Swine Flu Outbreak May Be Subsiding, CDC Says

May 22 -- THURSDAY, May 21 (HealthDay News) -- While many states are still reporting new cases of infection with the H1N1 swine flu virus, there seems to be an overall decline in visits to doctors and hospitals by people with the disease, indicating that the outbreak might be subsiding, a U.S. health official said Thursday.

"At the national level, we're seeing that the percent of visits [to doctors and hospitals] for influenza-like illness is starting to turn down," Dr. Anne Schuchat, the Centers for Disease Control and Prevention's interim deputy director for science and public health program, said during an afternoon teleconference. "That's a good sign. It's consistent with the idea that the worst may be over."

"We are seeing activity decline in some areas, but we are seeing increased or localized outbreaks in other areas," she added.

Schuchat also discussed the finding, first reported Wednesday, that 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.

The finding "suggests that older adults might have some preexisting antibody against this new H1N1 strain," Schuchat said. "But we don't know yet what that will mean in terms of actual immunity or clinical protection."

So far, 64 percent of cases of swine flu infection are among people aged 5 to 24, while only 1 percent involves people over 65, she said.

On Wednesday, Dr. Daniel Jernigan, deputy director of the CDC's Influenza Division, said that 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.

"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," he said.

That could 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, 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 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.

He also suggested the possibility of making the regular seasonal flu vaccine available earlier to Americans than its usual late September or early October introduction this year, to be followed by a second vaccine for swine flu, if needed.

"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.

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 that, as the H1N1 virus moves into the Southern Hemisphere, where the flu season is about to start, it could mutate and return in a more virulent form in the Northern Hemisphere next fall.

On Thursday, the CDC was reporting 5,764 U.S. cases of swine flu in 48 states, including nine deaths, although health officials said the death toll could be as high as 10.

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.

(As of May 21, 2009, 11:00 AM ET)

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