At least 855 persons around the nation died of influenza and pneumonia during the week ending Jan. 14, prompting the federal Center for Disease Control to classify this year's flu outbreak an epidemic.

Almost all of the deaths occurred among the elderly and persons with chronic respiratory condition. The number of deaths reported from 121 cities far exceeds the 500 flu and pneumonia deaths expected during the same week in any given year.

At the same time, the CDC announced that the so-called Russian flu, which has not yet reached this country, infected 40 U.S. Air Force personnel stationed in Britain.

A CDC spokesman said the virus, A/USSR 77, was first isolated at the American aribase at Upper Heyford, England, Jan. 4, and the outbreak there peaked Jan. 9.

According to the CDC, the flu primarily affected persons in the 18- to 22-year-old age group, and struck at a rate of 10 cases per 100,000 people.

National experts on infectious diseases have met twice this flu season, once in Atlanta at CDC headquarters and once in Bethesda at the National Institute of Health, to decide what to do about the new flu strain.

It was the concensus of those attending both meetings that the Soviet strain would definitely reach this country, the only question being whether the outbreak would occur late this winter or spring, or durng next fall and winter's flu season.

While pharmaceutical manufacturers have begun work toward producing a vaccine against A/USSR/77, those familiar with vaccine development have predicted that the drug cannot be produced in time to protect against an outbreak of the new flu strain this season.

According to the issue of the CDC's Morbidity and Mortality Weekly Report released yesterday, the new flu strain was first observed in mainland China, and not the Soviet Union.

About 10 days ago the Chinese notified the World Health Organization in Geneva that the virus was isolated outbreak in the northern Chinese city of Tientsin. The report states that "from July to October the epidemic spread south to many other areas of China . . . Illness was seen most often in children, ages eight to 20."

The new strain, which is somewhat similar to a virus reported in the late 1940s, also has been reported in Hong Kong, Taiwan, the Philippines, Czechoslovakia and Finland.

It reportedly causes no more serious an illness than any other strain of flue but is apparently quite infectious, and is said to have spread across the Soviet Union in a matter of weeks, infecting 35 million people.

According to influenze experts, those Americans in the 24- to 33-year-old age group, the heartiest segment of the population, have the best natural immunity to the new flu strain because it resembles the flu many of them caught when they were children.

On the other hand very young children and the elderly, the two groups for whom influenza can pose the most serious threat, have virtually no anti-bodies against the new strain.

The virus causing most of the flu in the United States at this time is the A/Texas strain, while the vaccine available now is aimed at the A/Victoria virus.

While the two strains have some similarity, and the vaccine can provide some minimal protection, most persons are being forced to depend upon their natural immunities to ward off this year's virus.

Influenza is primarily a respiratory infection, which causes headaches, body aches, fever, chills, a cough, "cold" symptoms and, in some cases, abdominal pains.

While many flu victims feel as if they are dying, very few do, and most recover in a one or two-week period.

The only practical way to treat the disease is to stay in bed, get plenty of rest, drink fluids and take medication to relieve the aches and reduce the fever.

Federal officials still have to decide whether they are going to call for a national immunization program to protect against the expected outbreak of the Russian flu.

If such an immunization program is begun, it is highly doubtful it would as widespread as the ill-fated swine flu immunization program of 1976-1977. It is more likely that any program would be aimed at the elderly and chronically ill.