The flu vaccine did a disappointing job last winter in the U.S., and officials worry that might be a bad sign for the fall.

Flu vaccines had been about 60% effective against the type of flu that caused the most lab-confirmed illnesses last winter, but last season’s vaccine was only about half that good, according to study results reported Wednesday.

Against another major type of flu, vaccines have already been only around 30% effective.

The new results may be a sign of shrinking effectiveness against a number of strains, and “that’s concerning,” said Brendan Flannery, who oversees the U.S. Centers for Disease Control and Prevention’s system for evaluating the vaccine.

The results come as health officials are gearing up for a critically important flu vaccination campaign, with a record 180 million doses being prepared. U.S. health officials fear a second wave of coronavirus infections, and reducing flu illnesses could help reduce patient traffic if COVID-19 causes doctors’ offices and hospitals to become overwhelmed.

CDC numbers indicate that last winter’s flu season caused somewhere in the neighborhood of 22 million medical visits, 575,000 hospitalizations and likely more than 40,000 deaths.

Vaccines against many infectious diseases aren’t considered successful unless they work at least 90% of the time, but flu is particularly challenging, partly because the virus can quickly change. Overall, flu vaccine effectiveness averages around 40%. Last season’s was 39% overall.

Last winter’s flu season featured two waves, each dominated by a different virus. Both flu bugs are considered dangerous to children, and it was a very bad flu season for kids.

Health officials reported 185 U.S. flu deaths in children this past flu season, the second highest total in a decade. And officials believe that because of reporting lags, additional cases may still come in.

Most of the children who died were not vaccinated. But how well the vaccine performs in kids remains a large concern.

A Type B flu strain ended up causing most early season illnesses and the bulk of illnesses in children over the entire winter, Flannery said. The vaccine was about 39% effective against that strain in children.

But the vaccine did an awful job against the Type A H1N1 strain that caused the second wave of illnesses. Protection was so low in kids ages 6 months to 17 years that — statistically speaking — it couldn’t be counted as working at all.

The vaccine’s performance was down in adults, too, and health officials don’t know exactly why. It may be due to changes in the virus, Flannery said.

Health officials have changed the components of what’s in the flu shot this year, in what they hope will be a better match against whatever flu strains circulate this fall and winter, he said.

The findings are drawn from a study of more than 2,700 people in five states with lab-confirmed flu infections, and presents estimates of vaccine effective in preventing flu illness severe enough to send a patient to a doctor’s office. They were presented during an online meeting of the Advisory Committee of Immunization Practices, an expert panel which the CDC looks to before making vaccination recommendations to U.S. doctors.