A new federal program to immunize 20 million children aged 14 and under against seven serious infectious diseases will be announced Wednesday by Health, Education and Welfare Secretary Joseph A. Califano Jr.

The aim will be to combat a rapid increase in cases of measles caused by a sharp lag in immunizations and prevent such an increase in cases of polio, diphtheria, rubella (German measles), whooping cough, tetanus and mumps.

Another goal, said an informed health official who declined to be named, will be "to jack up public interest in immunizations, which has suffered, fairly or unfairly, because of what happened in swine flu."

Speakers at a National Immunization Conference at the National Institutes of Health yesterday repeatedly stressed the need for a new national effort to vaccinate the unprotected and restore public credibility in vaccinations.

Attending the conference, which concludes Wednesday, are scientists, doctors, government health experts, insurance and drug firm officials and representatives of consumer, PTA and Catholic women's groups.

Four of five conference committees that have been working for several months on the subject called for a long-range national immunization policy to be developed by a new National Immunization Commission or immunization Policy Council that would report on the HEW Secretary and Congress.

HEW officials yesterday were still completing final details of their new vaccination program, Dr. James Dickson III, acting assistant secretary for health, told the conferees.

The object, other health officials said in interviews, will be to reach two primary groups - some 13 million children at or above the poverty level and 7 million in higher income groups. The needy children are those in families now eligible for federal aid programs.

Together, the 20 million youngsters lack one or more essential shots and represent more than a third of the 52 million children in this country under age 15.

Implementing a planned $6 million program announced Feb. 21 as part of President Carter's fiscal 1978 budget request, HEW will first seek to immunize at least 3 million of the 13 million needy children by July 1, 1978, officials said. Congress is expected to add a possible $4 million to increase this group.

These children will be immunized in large part by state and local health departments at public health clinics Local citizens' groups will be asked to help get youngsters to the clinics.

But there also will be a strong effort, officials added, to reach better-off families, partly by educating parents, children and doctors on the need for more vaccinations and partly by providing doctors free vaccine so they would charge their patients only for administering the shots.

Three diseases - measles. German measles (unrelated despite the similar name) and diphtheria - are on the increase this year" and all unneccessarily because we can vaccinate for them," said Dr. Saul Krugman, noted vaccine developer and chairman of two conference working groups.

"Children are dying," he told the NIH conferees. "In New York City last month two unimmunized children died of diptheria."

He showed slides of brain damaged, retarded, cerebral-palsied and blind children born to mothers never protected against rubella and said, "This is what we shall return to if we do not improve immunization."

The 1976 total of slightly fewer than 40,000 measles cases will "probably" double this year, he estimated. Doctors report that 1 child per 1,000 measles victims develops encephalitis of brain inflammation and 1 per 1,000 dies of encephantis or pneumonia.

The vaccine conference working groups largely agreed that vaccine efforts have been hindered by lack of continued financial support for vaccine research and development, lack of a federeal policy to protect vaccine maker from damage suits in cases where vaccines do harm through no fault of manufacturer and lack of public understanding that vaccines may sometimes do harm as well as good.