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But it's too early to know whether a second wave will occur this winter.
The influenza season, which got off to an unusually early start, appears to have reached its peak in this country, an official with the Centers for Disease Control and Prevention in Atlanta said Wednesday.
But it is too early to know whether the number of influenza cases will decline sharply or stay at a plateau for another few weeks, said the official, Dr. Stephen M. Ostroff.
He also said that it was still too early to know whether a second wave would occur later in the winter.
"We are at the peak right now nationally, but not every region sees their peak activity at the same time," Ostroff said in a telephone interview.
"So it certainly appears that there are some areas where they have already peaked, some regions that have not quite hit their peak yet and some that are in the midst of their peak now," Ostroff said.
He also said: "We have had indications from the states that felt the impact earliest, like Texas and Colorado, that the amount of influenza activity has backed off considerably. The way that I read our monitoring systems, this is what we see when we have hit the peak of a season."
But influenza is unpredictable.
"We have seen spikes go up and come right back down again within a week or two, while in other years there has been a zig-zag pattern where the peak stays up for weeks," Ostroff said.
The CDC monitors influenza activity in a number of ways. One is a statistical calculation based on the number of deaths from pneumonia and influenza in 122 cities. By that measure, influenza exceeded the statistical threshold for a seasonal epidemic for this first time for week ended Dec. 27. The calculation shows that number of deaths from pneumonia and influenza have exceeded the usual number for recent weeks. But it will take a few more weeks to determine how many there will be.
At this point, he said, "there is no reason to believe that this season should be significantly dissimilar from other influenza seasons."
The duration of the peak period could be a crucial determinant in the severity of this season's epidemic. In some seasons, a second wave of influenza occurs, often in late February.
Because of the holiday period, CDC is not releasing data from the period since Dec. 27 until Thursday.
On average, influenza causes about 36,000 deaths each year.
This season, the majority of influenza has been caused by a new variant of the virus known as the Fujian strain, which this year's vaccine was not developed to combat. Officials believe, however, that there is some cross-protection. The strain seems to have hit young children hard. CDC earlier reported 42 deaths in children from influenza this season. According to one statistical model, about 92 deaths in children occur on average in this country.
In a step that might eventually reduce problems of the type experienced with the flu vaccine this year, Aventis, the world's leading manufacturer of flu vaccines, announced Wednesday a move toward improving the way its vaccine is produced.
Aventis, which is owned by Aventis SA, said it would team up with Crucell, a Dutch biotechnology company, to produce flu vaccine in cultures of human cells, rather than in hen's eggs, as is done now. However, the new vaccine won't even begin to be tested in people until 2005 and won't be approved for use for several years after that.
Current production requires millions of eggs and many months. That makes it hard to quickly make new doses when supply falls short, as happened this year. Also, the viral strains to be used in the vaccine have to be decided months in advance, in some cases before scientists know which viral strains will be prevalent that year.
Government officials have been encouraging the development of new techniques like cell culture, and other manufacturers besides Aventis are also moving in that direction.
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