There is no evidence the flu epidemic sweeping the country is being caused by new strains of virus, or by strains not "covered" by the vaccine in use, according to a report released yesterday by the Centers for Disease Control and Prevention (CDC).

More than 99 percent of virus samples gathered in the last two weeks of December were in the broad family of influenza A (H3N2), which has been responsible for virtually all flu cases here and in Europe in recent months. About 97 percent of samples collected since Oct. 1 are very similar to the influenza A (H3N2) virus used in this winter's vaccine. That means the current vaccine (which contains killed samples of three different flu viruses) should protect people to the maximum extent possible.

However, when given to a large population, no vaccine is 100 percent effective. About 10 percent of young people fail to gain protection from flu shots, and that fraction rises to 30 percent or more in the elderly. About 80 million doses of vaccine were produced this year. How many shots were actually given, however, won't be known for months.

Yesterday's report documented a steady worsening of the outbreak in most parts of the nation.

Nationally, 6 percent of doctor visits in the last week of December were for influenza-like illnesses, compared with 3 percent two weeks earlier. In parts of the Southwest, 15 percent of visits were for influenza.

The proportion of deaths attributable to influenza or pneumonia in the last week of December was 8.4 percent, compared with 7.8 percent two weeks earlier.

Influenza, however, is far from the only cause of respiratory illness, even now, close to the peak of the season. Of about 4,700 throat samples taken from ill people and tested at laboratories in Europe and North America, only 31 percent turned out to be influenza, according to yesterday's report.

Numerous other microbes can cause infections with symptoms similar or identical to influenza. They include respiratory syncytial virus (RSV), coronavirus, adenovirus, rhinovirus, and mycoplasma. RSV, which mostly strikes infants, increases markedly in the winter, as flu does. Neither it nor the other microbes tend to cause regional epidemics, as influenza does.

There's no evidence that any of those diseases are occurring with unusual or unexpected frequency, said Larry J. Anderson, head of the respiratory and enteric viruses branch of the CDC.