We all know flu is bad. But after a historically severe season, public health researchers are warning it's even worse than many think, leading to increased risk of heart attack and stroke during recovery and an inability for some older people to recover completely.

"If you needed more reason to be vaccinated, how about those two?" Dr. William Schaffner, medical director of the National Foundation for Infectious Diseases, asked during a news conference Thursday.

Public health officials' annual push for everyone older than 6 months to be vaccinated follows a record number of deaths and hospitalizations from the flu last season: more than 80,000 deaths and 900,000 hospitalizations in the United States. Typically public health researchers have used 56,000 as the upper bound on the number of deaths caused by flu each year.

"It was a significantly bad year," said the Centers for Disease Control and Prevention's Dr. Daniel Jernigan.

What's more, vaccination rates in certain groups declined last year, Surgeon General Dr. Jerome Adams told reporters. Rates among children ages 6 months to 4 years decreased slightly last season, particularly troubling because kids that age are vulnerable to severe complications when they get sick, he said.

Last season, 180 flu deaths in kids were reported to the CDC, topping a previous record of 171. And researchers note that number is likely an under-estimation, given not all flu-related deaths are reported.

The disease is especially dangerous for the elderly. The flu causes a systemic inflammatory reaction that continues past the time people are in the acute stage of illness, Schaffner explained.

"It's why many people feel wiped out for two weeks or more after they have stopped coughing," Schaffner said. "This lingering inflammation can involve blood vessels, particularly to the heart and brain," which can lead to increased risk of heart attack and stroke during the two to four weeks of recovery from flu.

The disease can also lead to incomplete recovery among older and frail patients, he said.

"Flu can knock down that first domino of progressive decline, progressive disability," Schaffner said. "When people who are already frail or on the edge get flu, they may never return to their pre-flu functional level."

And while those groups, along with pregnant women, are most at risk, officials warned that the young and healthy shouldn't be too cavalier about getting a flu shot: Flu can be severe for them, and not getting vaccinated can put others who are more vulnerable at risk.

"Getting the flu shot isn't just about keeping you safe and healthy, getting the flu shot is about community," Adams said. "Those 80,000 people who died last year from the flu — guess what? They got the flu from someone. Someone passed it along to them."

Officials acknowledged concerns about how effective the vaccine is in protecting people from influenza. The effectiveness of last year's shot was thought to be particularly low, and the CDC reports it was about 40 percent overall; however, in a predominant strain, H3N2, the protection was just 25 percent.

Food and Drug Administration Commissioner Dr. Scott Gottlieb, in a separate statement Thursday, said the FDA, CDC and other agencies have made changes to try to strengthen the vaccine: replacing the H3N2 strain for one they hope will be a better match, and swapping out one of the B strains as well. (Each flu vaccine contains two A strains of virus and one B strain; some contain two B strains.)

"We have some confidence, based on the pattern of influenza circulating now in the Southern hemisphere, that the flu strains chosen for this year's U.S. seasonal flu vaccine should offer Americans good protection," Gottlieb said.

Nonetheless, officials said, the protection the flu shot provides is better than nothing.

"You know that old thing, 'I got the flu shot and I got sick?'" Adams said at the news conference. "Well, know what? You didn't die."