Older Adults May Have Some Immunity to Swine Flu

THURSDAY May 21, 2009 -- Some older people may have partial immunity to the new H1N1 swine flu virus because of possible exposure to another H1N1 flu strain circulating prior to 1957, a U.S. infectious-disease expert said Wednesday.

"The further back you go in time, the more likely you are to have been exposed to H1N1 virus back before 1957, and there is a possibility that having exposure to that virus many years ago may allow you to have some [antibody] reaction to the new H1N1 that's now circulating," Dr. Daniel Jernigan, deputy director of CDC's Influenza Division, said during a teleconference.

That may explain why the new swine flu outbreak is striking a disproportionately large number of children and young adults. The regular seasonal flu typically takes the biggest toll among the very young and the elderly.

The current H1N1 virus is a genetic cousin of the more virulent H1N1 "Spanish flu" virus of 1918 that killed 20 million to 50 million people worldwide, and up to 500,000 in the United states. Seasonal versions of this virus circulated throughout the United States until it was replaced in 1957 by the H2N2 "Asian flu" pandemic virus, which caused 70,000 deaths in the United States.

In 1977, the H1N1 "Russian flu" virus emerged, but people exposed to H1N1 before 1957 were largely immune to this strain, according to the U.S. National Institute of Allergy and Infectious Diseases.

Jernigan said studies have found evidence of H1N1 antibody activity in blood from older people. "We can infer from that, to some degree, that there is some level of protection," he said.

But he added a note of caution, saying that many years have passed and the new virus, although the same subtype, is different from the H1N1 seasonal flu virus circulating before 1957.

Jernigan also disagreed with an assessment Tuesday by the World Health Organization's director-general, Dr. Margaret Chan, who said production of an H1N1 swine flu vaccine could not begin until mid-July at the earliest, weeks later than previous estimates.

Chan's reasoning: swine flu virus isn't growing very fast in laboratories, making it hard for scientists to get the key ingredient they need for a vaccine -- the "seed stock" from the virus. And she said it would then take months before a vaccine would be available, the Associated Press reported.

However, Jernigan said that in the United States, the steps needed to produce a swine flu vaccine are moving ahead on schedule, and a vaccine should be ready by the fall. "We are moving along and have not had significant delays here in the U.S. with the development of the vaccine candidates," he said.

Jernigan noted that creating a vaccine for the new H1N1 flu is a complicated process. He also suggested making the regular, seasonal flu vaccine available earlier to Americans than its usual late September or early October introduction.

"At this point when to vaccinate [with the new H1N1 vaccine] is going to be driven largely by when it's available," Jernigan said. "If possible we want to have an earlier roll-out of the seasonal influenza vaccine, to make it easier for an additional vaccine if that's the ultimate policy," he said.

"For that reason we will be working closely with manufacturers and with multiple advisory committees through the federal government and with other partners to make sure that what is recommended in terms of timing is feasible and can be initiated, to offer the most protection to the most folks," Jernigan said.

In the United States, most cases of swine flu continue to be no worse than seasonal flu, health officials said. Testing has also found that the swine flu virus remains susceptible to two common antiviral drugs, Tamiflu and Relenza, according to the CDC.

The new swine flu is a highly unusual mix of swine, bird and human flu viruses. Experts worry that, if the new flu virus mutates, people would have limited immunity to fight the infection.

The CDC is concerned with what will happen as the H1N1 virus moves into the Southern Hemisphere, where the flu season is about to start. The agency is also preparing for the virus' likely return in the fall to the Northern Hemisphere.

On Tuesday, U.S. health officials said that people hospitalized with the swine flu who have underlying health problems fare worse than otherwise healthy people who also have been hospitalized. This buttresses the belief that the swine flu is no more dangerous than regular flu, the officials said.

On Wednesday, the CDC was reporting 5,711 U.S. cases of swine flu in 48 states, including 10 deaths. The three latest deaths include a 44-year-old man in Missouri, the state's first such fatality, a 57-year-old woman in Arizona, that state's second confirmed swine flu death, and a 21-year-old man in Utah who was overweight and had underlying medical conditions, according to the AP.

The World Health Organization on Thursday was reporting 11,034 diagnosed cases in 41 countries, including 85 deaths, mostly in Mexico, believed to be the source of the outbreak.

U.S. Human Cases of H1N1 Flu Infection
(As of May 20, 2009, 11:00 AM ET)
States # of
confirmed and
probable cases
Deaths
Alabama
64
 
Arkansas
3
 
Arizona
488
2 deaths
California
553
 
Colorado
55
 
Connecticut
59
 
Delaware
88
 
Florida
122
 
Georgia
25
 
Hawaii
26
 
Idaho
8
 
Illinois
794
 
Indiana
105
 
Iowa
71
 
Kansas
34
 
Kentucky**
20
 
Louisiana
73
 
Maine
9
 
Maryland
39
 
Massachusetts
175
 
Michigan
171
 
Minnesota
39
 
Mississippi
5
 
Missouri
20
1 deaths
Montana
9
 
Nebraska
28
 
Nevada
33
 
New Hampshire
22
 
New Jersey
22
 
New Mexico
68
 
New York
284
1 deaths
North Carolina
12
 
North Dakota
5
 
Ohio
13
 
Oklahoma
43
 
Oregon
94
 
Pennsylvania
55
 
Rhode Island
8
 
South Carolina
36
 
South Dakota
4
 
Tennessee
86
 
Texas
556
3 deaths
Utah
72
 
Vermont
1
 
Virginia
23
 
Washington
411
1 death
Washington, D.C.
13
 
Wisconsin
766
 
TOTAL*(48)
5,710 cases
8 deaths
*includes the District of Columbia
**One case is resident of Ky. but hospitalized in Ga.Source: U.S. Centers for Disease Control and Prevention

More information

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

Posted: May 2009


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