Babies younger than six months — the age at which they can receive their first flu vaccine — are hospitalized from flu much more often than older children and are at greater risk of dying from it. Dr. Schuchat said infants with flu can develop problems like pneumonia, dehydration and swelling of the brain.

“Maternal immunization rates have been steadfastly stuck at about 50 percent,” said Dr. Denise Jamieson, chairwoman of the department of gynecology and obstetrics at Emory University School of Medicine, who was not involved in the new report. “We really haven’t moved the needle at all.”

The new report analyzed data on hospitalization and death from flu and whooping cough between 2010 and 2018. The researchers also conducted an online survey this past spring of about 2,600 women who reported being pregnant any time since August 2018. It asked whether the women’s health care providers recommended the vaccines, either by offering to provide them or referring the patients to someone who could, and whether the women agreed to get vaccinated.

About three-quarters of the women surveyed said that during pregnancy their providers recommended the flu vaccine and the Tdap vaccine, which protects against whooping cough, tetanus and diphtheria. But even among those women, about a third did not get vaccinated, the report said.

The most common reason the women gave for not getting the flu vaccine was a belief it was not effective. The reason they gave most often for not getting the Tdap vaccine was not knowing it is necessary during each pregnancy. For both vaccines, the second most common reason women refused it was concern about whether it was safe for their babies, the report said.

Dr. Jamieson, a former C.D.C. official who now practices obstetrics and gynecology at Grady Memorial Hospital, said that pregnant patients in her practice who declined to get the flu vaccine often said, “they heard bad things about the vaccine, misconceptions that it makes you sick or wasn’t safe,” she said. Some didn’t think they were at risk for flu she said.

Dr. Jamieson said women were generally more likely to accept the Tdap vaccine, possibly because it is newer so there are fewer misconceptions about it. The flu vaccine has been recommended in pregnancy since 1960, Dr. Jamieson said, but the Tdap has only been recommended for pregnant women since 2012.