No information was available for contacts of the remaining 18 students. Those who were reported by students to have developed influenza-like symptoms a few days after the students arrived home were asked to provide diagnostic samples. Four contacts of three confirmed cases reported ILI and all were tested. Only one of these contacts tested positive

for influenza A(H1N1). Thus, the secondary attack rate was 0.5% (1/188) for confirmed influenza and 2.1% (4/188) for probable influenza. Of the 188 contacts, 137 were contacts of students with symptoms, and 4 (2.9%) of these reported illness. The 39 students with Quizartinib confirmed influenza had 98 contacts so, for this group, the secondary attack rate for confirmed influenza was 1% (1/98), and the secondary attack rate for probable influenza was 4% (4/98). All patients seen were advised about preventive measures to avoid further transmission: the use of masks, home isolation, and hand washing. No student received prophylaxis or antiviral FG-4592 in vitro treatment with oseltamivir, as all had mild illness

and none had underlying diseases or risk factors for severe illness. The symptoms resolved without specific treatment after a mean of 4–5 days. All confirmed and probable cases recovered satisfactorily and none required hospitalization. Only seven students had been vaccinated with the seasonal influenza vaccine before the

trip, of whom four developed laboratory-confirmed A(H1N1) 2009 pandemic influenza. Figure 3 shows the aircraft seats occupied by Cediranib (AZD2171) the 113 students on the return flight from the Dominican Republic. Students who became ill were seated throughout the aircraft with no apparent clustering. We were not able to obtain information on illness among other passengers who shared the return flight. The viral nucleotide sequences obtained from infected students were indistinguishable from sequences of viruses in GenBank from the Dominican Republic. However, the sequences were also indistinguishable from other viral sequences identified in Spain. We describe here a high primary attack rate of pandemic A(H1N1) influenza among a group of medical students who traveled to the Dominican Republic during a period of epidemic influenza transmission, followed by a low secondary attack rate among household contacts after the students’ return to Spain. The relatively mild clinical presentation of pandemic A(H1N1) influenza in this group of students is consistent with previously described outbreaks.6 However, we found a higher frequency of gastrointestinal symptoms than reported in previous studies in cases of influenza,6,13 but the high percentage of gastrointestinal symptoms in students with (64%) and without (43.2%) ILI suggests the possible coexistence of travelers’ diarrhea in this group.