flu numbersThe Centers for Disease Control and Prevention estimates that an average of 5 to 20 percent of the U.S. population gets the flu each year and more than 200,000 are hospitalized from flu-related complications.

But those are only estimates. Precise assessments of the severity of influenza each year are impossible to obtain because states are not required to report individual seasonal flu cases or deaths for people older than 18 years of age.

“The reason why the CDC does not require this is because the number is too large,” said Lynnette Brammer, ‎CDC epidemiologist in the influenza branch. “You’re not going to pick up such large amount of information.”

Most people who die from flu-related complications are not tested for the flu. Current tests are also only able to detect the flu within one week of catching it, meaning that if someone gets medical care after it is no longer detectable from respiratory samples, it may not show positive.

The tests are not sensitive enough to pick up the flu and can misdiagnose the virus as not being the flu, according to the CDC. So, a majority of flu-related deaths are not recorded on death certificates, according to Brammer.

Experts at the CDC believe that even if they did attempt to gather data for all deaths, flu would still be underreported each year.

Many flu-related deaths occur one or two weeks after a person’s initial infection, either because the person develops a secondary bacterial infection or because the flu can aggravate an existing chronic illness like heart disease.

“Influenza can lead to secondary infections or chronic diseases like pneumonia,” said Brammer. “So, it wouldn’t test positive for the flu. The results would undercount the flu and wouldn’t reflect flu-related deaths.”

Instead, the CDC and other health organizations currently use statistical models to estimate how many people die each year from the flu. They do this by gathering information from death certificates during the flu season.

“We look at death certificates for respiratory and circulatory deaths,” said Brammer. “Then compare those to what you expect to see and we come up with what we call ‘excess deaths,’ which is an estimate on the impact that the flu has during that season.”

Meanwhile, flu deaths in children have been based on reports for the past decade.

The deaths were made a nationally “notifiable condition” in 2004 after widespread publicity about children’s deaths due to flu that year underscored the need to learn more about how the virus affects young people.

The CDC said it differentiates by age because it is easier to gather data for flu-related deaths for people under 18 years of age because the disease progresses faster in children and the child tends to not develop secondary aliments like adults.

States report these flu-related deaths through the Influenza Associated Pediatric Mortality Surveillance System.

There is no plan to change how things are recorded for adults.

“It’s not that we don’t want to do it,” said Brammer. “It’s just that it’s very difficult to do.”