How Did Swine Flu Spread?

When a team of medical detectives from New York City's health department first arrived at a Queens school stricken by swine flu on April 24, the situation looked bad.
    
In a matter of days, nearly 660 kids at the St. Francis Preparatory School had developed fever and a wicked cough. A third of the student body was sick. Two dozen teachers and staff also fell ill. Students brought the illness home, and hundreds of their relatives got sick too.
    
Within days, 1,000 people connected to the school had flu symptoms. The virus appeared poised to break out and infect millions.
    
But then, the bug took a break.
    
Only a handful of new swine flu cases have been reported in the city since April 26. Among those who did become ill, almost all had either been to Mexico recently or had a relative at St. Francis Prep.
    
New York City seems like a perfect lab for cultivating the flu virus, with more than 8 million people packed together on subways and sidewalks. It seems remarkable, then, that a flu so infectious in one school didn't spread quickly across all five boroughs. Health officials are still trying to figure out why.
    
"That is one of the great mysteries of influenza,'' said Dr. Don Weiss, director of surveillance in the health department's bureau of communicable disease.
    
Sometimes flu picks up steam and races through a population, he said. Other times, outbreaks lose their punch as the virus is passed to its second and third generation of victims.
    
Just why the illness spread at St. Francis Prep, but not elsewhere, is likely to be unclear for some time.
    
Weiss said public health researchers are trying to answer a variety of questions: Was the teeming St. Francis campus, with nearly 2,700 kids, simply ripe for an outbreak? Did the school's ventilation system play a role? Or is the virus, perhaps, less contagious as time goes on?
    
"Now, we start to dig deeper,'' he said.
    
Some experts said if the number of new cases remains low, the city may have to wait until next flu season to learn whether the virus is any more dangerous than other strains.
    
"What these guys really want to know is, how many people will be infected three, six, nine days from now?'' said Peter Palese, a microbiologist and flu expert at Mount Sinai Medical School. His prediction: "It will peter out in our area in the next week or two, and then linger on in the Southern Hemisphere, and then come back next year.''
    
City public health officials seem to agree. They have continued to monitor the spread of the disease, but their advice to physicians over the past week has been telling.
    
Patients complaining of flu-like symptoms don't need to be tested for the new virus unless they are very ill, they say. Doctors shouldn't prescribe antivirals either, unless patients are very sick or have underlying illnesses that could make the flu more dangerous.
    
Still, the investigation will continue.
    
New York City's attempt to assess the danger began with a visit to the school April 24, the day after a nurse phoned in alarm about scores of students going home sick.
    
The Department of Health team took samples from a handful of sick kids and canvassed local doctors' offices for more. By the next afternoon, a lab had confirmed that at least eight of them had a new type of influenza -- almost certainly swine flu, though it would take slightly longer for the U.S. Centers for Disease Control to confirm the result.
    
The city's massive public health agency, one of the world's largest, with a $1.7 billion budget, began an all-out press to learn more about how quickly the disease was spreading.
    
"Our first question was, is it contained within the school?'' Weiss said. "How infectious is it? How high is the secondary attack rate?''
    
Investigators were able to reach 44 of those children who initially tested positive for swine flu. They assembled a quick demographic profile of the group: Their median age was 15. Seventy percent were female. They were, on average, running a temperature of 102 degrees.
    
The city distributed influenza test kits to some physicians' offices near the high school and e-mailed the student body, asking them to complete an online medical survey.
    
From almost the start, there was good news. Most of the sick students were getting better. Some were already back to normal. None had been very seriously ill.
    
But questions remained about whether the illness could cause havoc, as any flu would if it had a high infection rate.
    
The department monitored hospital emergency rooms for spikes in visits from patients with flu-like symptoms. That data were reassuring too. The bump in cases appeared to be mild and possibly motivated by public nervousness.
    
The city's health lab was staffed 24 hours a day to process tests for the flu, and samples were regularly flown to the Centers for Disease Control in Atlanta for confirmation.
    
So far, researchers have reached several conclusions about the city's outbreak, but many questions remain.
    
It is spreading, they say, albeit slowly. As the weekend began, the first cases emerged among people who hadn't been to Mexico recently and had no tie to St. Francis Prep. One was confirmed as swine flu. Tests are pending on two more.
    
But the transmission rate did not appear dangerously high, and the symptoms remained mild, Weiss said.
    
"A week is not enough time for us to be confident that we won't see serious illness,'' he added.
    
It also isn't enough time to understand why the illness clobbered 1,000 New Yorkers in just a few days, then went quiet.