In Some Cities, Spring Swine Flu May Be Easing Fall Outbreak

Oct. 9 -- THURSDAY, Oct. 8 (HealthDay News) -- New York, Philadelphia and other cities hit hard by H1N1 swine flu last spring aren't seeing as much of it now, even though outbreaks are occurring in all 50 states.

The possible reason: Many people in these spring-outbreak epicenters have already gained some immunity to H1N1, and this "herd immunity" is keeping a wider fall outbreak at bay, experts say.

According to a report released Thursday by the New York Times, health officials in New York City, Boston and Philadelphia say they are seeing less swine flu now than they did during the initial outbreak.

"This is very much in keeping with what I am seeing here in New York," noted one flu expert, Dr. Marc Siegel, an associate professor of medicine at New York University School of Medicine.

"Not only is there not a resurgence of flu in areas where there were previous large outbreaks, but there are probably very mild cases going around that are going under the radar, because people don't even realize they are flu," he said.

Siegel agreed that widespread immunity from the spring outbreak may have led to a different kind of autumn outbreak. That's because many people exposed to the flu don't get seriously ill, but they do build an immunity to it. "There's a large percentage of patients who don't get ill -- we only focus on those who do," Seigel said.

In New York City, health officials say that while 10 percent to 20 percent of the population fell ill with the H1N1 virus in the spring, up to 40 percent of New Yorkers may have been exposed to the flu. These people may have become immune to the disease and are thus preventing it from spreading now.

This high level of immunity may make a second wave of the H1N1 much less extensive, the experts said.

"We're not seeing illness in the city right now," Dr. Thomas A. Farley, New York City's health commissioner during a flu conference last Friday led by Kathleen Sebelius, the health and human services secretary. "We're seeing essentially no disease transmitted in the city. We had 750,000 to one million sick people last spring. We were the hardest-hit city then. So we have a lot of immune people right now," the Times reported.