(CBS/AP)-- A new global study of a century's worth of flu pandemics gives new meaning to that "first kiss." Influenza's first kiss, that is.
"Your first kiss by the influenza virus produces original antigenic sins," the study's lead author, Dr. Thomas Reichert of the Entropy Research Institute in Lincoln, Mass., tells CBS News. "We remember our first kiss forever."
The study, published Dec. 12 in the peer-reviewed BMC Medicine, looks at all five influenza pandemics of the past 100 years. It finds a variable but often large number of elderly individuals were immune to influenza because their bodies had been infected with a similar virus in the past. In other words, the viruses were recycled.
Reichert says that during the 2009 influenza pandemic, most people over age 62 were immune because the flu virus closely resembled viruses they'd been exposed to before 1947. In 1969, people over age 78 had immunity, the study found. In 1918, it was those over age 45-55 who were best protected.
In Reichert's view, the "immunity of past experience" has important implications. First, he says in pandemic seasons, flu shots and other resources should be diverted to younger people who aren't naturally protected and not wasted on the elderly, many of whom will already be immune.
Second, he says American businesses should give serious thought to cultivating a cadre of retired, elderly to bring into the workforce in the event of a severe flu pandemic. "If your factory risks being down for a year because so many workers are stricken, the immune elderly could be a tremendous resource," says Reichert.
"Any employer who motivated his retirees could bring them in and only a few would get sick." He concludes that competitors who prepared this way would have a huge advantage over those who don't.
Dr. Thomas Tallman, an emergency medicine physician at the Cleveland Clinic who was not involved in the research, tells CBS News that the study is "very intriguing."
He agrees that exposure to previous flu viruses or viruses in a vaccine could build up an individual's immunity. The problem, he said, is that it is difficult to determine how much protection a person may have.
"The measure of how much immunity there is extremely unpredictable," said Tallman. He would still recommend a flu shot for older adults because they are more susceptible to repiratory diseases like COPD and pneumonia, which could make flu more dangerous.
The new research is a follow up to Reichert's study with the National Institutes of Health in the Archives of Internal Medicine in 2005, that surprisingly showed flu shots don't work well in the elderly.
In fact, the results were the opposite of what the government and the researchers expected. As more and more elderly have gotten flu shots, death rates haven't gone down, they've gone up.
"We could not correlate increasing vaccination coverage after 1980 with declining mortality rates in any age group," that study concluded at the time.
With more data on more countries, Reichert adds, "We can now see that mortality rates declined in younger people -- most of whom were not vaccinated -- but not at all in the elderly (an increasing fraction of whom were vaccinated)." That was seen especially in those who were immune in the pandemic seasons, he says.