No matter how urgently public health officials warn Americans to get the flu shot, still only about half of the population does it each year. Barring the symptoms you get when you actually catching the illness, few healthy adults really have an idea of how great a danger the influenza virus can be. As a matter of fact, even scientists had greatly underestimated it for years, according to a new study published Wednesday in The Lancet.

In the study, a team of global health experts together with researchers from the United States Centers for Disease Control and Prevention report that many more people die from the flu around the world than we once thought. While in the past, scientists thought about 250,000 to 500,000 people worldwide died from the flu, a new analysis shows that the number of deaths is actually somewhere between 291,000 and 646,000.

“These findings remind us of the seriousness of flu and that flu prevention should really be a global priority,” said Dr. Joe Bresee, a study co-author and an associate director for global health at the CDC’s influenza division, in a statement.

School on Kenya-Somali border
People living in sub-Saharan countries were the most likely to die from the flu.

Bresee and his team came to this conclusion after looking at the number of flu-related respiratory deaths in 33 countries, whose combined populations account for 57 percent of the world’s total. The data they combed through for each country spanned at least a four-year period that occurred between 1999 and 2015. Taking those statistics into account, the researchers built a statistical model that allowed them to predict the number of flu-associated deaths that would occur in 185 countries in the world, which is how they came to the inflated estimates.

Because their data included demographic information as well, the researchers were able to determine which subpopulations were most likely to die from the flu. Their findings confirmed what public health officials already know: While healthy adults aren’t likely to have a fatal case of the flu, the people who are most at risk are the elderly and the very poor — especially those living in sub-Saharan African countries and, to a slightly lesser degree, people in Eastern Mediterranean and Southeast Asian countries.

In addition, the CDC warns that very young people, pregnant mothers, and immunocompromised people are also very much at risk of having life-threatening cases of the flu. The death estimates produced by this study, the researchers note, only encompass deaths that occur because of flu-related respiratory problems, so the numbers might actually be much higher — the flu is known to trigger or exacerbate other health issues, like cardiovascular disease and diabetes.

The annoying side effects of getting a flu shot can make getting one seem futile or even harmful, but studies like this one drive home how important it is that everyone gets one. You may not be the most at risk of dying from the flu, but when you don’t get one, you put everyone else — including the most vulnerable people around you — at risk.