H1N1 flu update: '76 flu vaccine provides some protection

An interesting article from Scientific American reports that people who were vaccinated with the 1976 flu shot have some immunity to the H1N1 (swine) flu that began spreading last spring.

As soon as the newest H1N1 virus burst onto the scene in the spring, it conspicuously assaulted the young and left the old mostly unscathed. To date, 79 percent of confirmed U.S. cases have been in people younger than 30 years and only 2 percent in people older than 65. In light of that lopsided attack pattern, investigators at the Centers for Disease Control and Prevention quickly started testing hun­dreds of human serum samples stored between 1880 and 2000, looking for evidence of past human experience with the novel H1N1 virus.

Data published in May showed a power­ful antibody response to the new virus in a third of the samples from subjects older than 60 and in a smaller number (6 to 9 percent) of samples from younger adults. The authors theorized that exposure to post-1918 H1N1 human flu viruses had primed the oldest subjects’ immune system to recognize the novel H1N1.

The CDC group procured serum samples collected from 83 adults and a handful of children who had received the vaccine against swine H1N1 that was given in 1976 to 43 million Americans. More than half of the samples from adults who received a single shot of that vaccine displayed a powerful immune response to the 2009 H1N1 virus, whereas little recognition of the new virus was seen in the serum of inoculated children, all younger than four at the time.

The discrepancy was an important clue, according to senior author Jackie Katz of the CDC’s influenza division, who published those particular findings in September. The adults, who were between 25 and 60 years old in 1976, would have been exposed to H1N1 flu before 1957, the year it stopped circulating for the next two decades. “We assume that by the age of five a person would have had at least one exposure to influenza,” Katz explains. That prior encounter with H1N1 seemed to be the key to a robust recognition of the 1976 vaccine virus, just as having had the 1976 vaccine seems to produce a strong response to the 2009 H1N1 virus. The very young children, in contrast, represent the responses of immune systems that have no past history with H1N1.

Katz cautions that high antibody levels in serum do not guarantee immunity from infection, but they serve as good indicators of protection when testing vaccines and are a fairly sure sign of earlier exposure to the pathogen. For people with some measure of previous immunity, a subsequent vaccine could act as a “booster shot.”

The researchers also found that over time and repeated exposure to flu viruses, people appear to build up a general immunity to influenza.

Analyses of infection rates in modern seasonal flu epidemics suggest that with age comes a subtle buildup of immunity to flu viruses in general. Although the external viral proteins hemagglutinin and neur­a­minidase (the H and N that designate a flu strain) are the main targets of vaccines, the human immune system may also recognize other viral parts. The resulting responses may not prevent infection, but they may reduce symptoms to a degree that people do not even realize they are infected.

Indeed, the seasonal flu peaks in kids and “then sort of declines with age,” says Jeffery Taubenberger, a virus expert at the National Institute of Allergy and Infectious Diseases. “The elderly have the highest mortality because they often have underlying conditions,” he adds, “but you find that people in their 40s and 50s get a lot less clinical flu than kids, so one possibility is that there’s a slow accrual of a wide variety of flu immunity.”