As swine flu spreads across the country and around the globe, its youngest victims may also be among the ones who are in the most danger of losing their lives, infectious disease experts say.
Their comments follow the first reported death in the United States from the swine flu -- a 22-month-old Mexican boy who had arrived at Texas Children's Hospital in Houston for treatment, but who died on Monday -- as well as the World Health Organization's decision to raise their pandemic alert level to phase 5 from phase 4.
"Classically, in seasonal influenza it is the very young and very old that die from influenza," said Dr. George Rutherford, director of the UCSF Institute for Global Health. "The CDC has not done full analysis on pediatric deaths this season, but last season 50 percent of childhood flu deaths occurred in those under 5 years old. The majority of these were in kids under age 2."
"Young children, especially those less than one year of age are particularly susceptible to influenza," agreed Dr. Rich Whitley, professor of medicine at the University of Alabama at Birmingham. "The younger the child, the more likely the severity will be greater, and risk for death increases."
Additionally, past research has shown that, in general, infants' immature immune systems make them more vulnerable to infection and death from viral infections like influenza.
But whether these past findings will be borne out with this new virus have yet to be seen, warns Ed Hsu, associate professor at the University of Texas School of Health Information Sciences and School of Public Health.
"We may need another week until the H1N1 outbreak runs the full course of its incubation and infectivity period," he said. "By this weekend we should have enough data to make some meaningful inferences from worldwide distribution of the disease, including susceptibility or vulnerability by age over time."
Will Swine Flu Show Death Spike in Young Adults?
But while many infectious disease experts expect very young children to be more vulnerable to the new virus, it is a spike in deaths among young adults that could be a sign of a much more troubling outbreak.
H1N1 Pandemic of 1918: Will History Repeat Itself?
Such a pattern was seen during the infamous 1918 influenza pandemic, which caused at least 675,000 U.S. deaths and up to 50 million deaths worldwide. Researchers say many of these deaths were among the young, whose healthy immune systems mounted a response to the virus that was so destructive it killed them.
"The 1918-1919 flu had a W-shaped mortality pattern, with elevated mortality rates in the young, old and 20- to 39-year-olds," Rutherford said. "It's this middle bulge in mortality that has everyone worried."
Thus far, patients in the United States with swine flu have ranged from infancy to 81 years old. But 64 percent of these cases have been in those under the age of 18. While the number of cases is still too low to draw any solid conclusions, some say this preponderance of younger patients is a potentially troubling sign.
"As was the case in 1918, young and otherwise healthy individuals seem to be disproportionately affected," said Dr. Oren Cohen, chief medical and scientific officer at Quintiles Transnational Corp. and consulting professor of medicine at Duke University Medical Center in Durham, N.C. "It is too early to know about death rates."
Protecting Your Child From Swine Flu
Jacqueline Civitano is just one parent who is dealing firsthand with the specter of swine flu. Her 17-year-old son, Frankie is a student at St. Francis Preparatory School in Queens, the site of the first reported cases in New York City. He tested positive for the illness on Sunday.
Since then, she said, three of her other children and her daughter-in-law have also come down with the disease.
She said everyone in her family is recovering. But she said she was particularly concerned about her infected 10-year-old son, who also has diabetes.
"Yesterday we thought we were going to have to take him to the hospital because he couldn't eat or drink," she said Tuesday. "It's a little scary when you don't know what's going on and you hear all this stuff about people dying in Mexico. It's extremely frightening."
Fortunately, infectious disease experts noted, there are steps that families can take to protect children from the worst consequences of the disease.
Simple Steps Help Protect Kids From H1N1
Rutherford said Tamiflu, one of the two antiviral medicines said to be effective against swine flu, is approved for use in babies as young as 1 year old, and because of an emergency order from the government, the FDA has already said that doctors may use Tamiflu to treat babies even under 1 year of age.
Relenza, the other antiviral that has shown promise against swine flu, is approved for use in kids age 7 and up; that prescribing information has not been altered.
But perhaps the best defense for babies is to keep them away from possible infection through social distancing -- in short, making certain they do not come into contact with infected individuals.
Parents of younger children can also make sure that their kids follow established hygiene practices in order to lower their risk of infection. These include frequent and proper hand washing, as well as staying clear from friends and family members who are known to be infected.
The ABC News Medical Unit contributed to this report.