THURSDAY, Oct. 8, 2009 (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.
Only about 150 to 250 people a day have been going to emergency rooms in New York City complaining of flu-like symptoms. Attendance in the city's public schools was 91 percent on Wednesday. Compare that to last spring, when 60 city schools closed and some 18 percent of students were absent, according to the Times.
Dr. Anita Barry, director of the infectious disease bureau of the Boston Public Health Commission, told the Times, that although 11 percent of teens in her city got swine flu in the spring, public schools and college health services have reported very little flu this fall.
In addition, Seattle, Connecticut and Utah -- where there was also a lot of swine flu in the spring -- are seeing less now, Donald R. Olson, research director for the International Society for Disease Surveillance, told the Times.
Another expert said this phenomenon isn't unexpected.
"As a result of last spring's outbreak in certain areas of the country, one may see a pattern of uneven geographic distribution of cases in the U.S. this fall, depending on what occurred in specific geographic areas last spring. There may be few cases in some areas and a significant number of cases in others," said Dr. Pascal J. Imperato, dean of the School of Public Health at the State University of New York Downstate Medical Center.
"The majority of cases, as was the case last spring, will probably be in children and young adults and will be clinically mild in nature," he added.
However, even in places where the swine flu appears to be less extensive than before, it is still important for people to get the H1N1 vaccine, Siegel said.
"This is even more of an argument for the vaccine," Seigel contended. "If people are immune based on previous outbreaks, imagine how good we can do with vaccinating the country. It seems that immunity is playing a pretty big role in areas where it spread previously, and if we vaccinate people we will really ante up on that."
This phenomenon has been seen before in other pandemic flu, Siegel said. "That was true of the 1918 flu," he said. "Areas that got heavy-hit in the spring didn't get hit again in the fall," he said.
Although areas hit hardest in the spring are seeing less-than-expected cases of flu, other parts of the country, where the flu was not as extensive, are seeing severe outbreaks, particularly in the Southeast, according to the U.S. Centers for Disease Control and Prevention.
For example, in Georgia since September there have been 81 hospitalizations and eight deaths from H1N1, according to the Georgia Department of Community Health, compared with 44 hospitalizations and one death from April through late July.
In Arkansas, seven pregnant women were in hospitals on respirators, the Times reported.
In addition, about half the children seen in the emergency room at Arkansas Children's Hospital in Little Rock have flu-like symptoms, which is almost double the usual number for this time of year Craig H. Gilliam, the hospital's director of infection control, told the Times.
More information
For more information on H1N1 swine flu, visit the U.S. Centers for Disease Control and Prevention.
