication, and reduced medical adherence [news.stanford.edu]. Because of the current distrust from health mis- and dis-information online, along with historically poor COVID-19-related communication from the government and public health [pubmed.ncbi.nlm.nih.gov], people may be overestimating vaccine side effect risk even more than in those studies. People are better calibrated when given precise numbers rather than vague descriptors (e.g. “1 in 5000 people will experience…”) but countless studies have shown that even then people still typically overestimate small risks.
Given the known biases in understanding the likelihood of side effects, how can we help people understand the true risks of medication adherence? One possibility is to provide comparisons to other scenarios where people have more accurate intuitions. For example, a woman is 10 times as likely to have three children with the same birthday (who aren’t triplets) as she is to get a blood clot from the Johnson and Johnson vaccine. Indeed, a woman is twice as likely to bear quadruplets, 20 times as likely to be killed by bees/hornets/wasps, and 60 times as likely to get struck by lightning over an 80-year lifespan, than to have a blood clot from the J&J vaccine. People are better at understanding relative likelihood than absolute likelihood, so learning that a side effect is much less likely than other extremely unlikely events that people aren’t worried about can help people calibrate the risks.
Another way to help people understand risks is to provide visual displays rather than verbal descriptors. Rather than presenting news using only verbal risk descriptors or titles that might elicit overestimation (“US Calls for Pause on J & J vaccine After Blood Clotting Cases,”) journalists and media should be encouraged to include an accompanying visualization to help improve reduce overestimation. For example, the risk of blood clot could be visualized using an image of a cup of salt, with blood clot illustrated with an arrow to one out of the million grains in the cup. A risky event with approximately 200 times higher risk—0.02%-- might be communicated showing a typical night sky image of 5000 stars, with one of those stars being the risky event. Using these methods won’t address all psychological barriers to vaccination, such as the role of people’s pre-existing experiences, beliefs in conspiracy theories, and the role of political views, but it does improve people’s understanding of the information, which can reduce overestimation, lead to more informed decisions, and increase adherence.
As society grapples with helping people make informed decisions about their health care in the wake of the pandemic, we need to be thoughtful about how we convey medical information. We’ve known for decades that we need to be careful in how we provide people with information about side effects, or else ironically, our attempts to help people become better informed inadvertently creates false impressions. People process risk information in predictably biased ways, and journalists and public health advocates need to keep that in mind when sharing information about side effect risks.
