The flu vaccinations that doctors hoped would save the lives of fragile elderly people have apparently failed to lower death rates, U.S. researchers said on Monday.

More people whose health could be put at risk by influenza have heeded the call to get vaccinated before flu season, but the death rate during the winter flu season remained the same rather than declining, they said.

Based on U.S. mortality rates from 1968 to 2001, the study by the National Institute of Allergy and Infectious Diseases found no correlation between increasing vaccination rates after 1980 and declining death rates in any age group.

“We conclude, therefore, that there are not enough influenza-related deaths to support the conclusion that vaccination can reduce total winter mortality among the U.S. elderly population by as much as half,” study author Lone Simonsen wrote in The Archives of Internal Medicine.

Previous studies have estimated that vaccine programs have cut mortality rates among the elderly by about half.

While the vaccination rate in 2001 rose to 65 percent among elderly Americans from around 20 percent before 1980, the rate of excess winter deaths has remained flat instead of declining by an expected 40 percent.

Because deaths spiked in some years when a virulent form of flu was epidemic, the findings may not be conclusive, the researchers said.

The report suggested earlier observational studies may have had skewed data. For one thing, influenza sufferers may have died from secondary complications brought on by flu after their symptoms had passed, it said. In addition, vaccines often fail to activate antibodies in the elderly.

Further evidence may come after this flu season since an initial U.S. shortage of flu vaccine should have an effect on mortality rates, one way or the other, the report said.

“Either way, this vast disconnect between conclusions from different studies must be sorted out,” it said.