A second study found that children are still shedding H1N1 virus nearly two weeks after symptoms first appeared, although the lead author of that study emphasized that this is not synonymous with the virus being infectious for that long.

The H1N1 virus is spreading rapidly, although it has not changed from the typically mild illness observed last spring and summer, experts said at a press conference held Thursday at the Infectious Diseases Society of America's annual meeting in Philadelphia.

"We have the same [H1N1] disease from the spring and summer but just a lot more of it right now," said Rear Admiral Dr. Stephen Redd, director of the Influenza Coordination Unit at the U.S. Centers for Disease Control and Prevention.

"An increasing proportion of people are visiting doctors with influenza-like illness, the disease is widespread and we are seeing more deaths in children in particular, and we would expect that to continue as the number of cases increases," he said.

Antiviral drugs have been dispatched from the U.S. government stockpile to treat children, Redd added.

So far, almost all strains of H1N1 have responded to both oseltamivir (Tamiflu) and another antiviral, zanamivir (Relenza), while displaying resistance to amantadine, a drug in a different class. As a result, Tamiflu and Relenza have been used widely for both the prevention and treatment of H1N1.

However, in June and July of 2009, 65 campers and staff at a summer camp in North Carolina became ill with H1N1 and were treated with Tamiflu, while 600 other campers and staff took the antiviral to prevent the illness.

Two females who shared a cabin developed symptoms after starting on Tamiflu and were later found to have a virus with two viral mutations that rendered them resistant to the drug. The mutated virus was not found in other people tested.

What's troubling is that one of the females appears to have transmitted the mutated virus to her cabin mate. "It is likely that this resistant virus was passed from one camper to the other based on the timing between the illnesses and 2 genetic mutations found in the virus in both campers," explained Dr. Natalie Janine Dailey, lead author of the study and an epidemic intelligence service officer with the North Carolina Division of Public Health Communicable Disease Branch. "A small number of cases of oseltamivir-resistant have been seen in the U.S. so far, but these were the first cases reported in otherwise healthy individuals and the first which appeared to have spread from one person to another."

"This suggests that using oseltamivir to prevent influenza in healthy people may increase the risk of resistance," she said. "If resistance became widespread, oseltamivir would no longer be effective."

With this in mind, Dailey believes that the H1N1 vaccine, instead of antivirals, should be used for prevention as it becomes available, although treatment with antivirals should begin immediately in people who are hospitalized or who are at high risk, such as pregnant women, children under the age of 2 and people with underlying health conditions.

A second team of researchers looked at 26 elementary-school students in Pennsylvania and their household contacts who had tested positive for H1N1 to assess virus "shedding patterns."

"We found the median duration of shedding to be six days, with a minimum of one day and a maximum of 13 days," said study author Dr. Achuyt Bhattarai, an epidemic intelligence service officer with the CDC.

The same numbers were found in children over the age of 9, representing a longer time frame that is typically seen in adults. Bhattarai said, "this is consistent with earlier studies of seasonal flu."

This and future data should help officials decide when children should be allowed to return to school.

The teleconference also addressed the current delays and shortages in available H1N1 vaccine.

"We're all disappointed and frustrated by the current situation with the vaccine supply but we need to recognize we're not alone. The situation is true globally," said Dr. Bruce Gellin, director of the U.S. Department of Health and Human Services' National Vaccine Program.

The situation points up problems in the current vaccine production system, which relies on eggs as incubators of the virus.

"There's certainly lots of room for improvement in these systems," Gellin said. "Some of the early issues are resolving, particularly real difficulties with yield and variability among manufacturers. Some yields were half what was expected, some were less than half. That was a large part of the issue. We're encouraged that many of these things are being optimized and it's the same with the seasonal vaccine every year. We continue to do tune-ups which are going to translate to more doses over the coming weeks and hopefully then, the lines will get shorter."

More information

There's more on H1N1 flu at the U.S. Centers for Disease Control and Prevention.