WASHINGTON - The level of swine flu activity in the United States appears to be declining, although officials are worried about another increase of cases during the Thanksgiving holiday when many people travel and families gather.
The number of states reporting widespread activity of the H1N1 virus dropped to 43 from 46 in the past week, and activity fell in all 10 regions of the country, according to the federal Centers for Disease Control and Prevention.
But flu cases are still rising in some states, including Maine and Hawaii, and it is too soon to know whether activity will surge again, said Anne Schuchat, director of the CDC's Center for Immunization and Respiratory Diseases.
"Influenza is unpredictable, and it is so early in the year to have this much disease. We don't know if these declines will persist, what the slope will be, whether we'll have a long decline or it will start to go up again," she said Friday.
The news came as scientists in Norway announced that they had detected a mutated form of the swine flu virus in two patients who died of the flu and in a third who was severely ill. It is the most recent report of mutations in the virus that is being watched closely for any change that could make it more dangerous.
In a statement, the Norwegian Institute of Public Health said the mutation "could possibly make the virus more prone to infect deeper in the airways and thus cause more severe disease," such as pneumonia.
The institute said that there was no indication that the mutation would hinder the ability of the vaccine to protect people from becoming infected or impair the effectiveness of antiviral drugs in treating people who became infected.
Scientists have been analyzing the H1N1 virus from "a number of patients as part of the surveillance of the pandemic flu virus" and have detected several mutations, the statement said. While the existence of mutations is normal, and most "will probably have little or no importance ... one mutation has caught special interest."
The two patients who had the mutation and died were the first swine flu fatalities in Norway. The third patient found to have the mutated form of the virus also became severely ill.
"Based on what we know so far, it seems that the mutated virus does not circulate in the population, but might be a result of spontaneous changes which have occurred in these three patients," the statement said.
Schuchat said the mutation is no reason for alarm.
"I don't think it has the public health implications that we would wonder about," she said, noting that some patients have gotten severely ill, including developing pneumonia, after being infected with strains of the virus without the mutation.
The World Health Organization said viruses with a similar mutation had been detected in several other countries, including Brazil, China, Japan, Mexico, Ukraine and the United States. "No links between the small number of patients infected with the mutated virus have been found and the mutation does not appear to spread," the WHO said in a statement.
"Influenza is a mutable virus, and changes are to be expected," said Arnold Monto of the University of Michigan in an e-mail. "This is typical early in the spread of a pandemic virus."
Scientists around the world have been tracking the virus carefully for any signs that it had mutated into a more dangerous form. While a variety of mutations have been detected, most have not appeared to have affected the virus in any significant way. There have been some mutations that make the virus more resistant to antiviral drugs, experts said. But like the mutation that may cause more severe illness, those too seem self-contained.
The CDC is investigating a cluster of four cases of patients at the Duke University Medical Center in Durham, N.C., who were found to be infected with H1N1 virus that was resistant to the antiviral drug Tamiflu. All four patients were treated with another antiviral drug known as Relenza.
William Schaffner, a medical professor at Vanderbilt University, said mutations that make episodes of swine flu more severe are most dangerous only if they are "easily transmissible."
"That would make it a more severe disease. Apparently this has that capacity. But in order for it to become really, quote, dangerous to the population, it also has to be easily transmissible," he said.
"That's a different characteristic And apparently that does not appear to have happened to this virus. It does not seem to be spreading in the general population."
Detection of the mutation should be reassuring, Schaffner said, because it shows the intensity of the global effort to monitor the virus. "The virologists are keeping an eye on H1N1 and this is evidence of that," he said.