The flu season is well underway, and if you haven’t got it, chances are you know someone who has gotten sick this season.

Across the country, overall hospitalizations are higher and approaching the final rate of hospitalizations observed at the end of the 2014-2015 influenza. In addition, this season has resulted in the death of at least 53 children from the flu, according to the CDC. 

So far, Chicago has seen 392 influenza-associated ICU hospitalizations and two pediatric deaths, according to the Chicago Department of Public Health's latest flu update.

Dr. Marielle Fricchione, a pediatrician and medical director of the Immunization program at Chicago Department of Public Health, says this year has seen the highest level of influenza-like illness in the outpatient setting in the U.S. since 2009.

“For Chicago specifically, the number of our ICU hospitalizations is the highest since 2010–2011 season,” Fricchione says. “The unique part of this season, from what the CDC is saying, is that the disease increased and the activity increased so quickly at the same time, in every state across the continental U.S.; and the amount of activity has been relatively consistent.”

So why has this flu season been particularly bad this season?

In order to understand the severity of this flu season, Dr. Michael Ison, an infectious-diseases specialist at Northwestern Memorial Hospital, says we first must put the season into context.

Three types of influenza viruses commonly circulate and cause infections in the U.S. Two are strains of influenza A, H1N1—which gained notoriety in the 2009 pandemic—and the H3N2 circulating this year. The third is influenza B.

A greater impact is to be expected from the H3N2 influenza strain, according to Ison. “What we know in terms of trends over time, is that years where the H3N2 virus is the predominant strain, we tend to have more cases, more severe infections, more hospitalizations, and more deaths,” Ison says.

We saw similar numbers the last time there was a H3N2-predominate season two years ago, Ison adds. “It might be a little bit heavier in some parts of the country but it's basically in line with years of H3N2. Maybe a little bit higher rates of illness, hospitalization, and death, but we'll have to wait until the end of the season to know for sure.”

This year, a suboptimal batch of the influenza vaccine, a relatively cold winter, and a virulent strain of the virus circulating has contributed to creating “somewhat of a perfect storm” that has led to this severe flu season, Ison says.

“We have the three things coming together in the same year: an H3N2-predominate virus, a vaccine that's less efficacious than we would have liked, and a relatively cold flu season—where large groups of people are congregating indoors, which produces a greater chance of a sick person spreading the virus,” Ison says. “These factors all kind of work together to make this a slightly worse season.”

Reasons why the H3N2 strain causes more severe illness aren't well understood, but CDC researchers are studying the strain and trying to come up with a more effective flu vaccine.

Every year from September to December, the Chicago Department Public Health hosts flu clinics in every ward at no cost. This year, the Department has donated the vaccine to Walgreens locations to give out for free for community members without health insurance or individuals who can’t afford the vaccine.

Dr. Fricchione says researchers and medical experts can never predict the severity of the flu season year to year but emphasizes the importance of the flu vaccination, especially for people greatly at risk like people over the age of 65 and children. “We've gotten much better data in the past few years that it really does reduce your chance of hospitalization if you’re an adult with chronic medical problems—and can actually prevent death in children from flu,” Fricchione says.

Despite the skepticism that surrounds the flu vaccine, both Fricchione and Ison point out that even if you get infected, getting the vaccine significantly reduces the risk of hospitalization and death if you get sick.

“This has been a tough few months for Chicago. I work for the hospital and I see it first hand,” Fricchione says. “We can use lessons from seasons like this to hopefully prevent disease in the future. I hope seasons like this where we get the word out  about the flu vaccine will help more people get vaccinated next year.”