While concern over the spread of the H1N1 virus sweeps the country, epidemiologists in New York and a few other cities that were awash in swine flu last spring are detecting very little evidence of a resurgence.
Although flu season will not peak until the weather gets cold, in New York, which was the nation’s hardest-hit city, officials say that flu activity is no higher than it normally is at this time of year and that school attendance is normal.
Last week, Dr. Anne Schuchat, the director of immunization at the federal Centers for Disease Control and Prevention, said, “Most states do have quite a lot of disease right now, and that’s unusual for this time of year.”
But public health officials say there appears to be a pattern of areas that had big outbreaks in the spring, like New York, Boston and Philadelphia, seeing less swine flu now.
New York City health officials now believe that while only 10 percent to 20 percent of New Yorkers were reported ill with flu last spring, as many as 20 percent to 40 percent may have been exposed to the disease and developed immunity that has prevented it from spreading.
Although it is too early to be sure, they said, the high level of immunity may mean that the second wave of swine flu infection ends up being far less extensive than expected.
The immunity theory has gained enough credence that Dr. Thomas A. Farley, New York City’s health commissioner, put it forward at a conference on the national preparations for H1N1 last Friday in New York, led by Kathleen Sebelius, the health and human services secretary, and Dr. Thomas R. Frieden, director of the disease control centers.
“We’re not seeing illness in the city right now,” Dr. Farley said at one session. “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.”
Officials say the conflicting data show the delicate balance public health officials are walking with swine flu. So far it has turned out to be less deadly than it seemed when a pattern of deaths was reported in Mexico last spring.
At the same time, officials fear that it could take a turn for the worse, and they want to maintain a high level of alertness without crying wolf too many times.
Dr. Martin S. Cetron, a flu expert at the disease control agency and the co-author of a 2007 study of how the 1918 flu hit 43 American cities, called the idea that flu is not big now because it was big in the spring “an interesting hypothesis, with biological plausibility,” but said that only the rest of the winter would tell.
“To say, Oh, all of us in New York are immune, we won’t have any more disease and we don’t need to take vaccine, is a dangerous conclusion to draw,” Dr. Cetron said.
New York City public health officials are in fact conducting an extensive immunization campaign, and they agree that it is far too early to draw any final conclusions.
“It’s like Sherlock Holmes looking at the evidence and saying, ‘Hmmm, that’s a plausible solution,’ ” Dr. Don Weiss, an epidemiologist with New York City’s health department, said Wednesday. “This sort of fits with what we’re seeing, but there could be other explanations.”
Still, Dr. Weiss added, “The theory that everybody’s talking about is that maybe because New York had such a bad outbreak in the spring, it won’t be so bad in the fall.”
The amount of immunity in a population, called the herd immunity, “will tell you whether or not you will have an outbreak,” Dr. Weiss said.
For highly infectious diseases, like measles, it is generally accepted that 90 percent to 95 percent of the population has to be immunized to prevent an outbreak, he said.
For flu, a virus that is constantly changing year to year, it is less clear what the herd immunity has to be to prevent a further outbreak, but it may be as little as half, and New York may be very close to that, Dr. Weiss said.
Since September, only about 150 to 250 people a day have been going to New York City emergency rooms complaining of flu-like symptoms, officials said. The presumption in New York and elsewhere is that most flu cases this time of year are swine flu, because it is still early in the year for so-called garden-variety flu.
Attendance in the New York City’s public school system, with just over a million students, was 91 percent Wednesday. Last spring, when the virus was rampant, nearly 60 schools were closed and about 18 percent of students were absent.
In Boston, where an estimated 11 percent of adolescents got swine flu in the spring, public schools and college health services have reported very little flu activity this fall, Dr. Anita Barry, director of the infectious disease bureau of the Boston Public Health Commission, said Wednesday.
But Dr. Barry said she was reluctant to draw any conclusions so early in the season, without taking blood samples to test for immunity.
Seattle, Connecticut and Utah also had lots of swine flu in the spring, but appear to have less now, said Donald R. Olson, research director for the International Society for Disease Surveillance.
Some states, including Georgia, Indiana and North Carolina, had “false waves” of swine flu in the spring, Mr. Olson said, which seemed to have been caused by the “worried well” flocking to hospitals.
“Of the places that had ‘real waves’ last spring,” he said, “none have really taken off.”
Georgia in particular took off when schools reopened in August. In the last week of September, there were 81 hospitalizations and eight deaths from H1N1 in the state, according to the Georgia Department of Community Health, compared with 44 hospitalizations and one death in the three-months from late April through late July.
As of Monday, seven pregnant women were on respirators in Arkansas hospitals, officials said.
“We’re hearing from doctors and clinics across the state that they’re swamped,” said Ann Wright, a spokeswoman for the Arkansas Department of Health.
Since August, about half the children seen in the emergency room at Arkansas Children’s Hospital in Little Rock have complained of flu, about double the usual number for this time of year, said Craig H. Gilliam, the hospital’s director of infection control.
Mr. Gilliam said the hospital had a record number of emergency room visits on Sept. 15, which he attributed to the flu.