(McClatchy-Tribune Information Services) -- Newborns whose mothers got seasonal flu shots during pregnancy were less likely to be born premature, underweight, and in need of hospital care for respiratory illnesses, three new studies found.

The new research, coming in the midst of the swine flu pandemic, was released yesterday at the Infectious Diseases Society of America annual meeting in Philadelphia. Experts yesterday said the work underscored the need to improve vaccination rates among pregnant women.

Expectant mothers are especially vulnerable to the flu because their immune systems are suppressed. The baby also can limit expansion of the mother's lungs.

Pregnant women with swine flu were admitted to intensive-care units in Australia and New Zealand at nine times the expected rate given their proportion of the population, researchers reported this month in the New England Journal of Medicine.

From April through August, 100 pregnant women across the country have been in intensive care with swine flu and 28 have died, according to the Centers for Disease Control and Prevention.

Despite the peril, vaccination rates remain low for expectant mothers in this country -- from 15 percent to 25 percent.

Of all the high-risk groups, "pregnant women have the highest rate of complications yet they have the lowest rates of vaccination," said Paul G. Auwaerter, of Johns Hopkins University School of Medicine, program chair for the association's annual meeting.

"When you prevent flu in a pregnant mother, you benefit the woman, and you also benefit the fetus," added Mark C. Steinhoff, director of the Global Health Center at Cincinnati Children's Hospital Medical Center and the senior author of one of the studies on the impact of flu vaccine during pregnancy.

The challenge is getting that message -- that the vaccine will protect, not harm, their children -- to resonate with women who for years have been told to take particular care during pregnancy.

"It does somewhat go against the other messages that we give . . . 'don't drink, don't smoke, don't put anything foreign or toxic into your body, but take this vaccine,' " said Neil Fishman, director of the department of health care epidemiology and infection control at the University of Pennsylvania Health System.

The bottom line, Fishman and doctors here said, is that the flu vaccine is safe and should be effective in protecting mothers and, through them, their young children against the H1N1 virus.

Three studies being presented at the infectious diseases conference show how babies benefit when their mothers are immunized.

While those studies were of seasonal flu vaccines, the experts all said the same would apply to the H1N1 vaccine, which they hope will be more widely available in the next month or so.

"This is clearly a novel virus, but this is not a novel vaccine," said Bruce Gellin, director of the national vaccine program office at the Department of Health and Human Services.

"The vaccine is made exactly as it is made all the time [for seasonal influenza] with every test and every safety precaution," Gellin said.

William Schaffner, chairman of the Division of Infectious Diseases at Vanderbilt University Medical Center, said the H1N1 vaccine had been tested in at least 50 pregnant women and other healthy volunteers -- including himself. "It appears to be safe," he said.

A randomized study of 340 women in Bangladesh found that women vaccinated for the flu in their third trimester delivered babies who were on average half a pound heavier than the control group.

By contrast, "maternal influenza infection reduces fetal growth," Steinhoff and his coauthors concluded.

In another study, Marietta Vazquez and colleagues at the Yale University School of Medicine found infants under a year old were 79 percent less likely to be hospitalized if their mothers were immunized against the flu during pregnancy. Moreover, for babies less than six months of age -- those who are too young to get flu shots themselves -- maternal vaccination during pregnancy reduced the risk of hospitalization by 85 percent.

Saad B. Omer of the Emory University School of Public Health examined 6,410 births in Georgia between June 2004 and September 2006, and found that maternal immunization for the flu significantly reduced the rates of both premature births and low birth weight.

Both were particularly pronounced during periods of widespread influenza outbreaks, Omer said.

Still, doctors are confronted by parents fearful that the flu vaccine could harm their children, born or unborn.

Paul Offit, chief of the division of infectious diseases at Children's Hospital of Philadelphia, said those fears were being stoked by a misguided movement that blames vaccines for a host of illnesses from autism to the flu itself despite overwhelming scientific evidence to the contrary.

"As H1N1 sweeps across this country, it has already caused more deaths in children less than 18 and it's only October," Offit said. "Yet 40, 45 percent of Americans will not vaccinate themselves or their children. It is just a terribly dangerous . . . thing to do."

Copyright (C) 2009, The Philadelphia Inquirer