All statistical analyses were performed using Stata 12.0 (StataCorp, College Station, TX, USA) statistical software. The study was conducted according to Ethical Principles for Medical Research Involving Human Participants of the World Medical Association, the Declaration of Helsinki, and the International Ethical Guidelines for Epidemiological Studies. The Ethic Research

Committee of the Directorate of Public Health and Public Health Research Center of Valencia approved the study protocol and provided the exemption from obtaining individual informed consent to obtain and merge individual data from the different registries. Overall, 438,024 adults aged 65 years and older on 1 October 2011 were vaccinated against influenza during the 2011–2012 season (51% of MEK inhibitor the total population ≥65 years PI3K inhibitor old in Valencia region). We excluded 252,372 who resided outside the nine HSAs under study, 5593 that were institutionalized, and 16,038 who had received a different vaccine to those being compared. This left 164,021 (19% of the total population ≥65 years old in Valencia region) subjects for the analysis (Fig. 1). The cohort mean age was 76.7 (standard deviation: 7.2) years, and 55.3% were female. A total of 49.7% of cohort members were recorded as suffering from “chronic cardio-respiratory conditions” in the Vaccine Information

System database, but only 8% were on chronic cardiovascular and respiratory medication. A total of 62,058 (37.8%) people were vaccinated with virosomal-TIV and 101,963 (62.2%) were vaccinated with intradermal-TIV (Fig. 1, Table 1). The age and sex distribution of patients vaccinated with each vaccine were similar (Table 1). Subjects vaccinated with virosomal-TIV were more likely to be reported as belonging to the “cardio-respiratory risk group” (59.3% for virosomal versus 43.8% for intradermal TIV; P < .001). However, pharmaceutical claim distributions were similar between both groups of vaccinees ( Table 1). During the time influenza

was circulating in the community, we identified 127 hospitalizations related to Tryptophan synthase influenza among subjects vaccinated with virosomal-TIV, out of 914,740 total person-weeks at risk. We also identified 133 hospitalizations related to influenza among subjects vaccinated with intradermal-TIV, out of 1,504,570 total person-weeks at risk (Fig. 1, Table 2). From the total of 260 cases, 241 were identified through the VAHNSI scheme, 12 were reported to the Microbiological Surveillance Network (RedMIVA) and 15 (0.6%) patients were ascertained from the CMBD because of a discharge diagnosis for influenza (ICD9-CM 487–488.89), seven of these (five virosomal-TIV and two intradermal-TIV vaccinees) lacked a laboratory result for the confirmation of influenza virus infection. The most frequent primary diagnosis among those with a positive laboratory result for influenza was chronic obstructive pulmonary disease (COPD) (24.5%), followed by pneumonia (21.3%). A total of 24.