The more facts that roll in about the spread of AIDS in America, the more accurate appear the tentative early assessments of United States health officials. Figures beginning to come in from various studies and surveys show that initial fears that a cataclysm would rush across the spectrum of America's population are proving unfounded, experts say - but the time for vigilance is far from ended.
Experts overwhelmingly agree that the disease is transmitted virtually exclusively by blood, sexual secretions, or from mother to unborn child. It is not spread by casual contact, they say.
Studies continue to show that the disease and the virus which is its precursor have made only limited inroads among heterosexual Americans who do not use intravenous drugs. ``There is the potential for the [AIDS] virus to move out into the general population,'' Dr. Harold Jaffe says. ``But at this point the extent is relatively limited.''
Dr. Jaffe, the government's top expert on the spread of the illness, is chief of the epidemiology branch of the AIDS program of the government's Centers for Disease Control.
For example, four of every 10,000 US blood donors test positive for the AIDS virus, and 15 of every 10,000 military applicants. But Jaffe says that ``probably 80 or 90 percent'' of those who tested positive belong to known risk groups, such as homosexuals and intravenous drug users.
Thus, he concludes, ``these figures almost certainly are overestimates'' of the penetration of the virus into the general heterosexual population.
Similarly, a study of 120 women conducted at an American sexual disease clinic did not find ``a single woman'' who had contracted the virus, apart from those who belonged to known risk groups.
The number of Americans afflicted with the disease, now 60,000, continues to rise at the rate anticipated by the US Public Health Service. Two years ago it forecast that 270,000 Americans would be dealing with the illness by the end of 1991.
On June 2 and 3 of this year top government officials will meet in Charlottesville, Va., to chart their expectations for the next year or two, based in part on the new information coming in about the prevalence of the AIDS virus among Americans.
The estimates for 1992 and '93 are likely to be higher than for '91. The numbers of AIDS cases in the early '90s ``are going to be increasing,'' says Dr. Peter Fischinger, who directs the national AIDS program office of the US Public Health Service.
Thus far, 65 percent of Americans afflicted with the illness are homosexuals or bisexuals; 17 percent are intravenous drug users. Only 4 percent are heterosexuals who do not use drugs.
Despite some successes, efforts nationally have been inadequate to educate Americans to the causes of the disease and the ways to prevent its spread.
``We're giving people kind of a mixed message,'' Dr. Jaffe says. Simultaneously Americans are being told that risk exists but that most of them should not be too worried, that they should take sexual precautions but that most heterosexuals probably will not contract the illness.
``I don't think,'' Jaffe says, ``that we've done a very good job'' of communicating the complexity of the situation and persuading people to be sexually cautious.
In general, homosexuals in large cities have changed their practices; but many young heterosexuals - believing they are not at risk - have not, nor have homosexuals in small cities. Neither have drug abusers nor their sexual partners.
Drug abusers are ``the most difficult to reach,'' Dr. Fischinger says; they and their sexual partners make up the principal route by which the disease is transmitted into the general heterosexual American population.