Tests of blood from medical staff and their spouses showed those who had been vaccinated in 1976 had evidence of extra immune protection against both the 2009 H1N1 swine flu and the seasonal strain of H1N1 that circulated the year before.
"We gave this vaccine to 45 million people and it was declared one of the greatest public health blunders of all time, and now we are finding out that it actually did some good," said Dr. Jonathan McCullers of St. Jude Children's Research Hospital in Memphis, Tennessee, who led the study published in the journal Clinical Infectious Diseases.
The study, published on Friday, supports a theory that different strains of flu virus cycle in and out of circulation and that getting a flu vaccine every year may protect people from as-yet unseen flu strains in the future.
"Our research shows that while immunity among those vaccinated in 1976 has waned somewhat, they mounted a much stronger immune response against the current pandemic H1N1 strain than others who did not receive the 1976 vaccine," McCullers said.
Every year, several different strains of influenza circulate. The viruses are mutation-prone and change a little bit every year, forcing vaccine-makers to reformulate the seasonal flu vaccine cocktail every year too.
"You never know with flu — more immunity is always good," McCullers said in a telephone interview.
Several times a century, a new strain pops up and causes a pandemic. That happened in 1918 with H1N1, 1957 with H2N2 and 1968 with H3N2. Usually, the new pandemic strain settles down and joins the seasonal mix, which may eventually happen with the 2009 H1N1 strain.
In 1976, a new strain of H1N1 broke out at an Army base in New Jersey and U.S. officials, worried about a pandemic, rushed out a vaccine and pushed hard to vaccinate the population.
The virus never spread off the base and the vaccine was linked with a rare but devastating side effect called Guillain Barre syndrome. Many Americans have been suspicious of vaccines ever since.
The 1976 virus was a distant cousin of the 1918 pandemic H1N1 and of the current swine flu pandemic strain. Before the 2009 swine flu reached Tennessee, McCullers and colleagues tested the blood of 116 St. Jude employees and their spouses who were 55 and older, including 46 people vaccinated in 1976.
It was not possible to follow the employees and find out if they were less likely to catch the 2009 H1N1 swine flu if they had been vaccinated in 1976, because all were vaccinated against the 2009 virus as soon as a vaccine was available.
But tests of their blood indicated they were protected.
There are two lessons, McCullers said. "It turns out the 1976 vaccine wasn't so bad after all," he said.
Health officials need to think about how vaccine effectiveness is measured. As it is not ethical to vaccinate people and then expose them to an infection, researchers usually just measure whether a vaccine makes the body produce antibodies.
But 97 percent of the St. Jude medical staff produced antibodies against the 2009 H1N1 virus, McCullers said. For most, "these antibodies were not terribly functional," he said.
Only the people who had received the 1976 vaccine had antibodies that actually neutralized the 2009 virus, he said.