We inhibitor price chose calendar time from 1 October 2009 as the time scale and we considered vaccination as a time varying covariate. Calendar time was chosen as the time scale for analysis to control for potential seasonal effects in incidences of the outcomes under study. In a second model we conditioned further on the number of in-hospital admissions and visits to specialist care one year before vaccination. The results are presented as hazard ratios with 95% confidence intervals, with 0.05 considered as the level of statistical significance. We estimated the number of cases attributable to vaccination in the vaccinated group as the number of observed cases in the vaccinated group multiplied by 1?1/hazard ratio. We estimated the absolute excess risk among vaccinated people as the number of cases attributable to vaccination divided by the number of vaccinated people.

The hazard ratios related to vaccination were stratified on time since vaccination and calendar time of vaccination. We used six weeks as a cut-off point for stratification according to time since vaccination. The early phase of the vaccination campaign was defined by vaccination within 45 days of 1 October 2009 and the late phase by vaccination after that period. The vaccinations started on 13 October (except for 10 people who had been vaccinated earlier); the cut-off point used includes the first 32 days of the vaccination campaign (43.6% of all vaccinated people) in the early phase. To investigate whether there was an acute effect of the vaccination (that is, non-proportional hazards on the time scale time since vaccination), we analysed estimates stratified according to both times (see table 4).

We used likelihood ratio tests to determine the interactions between calendar time of vaccination and time since vaccination. Post hoc analyses In an attempt to further estimate the influence of underlying comorbidity on health outcomes in patients undergoing H1N1 vaccination, we also estimated short term mortality according to vaccination status. Results By 31 March 2010 virtually all vaccination activity had been completed, with a cumulative 1024019 people vaccinated (52.6% of the study population; figurefigure).). In all, 222388 people, of whom 66.4% received a second dose, were vaccinated between the ages of six months and 12 years. Those belonging to a high risk group were largely over-represented compared with the total population during the first six weeks of the campaign (mid-October to November 2009); 74% of all those Drug_discovery vaccinated in the first week and 30% in the sixth week. In a second phase (from the beginning of December) the remainder of the population was offered vaccination.