SUNDAY, Aug. 17 (HealthDay News) -- People who lived through the 1918 flu pandemic that killed 50 million worldwide are still producing antibodies to the virus 90 years later, researchers report.

"Most people have a notion that elderly people have very weak immunity or they have lost immunity," said lead researcher Dr. James E. Crowe Jr., a professor of pediatrics, microbiology and immunology at Vanderbilt University.

"This study shows that extremely elderly people have retained memory of being infected with the 1918 flu, even 90 years later," Crowe said.

This is the first evidence that shows that people developed significant immunity to the 1918 flu virus, Crowe said. "It's important to know that you can develop immunity to such a pandemic virus. That has implications for new pandemic viruses," he said.

The report is published in the Aug. 17 issue of Nature.

For the study, Crowe's team studied antibodies in the blood of 32 people in their 90s and 100s, born during or before 1915. They found that all 32 people had antibodies to the 1918 strain of flu virus. In fact, several of these people were still producing the antibodies to the virus.

In experiments with mice, the researchers found that these antibodies continue to protect the mice from infection with the 1918 flu strain.

The study also shows that people have a "surprising ability" to maintain immunity to things they saw a long time ago, Crowe said.

Whether this long-term immunity is peculiar to the 1918 flu virus isn't known, Crowe said. He believes more work needs to be done understand the full extent of this immune response. "The elderly might be a very good donor source for finding antibodies against viruses," he said.

"This study shows that humans can develop very potent immune responses against dangerous influenza that cause pandemics," Crowe said. "It gives us hope that we can develop vaccines and antibody treatments for any other pandemic viruses that come along," he said.

Dr. Marc Siegel, an associate professor of medicine at New York University School of Medicine in New York City, thinks that people who developed this strong immune response may have been infected with a less deadly strain of flu before 1918.

"The implication of this study is the 1918 virus was so powerful that the immunity you had to have in order to survive was so prominent that it lasted for the rest of your life," Siegel said.

However, Siegel noted that some people may have had experience with a similar less deadly flu virus that prepared their immune system to handle the 1918 strain.

"So, those in certain age groups who had seen a related virus had the strongest responses," Siegel said. "Either they died, or they developed a profound immune response," he said.

Siegel expects if there is another flu pandemic, some people will develop a lifelong immunity as they did in 1918.

More information

For more on avian flu, visit the U.S. Centers for Disease Control and Prevention.