The vaccine dilemma: how experts weigh benefits for many against risks for a few

You’ve likely seen a version of the image above. Some people observe two faces in profile. Others see a vase.

The same phenomenon can occur when scientists look at data, particularly when they try to weigh the benefits and risks of individual vaccines. Vaccines protect huge numbers of people, generally children, from serious diseases, but in rare cases, certain vaccines can tragically cause harm. How do those scientists figure out which to value more?

This dilemma was at the center of last week’s decision by an expert committee advising the World Health Organization to sharply scale back use of a controversial vaccine called Dengvaxia, the first to protect against dengue infection.

Two years ago, many of the same experts concluded the vaccine was safe to use in children 9 and older in places where dengue infection is almost unavoidable — even though there were strong theoretical concerns the vaccine might put some vaccinated kids at higher risk of developing a severe form of dengue. Severe dengue can lead to internal bleeding, shock, and even death.

Late last year, theory was . After reviewing the data, the WHO’s Strategic