THURSDAY, Oct. 1, 2009 (HealthDay News) -- People exposed to the deadly 1918 Spanish flu pandemic while still in their mother's womb were about 20 percent more likely to have heart disease 60 years later, a new study has found.
The flu outbreak in 1918 killed 20 million to 40 million people worldwide, including 500,000 in the United States. That flu, like the current H1N1 swine flu pandemic, began as a mild disease, but it then came back in a much more lethal form. What the current H1N1 flu will do is unknown, but so far its genetics have not changed and there is a vaccine to protect against it, researchers say.
That's especially good news for pregnant women.
"There are long-term effects of being exposed prenatally to flu," said lead researcher Caleb Finch, director of the Gerontology Research Institute at the University of Southern California. "There is a danger to the fetus from exposure to maternal flu that has shown up 60 years later from the 1918 influenza epidemic."
Why exposure to flu in the womb has this effect is not known, Finch said, but he added that it's "a likely outcome of maternal stress."
Maternal stress increases a number of developmental problems, including the risk for autism and schizophrenia, Finch said.
Whether these same effects can be traced to other flu strains also is unknown, he said. "Each flu is different, and the 1918 epidemic remains the most virulent," he said. "Subsequent epidemics have not been as severe. This could have been something unique to that, but we can't tell. It took 60 years to find this out."
The findings are reported in the Oct. 1 issue of the Journal of Developmental Origins of Health and Disease.
For the study, Finch's team collected data on 101,068 people born around the time of the 1918 flu pandemic -- specifically, between 1915 and 1923. Information came from national surveys conducted from 1982 to 1996, when most participants were 63 to 78 years old.
The researchers found that men born in early 1919 -- meaning their mothers were in their second or third trimester of pregnancy during the height of the epidemic -- had a 23 percent increased risk for heart disease at age 60, compared with the general population.
Yet women born in early 1919 were not significantly more likely to develop heart disease, which may have to do with gender differences in the effects of flu exposure, Finch's group said.
However, women born in the second quarter of 1919 -- whose mothers, then, were in the first trimester of pregnancy during the height of the epidemic -- were 17 percent more likely to develop heart disease than the general population, the study found.
In addition, among 2.7 million men born between 1915 and 1922, the researchers looked at their height at the time they signed up for service in World War II. They found that the men's height increased every year, except among men born during the flu pandemic.
Moreover, men who'd been exposed to the 1918 Spanish flu while in the womb were slightly shorter than men born just a year later or a year before. The findings remained significant even after controlling for season-of-birth effects and any malnutrition among the mothers, the study reported.
Dr. Marc Siegel, an associate professor of medicine at the New York University School of Medicine in New York City said that "it is reasonable that, if you had the flu in 1918, it could lead to a maternal disruption that would increase the incidence of long-term medical problems if you're a fetus."
The 1918 flu, he said, was particularly deadly, which is not likely to be the case with every flu variation.
"The current H1N1 flu is mild and certainly has less teeth than the 1918 flu, in terms of its virulence," Siegel said. "You cannot conclude that the 2009 swine flu pandemic is going to lead to heart disease 60 years later."
But the study is a reminder of just how problematic and tricky flu can be, especially in pregnancy, Siegel said. "Pregnant women should get flu shots, especially the H1N1 shot," he said.
The U.S. Centers for Disease Control and Prevention agrees that the best protection against the flu is to get vaccinated. This year, that means getting a seasonal flu shot and an H1N1 flu shot when the vaccine is available.
Because pregnant women are at a high risk for complications from the flu, the CDC has put them at the front of both the seasonal and H1N1 vaccine line.
"If you are a woman of reproductive age and likely to be pregnant, be very sure you have gotten vaccinated," Finch advised.
The U.S. Department of Health and Human Services has more information on flu at its Web site flu.gov.