The "Russian flu." which spreads extremely quickly although it does not seem to cause more serious illness than any other influenza strain, has been reported in this country for the first time.

Dr. L.J. Cohen, director of the Wyoming health department, said the virus caused an "explosive" outbreak at a high school in Cheyenne, Wyo., where the flu spread to 500 of the school's 1,500 students within a 10-day period.

Scientists at the federal Center for Disease Control in Atlanta identified the A/U.S.S.R./77 strain in cultures from five throat swabbings sent to Atlanta by Wyoming officials.

he outbreak of the new flu comes at a time when the nation, including the Washington area, is in the grip of a major influenza epidemic caused by other viruses. Physicians have been treating up to 180 cases of flu a day in the Children's Hospital National Medical Center emergency room.

Cohen described the illness from the Soviet virus as "relatively benign. The kids are sick, and they tell us they feel awful, but there have not been any complications." The disease only last three to five days, a relatively short time for influenza.

While the virus has not been isolated anywhere else in the United States. Cohen said he is convinced it is "probably widespread in the U.S. I'm sure you'll find the trends of spread from here now, but I don't like to leave you with the impression it began here. We just found it first."

The health department official said the only reason his workers took throat cultures from the students in Cheyenne was that the outbreak there "was so remarkable and so sudden."

The influenza epidemic now spreading through the country has been caused by the A/Texas and A/Victoria virus strains.

The only vaccine in use this season provides primary protection against the A/Victoria strain, and very minor protection against the more prevalent A/Texas. It provides no protection against the Russian flu.

Several pharmaceutical firms said yesterday there is no way they can have a sufficient amount of vaccine produced this season to provide immunity to the A/U.S.S.R./77 virus.

The manufacturers are working on the vaccine after being given samples of the virus by the CDC and after a panel of experts twice concluded that production of a vaccine should begin.

Secretary of Health, Education and Welfare Joseph Califano announced yesterday that yet another group of experts will convene Monday in Washington to advise Califano on the way in which HEW should respond to the threat posed by the new virus.

At the two previous meetings, one in Atlanta and one at the National Institutes of Health in Bethesda, a panel of experts was reluctant to propose a national immunization program after the 1976-77 Swine flue debacle.

That program, which involved the federal government immunizing some 30 million Americans against a disease that never materialized outside the gates of Fort Dix, N.J., left the government with a stockpile of $40 million worth of now worthless vaccine, and many millions of dollars in lawsuits filed by persons who suffered side effects from the injections.

One of the possible options under consideration this year is the use of the antivirus drug Amantadine, which can be take prior to infection in order to prevent disease and while one is ill to reduce the seriousness of the infection. If this drug is used, it undoubtedly will be limited to those with chronic respiratory diseases and the very old, the two groups for whom flu poses the greatest threat.

So far researchers have been unable to come up with a single, all-encompassing flu vaccine because there are a large number of influenza viruses.

The A/U.S.S.R./77 virus that swept through China and the Soviet Union is identified as an H1N1 virus, while A Texas and A-Victoria are H3N2 viruses.

The H stand for hemaggultinin and the N stands for neuraminidase, the factors in the virus that trigger an antibody response in the person infected.

Because the A/U.S.S.R./77 is an H1N1 virus, those persons in the 23- to 33-year-old age group have some resistance against the virus because they were probably infected by a very similar H1N1 virus in the late 1940s and the 1950s. Therefore they have antibodies against that type of infection.

Those younger than 23 have probably had significant exposure only to H2N2 and H3N2 viruses, and thus have no immunity to the H1N1 virus. The elderly are also defenseless, because they are unlikely to have had flu in the late 1940s - children are the ones most likely to be infected by influenza - and their exposure to a similar virus in 1918-19 is probably too long ago to do them any real good now.

One of the interesting questions raised by this new outbreak is what will happen when two very dissimilar influenza viruses are in circulation at the same time.

Dr. Robert Parrott, director of Children's Hospital and a nationally known expert on viruses, said this is the first time that there have been two distinct, major, outbreaks simultaneously.

It is possible, said Parrott, for a person to be infected by both viruses at the same time. However, he said, all that would probably happen under those circumstances is that the person would have a single case of flu.

The prospect of a double influenza outbreak does pose serious problems because of the numerous days that could be lost from work and school if large numbers of people were afflicted by first one virus and then the other.