Government officials say it's likely that 100,000 Americans are infected with the new influenza strain that is worrying health experts around the world.
Dr. Anne Schuchat of the Centers for Disease Control and Prevention said the more than 5,400 confirmed and probable swine flu cases and at least six deaths in the United States were "the tip of the iceberg."
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The new strain of H1N1 is also putting a worrying number of young adults and children into the hospital — as many as 200 people have been hospitalized in the U.S. — and hitting more schools than usual.
"That's very unusual, to have so many people under 20 to require hospitalization, and some of them in (intensive care units)," Schuchat told reporters in a telephone briefing.
"We are now experiencing levels of influenza-like illness that are higher than usual for this time of year," Schuchat added. "We are also seeing outbreaks in schools, which is extremely unusual for this time of year."
It killed a vice principal at a New York City school over the weekend and has spread to 48 states. While it appears to be mild, it is affecting a disproportionate number of children, teenagers and young adults.
It is also suspected in the death of a 44-year-old Missouri man and a New York City toddler, health officials said Tuesday.
New York City Health Commissioner Dr. Thomas Frieden agreed with Schuchat.
"We're seeing increasing numbers of people going to emergency departments saying they have fever and flu, particularly young people in the 5 to 17 age group, " Frieden, who has been named by U.S. President Barack Obama as the new CDC director, told a news conference.
About half of all cases of influenza are being diagnosed as the new H1N1 strain, while the rest are influenza B, or the seasonal H1N1 and H3N2 strains. Flu season in the United States is usually almost over by May.
MORE ILLNESS OVERALL
"We are seeing more reports of influenza-like illness from outpatient visits that we monitor than is typical for this time of year," Schuchat said.
Because doctors usually treat symptoms and only occasionally give flu tests to patients, the CDC must monitor reports of symptoms such as fever, cough and muscle aches to track flu activity. Some centers are doing actual influenza tests to confirm the patterns that are seen.
Influenza is a factor in 36,000 deaths a year in the United States and 250,000 to 500,000 deaths globally, the CDC says.
"Unlike the seasonal flu, we are seeing relatively few cases or hospitalizations in people over 65," Schuchat said. Usually flu kills the elderly and people with chronic diseases.
There is no evidence that a second, bacterial infection is worsening the H1N1 cases, Schuchat said.
When family members are questioned, it seems clear that children and teens are more prone to infection than older adults, Schuchat said. "People under 18 are more likely to have infections when another person in the family is infected," she said.
"One of our working hypotheses is that older adults may have some pre-existing protection against this virus due to their exposure long ago to some virus that may be distantly related," Schuchat said.
An alternative hypothesis is that it just has not had a chance to make its way into the older population yet.
The number of confirmed cases of the new Influenza A (H1N1) flu has risen to 10,243 and the death toll has edged up to 80, the World Health Organization (WHO) said on Wednesday.
Most of the new cases are in the United States, which has seen 5,469 outbreaks of the virus so far, the WHO said as it focuses on the H1N1 virus that has brought the world to the brink of a pandemic.
Another 51 cases have also been reported in Japan, bringing the total number of cases there to 210 and potentially making it more likely that the WHO will declare a full pandemic after it raised its pandemic alert last month to 5 on a 6-level scale.
Health ministers and experts at this week's WHO annual assembly have been discussing how to fight the virus with vaccines and drugs as well as what criteria the WHO should consider when deciding whether to raise the alert level.
Under WHO rules, signs the disease is spreading in a sustained way in a second region of the world outside its North American epicenter would prompt a declaration that a full pandemic is under way.
Ministers have urged the WHO to consider other factors such as the severity of the virus before moving to the highest alert.
Forty countries have confirmed cases of the new strain and nearly all of those who have died were in Mexico, but most patients globally have had relatively mild symptoms.