Public health experts warn that the world might be close to a repeat of the flu pandemic of 1918, which killed millions. But could it instead be close to a reprise of the 1976 pandemic that never happened?

That is the year President Gerald R. Ford announced a crash program to "inoculate every man, woman and child in the United States" against swine flu. But the virus never became a killer, and vaccinations were halted two months after they began after reports that 500 people who received the shot developed a paralyzing nerve disease and more than 30 of them died.

If scientists were wrong in 1976, could they be wrong now? Some arguments being made about why a pandemic is looming echo those made three decades ago. But many experts say the situation now is different enough that a false alarm is less likely.

"We just know a lot more about the influenza virus than we did in 1976," said Ira M. Longini Jr., a professor at Emory University who is an expert on epidemics.

Still, a lot can be learned both from what did and did not happen back then.

The 1976 scare started in February when a handful of soldiers at Fort Dix in New Jersey got sick and one of them died. Scientists determined that the virus was one that infected pigs and was different from the human influenza viruses circulating then. On March 24, barely a month later, Ford announced the vaccination plan.

One reason for the concern was that scientists thought the 1918 pandemic had been caused by a swine virus and that the Fort Dix outbreak marked its second coming. Furthermore, experts warned that pandemics tended to be cyclical and that another one was about due.

Today, thanks to genetic analysis — a technique not available in 1976 — scientists know the 1918 virus was a bird virus that mutated. So now there is concern that the H5N1 avian strain ravaging birds in Asia could in like fashion evolve into a form that can spread easily among people. The avian virus shows some mutations similar to those in the 1918 virus, said Dr. Jeffery K. Taubenberger of the Armed Forces Institute of Pathology. And experts are again warning that the world is overdue for a pandemic.

Edwin M. Kilbourne, a professor emeritus at New York Medical College who argued for the vaccination program in 1976, said there was actually less reason to be concerned about a pandemic today. That is because the swine flu virus at Fort Dix clearly passed easily from person to person, while the current avian flu has not.

Many experts disagree, however. In retrospect, they say, the 1976 decision to vaccinate was based on much less solid evidence than is available today.