Swine Flu Less Severe for Over-50s?

Daniel J. DeNoon
from WebMD — a health information Web site for patients

May 20, 2009 — People born before 1957 may be less susceptible than younger people to the H1N1 swine flu.

CDC researchers have detected antibodies in the blood of older people that neutralize the new flu bug now sweeping the nation, Daniel Jernigan, MD, MPH, deputy director of the CDC's flu division, said today in a news conference.

"We infer from that, there is some level of protection," Jernigan said. "But to prove protection, we look at the effect [the virus has] on the population, and at this point we don't have that information."

Why is 1957 a key year? Every flu season after it first appeared, the deadly 1918 pandemic H1N1 flu bug circled the globe. Each year, the virus acquired changes that made it different from the original virus. But in 1957 there was a new pandemic, this time with an H2N2 virus. The new virus took the place of the old H1N1 bug.

"And so when we talk about the pre-1957 exposures, we are referring to those exposed to the past H1N1 virus that went away in 1957," Jernigan said. "The farther back you go in time, the more likely you are to have been exposed to an H1N1 virus before 1957 -- and exposure to that virus many years ago may allow you to have some reaction to the new H1N1."

The new H1N1 swine flu bug is much different from the 1918 H1N1 virus. It's also much different from the H1N1 seasonal flu virus that still circulates. But something about that pre-1957 bug seems to have left older people with antibodies that neutralize the new flu -- and might offer some protection against it.

Swine Flu Hits Youths Hardest

Whether or not ancient antibodies are protective, many older people are getting sick from the new flu. Some of these illnesses are severe: 13% of people hospitalized with swine flu are 50 or older. And the number of H1N1 cases among older people is increasing.

But H1N1 swine flu is hitting young people hardest. More than 60% of cases are in 5- to 24-year-olds.

Remarkably -- since this is usually the healthiest age group -- 37% of people hospitalized with swine flu are 19 to 49 years old. The median age of a person hospitalized with the new flu is 19.

Those aged 5 to 18 make up 29% of swine flu hospitalizations. Because so many cases of H1N1 swine flu have been transmitted in schools, it's possible that older people only seem to be protected because they've had less contact with younger people.

In past flu pandemics, however, the same pattern has emerged: the illness tended to strike young people hardest.

Jernigan said that the CDC will soon release a detailed report on the neutralizing antibody study.

Meanwhile, the new flu continues its spread while the seasonal flu wanes. Jernigan said that nearly 80% of people who test positive for flu now turn out to have the new H1N1 bug.

SOURCES:

Daniel Jernigan, MD, PhD, deputy director, influenza division, CDC, Atlanta.