WOMELSDORF, Pa. - Infectious-disease investigators began this week the nitty-gritty phase of tracking back a flu outbreak among fourth graders: Who plays kickball with you? Who was coughing? During, say, arts and crafts, did you touch a piece of paper? Pass it? Lick it?

A classroom seating chart already showed a cluster of sick kids. But members of the investigative team, most sent here to Berks County from the U.S. Centers for Disease Control and Prevention in Atlanta, needed more definitive evidence.

The CDC has deployed dozens of teams across the country, including at the University of Delaware, to gather facts about a type of flu that was virtually unknown just six weeks ago. The dearth of information led to differing, sometimes confusing, recommendations on when to close schools.

So, part of the objective of the Berks County team, the only one in the state, is to examine the impact of a one-week closure of Conrad Weiser West Elementary School - not just if it slowed the spread of disease, but to what extent it hurt families economically. Other goals are to measure how long the students remained contagious, and how the disease spread within households and around the school.

Even seasonal flu generally "hasn't been studied before to this level," said Ryan Fagan, an infectious-disease specialist who is leading the study.

Conrad Weiser West is a sprawling, gray-brick building with 456 children in kindergarten through fourth grades in semirural Womelsdorf, 16 miles west of Reading.

Absenteeism, normally 3 percent, jumped suddenly to 20 percent on May 11, a Monday, and 30 percent on Tuesday, with most parents reporting their children had influenzalike symptoms, said Superintendent Robert L. Urzillo.

After consulting with state health officials, he closed the school on Thursday. It reopened the following Thursday, May 21, with about 12 percent absent; attendance returned to normal over the next several days, and no students have been hospitalized.

Confirmed cases of H1N1 have continued to rise in the county as a whole. Yesterday the number was up to 75 - twice that of far more-populous Philadelphia and well over a third of the statewide total - although health officials noted that investigators had been actively looking for the virus here and not elsewhere. (Five schools have been closed this week in York and Lancaster Counties.)

Stephen Ostroff, director of the Pennsylvania Department of Health's Bureau of Epidemiology, told CDC officials about the Berks County outbreak around the time the school closed. They quickly asked to do an intensive investigation with the state to "answer some questions that up until that point there really hadn't been an opportunity to answer," Ostroff said.

Conrad Weiser was more rural than most other schools studied. The students were younger. And while other communities had cases both inside and outside the school, here it was largely limited to students at first.

The objectives were somewhat different as well.

In the first frenetic days, amid reports that emphasized the "swine" in swine flu, teams examined possible animal-to-human transmission, Stephen Redd, who is managing the CDC's response to H1N1, said yesterday. Then they focused on human-to-human transmission.

"In this, we have really been working to understand the effectiveness of school closure," Redd said, adding that the early recommendations were based almost entirely on archival data from the 1918 pandemic and on theoretical models.

Fagan, the Berks County team leader, got the call to fly up here the day the school was closed, and arrived the next day. All members of the team - now four, down from a peak of eight, working out of a state Health Department conference room in Reading and assisted by dozens of people in Atlanta - have experience in disease detective work but normally work elsewhere.

They called every household with children in the school and asked permission to take nose and throat samples from anyone who had experienced symptoms within the previous seven days.

Cameron Driesbach, 7, was beyond that time limit, but her sister Taylor, 5, who got sick later, tested positive. The investigators returned to their home for another swab every other day until two consecutive samples came up negative, after about 10 days.

"I thought that it was something good for us to follow through with," said the girls' mother, Lisa, adding that Taylor did not. "At the end, when the doorbell rang, she ran." Neither parent nor the other three siblings got sick.

In the conference room in Reading is a whiteboard with boxes and arrows indicating patterns of flulike symptoms based on the team's first round of interviews. Sixteen children had symptoms in one fourth-grade class and 10 in a second. But only six got sick in a third class and five in a fourth.

The team used rough data like that early on to generate hypotheses and develop a second, far more detailed, telephone questionnaire - perhaps 30 minutes per household, depending on the number of children - that would gather information to test the theories.

"If two homerooms came together and did a certain activity together, were they more likely to get sick as compared to others who did not do anything together?" said CDC investigator Achuyt Bhattarai. Why was the fourth grade disproportionately affected?

"And why did we see a wave of boys at the beginning, and then the girls?" Bhattarai said, an observation that prompted questions about specific activities that are now being asked directly of fourth-grade children in a third set of interviews.

Many of the questions in the detailed phone survey were generated from scratch by the team, guided by daily conference calls with experts, including an economist, in Atlanta.

"When you close a school," said Roodly Archer, who is handling the cost-effectiveness part of the study, "if parents have to miss work, do they get sick leave?" Without pay, they may be less supportive of the decision and their children may be more likely to congregate elsewhere, defeating part of the purpose of the closure.

Urzillo, the superintendent, said he got fewer than a dozen calls about the closure and no panic, perhaps because of regular efforts to keep parents informed.

Outsiders appeared to be more concerned, with at least two baseball games canceled and one team resorting to glove-taps instead of handshakes, said Lori Carman, head of the district athletic association.

With interviews still being conducted, no data from the epidemiology study are available. Early impressions, team members said, is that this flu seems to be behaving like most other flu.

Team leader Fagan, who wears the beige of the Public Health Service, a uniformed service that is headed by the Surgeon General and has members in various agencies, conducted several investigations during a two-year posting in Alaska. After three weeks without a day off, he hopes to wrap up work in Berks County as early as this weekend, then finish data analysis in Atlanta and return to his normal job supervising the CDC's emergency botulism response.

The findings will be combined with information from elsewhere, he said, and members of his team might publish four or five of their own papers in medical journals.

Things are back to normal at Conrad Weiser West, with one big exception: Instead of today, the last day of school will be next Thursday.

Contact staff writer Don Sapatkin at 215-854-2617 or dsapatkin@phillynews.com.