Originally published in MedPage Today
by Todd Neale, MedPage Today Staff Writer
Pandemic H1N1 influenza behaved much like seasonal and previous pandemic flu viruses in an early, isolated outbreak at a New York City high school, researchers found.
Natural history and transmissibility characteristics were similar in an April outbreak that sickened about 800 students and staff members, according to Justin Lessler, PhD, of Johns Hopkins Bloomberg School of Public Health, and colleagues from the New York City Department of Health and Mental Hygiene.
The reproductive number — the average number of secondary cases of disease generated by a typical primary case in a susceptible population — was slightly higher than seen with previous viruses, they reported in the Dec. 31 issue of the New England Journal of Medicine.
But “it’s sort of accepted that transmission is much more efficient within an environment like a school than it would be in the general community,” Lessler told MedPage Today.
By the third week of April, there were hints that a new H1N1 influenza virus was emerging, prompting the CDC to brief state and local health officials on the possibility on April 23.
Later that day, the school nurse at a high school in the borough of Queens called the city health department saying about 100 students had been sent home with flu-like symptoms.
The health department began an intense investigation and the school was ultimately shut down from April 25 to May 3.
Although Lessler wouldn’t name the school, it’s likely that this was the outbreak that occurred at St. Francis Preparatory School. (See Possible Swine Flu Identified in New York)
According to Lessler and his colleagues, between April 24 and May 8, infection with the new H1N1 strain was confirmed in 124 students and staff members. However, not all people with symptoms were able to be tested.
Of those reached by phone, only two had to be hospitalized — a 14-year-old who had received a diagnosis of a viral syndrome and a 17-year-old who had been hospitalized after an episode of syncope. Both were discharged after one day.
Among those who filled out an online questionnaire, 35% of the students and 10% of the staff reported having flu-like symptoms during the outbreak.
No severe symptoms were identified in anybody associated with the school.
The attack rate was significantly higher among respondents younger than 60 than in older individuals (11% versus 0%, P=0.04).
A possible association with travel to Mexico was identified: 14 students had traveled to the country — five reported flu-like symptoms, one of whom had confirmed infection with the new virus.
Estimated median incubation period for confirmed infection (1.4 days) and estimated median generation time (2.7 days) were both similar to seasonal and previous pandemic viruses, Lessler said.
Three-quarters of those with flu-like symptoms recovered within nine days of onset.
The estimated within-school reproductive number was 3.3, assuming that the number of incident cases of flu-like illness doubled every 1.3 days.
This number is high compared with pandemic and interpandemic influenza outbreaks and compared with estimates of 2009 H1N1 influenza transmission in Mexico, Lessler and his colleagues noted in the journal.
“Very efficient transmission within the school suggests that schools are very much a driver of the transmission of this virus and that seems to have been borne out when the fall wave of the epidemic coincided with the start of the school term,” Lessler told MedPage Today.
The estimated secondary attack rate of flu-like illness in households with at least one confirmed infection was 17.7%, which is “somewhat lower than had previously been seen,” he said.
Although H1N1 flu appears to be similar to seasonal viruses, “this should not make us complacent about the potential impact of 2009 H1N1 influenza,” Lessler and his colleagues wrote.
“The reproductive number, incubation period, and generation time that were seen in the influenza pandemic of 1918 were also similar to those seen in interpandemic periods, and the fact that a large number of persons are probably susceptible to the 2009 H1N1 virus could mean that there will be substantially more cases than are seen in a seasonal epidemic.”
They cautioned that “even modest increases in the rate of transmission or the severity of disease over the levels seen with interpandemic influenza strains could have a substantial impact on public health.”
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