The observed frequencies of seeding events between clusters were compared to expected frequencies based on the prior probability of randomly choosing a sequence from a given geographical zone in the past. Using the binomial distribution with the proportion of prior NCBI sequences as a binomial probability, a p-value was calculated for observing more seeding events than expected. The best predictor of a seeding region for each season had the greatest ratio of observed to expected seeding events with a p-value smaller than 0.05 ( Figure 2 ). Figure S4 Rankings of significant seeding and seeded climate zones for H3N2 and H1N1 using different graph theory metrics. (A) The indegree and outdegree of a node represent the total number of seeding events into and out of a region, respectively. Local seeding events depicted in gray play little role in overall seeding except in the tropics. (B) Degree flow measures the difference between seeding events out of and into a node and determines whether it is a terminal sink or source. (C) PageRank uses an algorithm similar to that employed by Google to categorize nodes based on the number and quality of links pointing to that node. Found at: doi:10.1371/journal.pcbi.1001005.s004 (0.88 MB EPS) Figure S5 Rankings of significant seeding and seeded continents for H3N2 using different graph theory metrics. (A) The indegree and outdegree of a node represent the total number of seeding events into and out of a region, respectively. Local seeding events depicted in gray play little role in overall seeding except in Asia. (B) Degree flow measures the difference between seeding events out of and into a node and determines whether it is a terminal sink or source. (C) PageRank uses an algorithm similar to that employed by Google to categorize nodes based on the number and quality of links pointing to that node. (D) Betweenness measures the number of shortest paths in a network passing through a given node. Text S1 Detailed description of the methodology, including evaluation of clustering, determining flu seasons, timing of observed seeding events, and network randomization. Detailed description of the methodology, including evaluation of clustering, determining flu seasons, timing of observed seeding events, and network randomization. Found at: doi:10.1371/journal.pcbi.1001005.s009 (0.02 MB DOCX) Willingness to accept H1N1 pandemic influenza vaccine: A cross-sectional study of Hong Kong community nurses BACKGROUND: The 2009 pandemic of influenza A (H1N1) infection has alerted many governments to make preparedness plan to control the spread of influenza A (H1N1) infection. Vaccination for influenza is one of the most important primary preventative measures to reduce the disease burden. Our study aims to assess the willingness of nurses who work for the community nursing service (CNS) in Hong Kong on their acceptance of influenza A (H1N1) influenza vaccination. METHODS: 401 questionnaires were posted from June 24, 2009 to June 30, 2009 to community nurses with 67% response rate. Results of the 267 respondents on their willingness to accept influenza A (H1N1) vaccine were analyzed. RESULTS: Twenty-seven percent of respondents were willing to accept influenza vaccination if vaccines were available. Having been vaccinated for seasonable influenza in the previous 12 months were significantly independently associated with their willingness to accept influenza A (H1N1) vaccination (OR = 4.03; 95% CI: 2.03-7.98). CONCLUSIONS: Similar to previous findings conducted in hospital healthcare workers and nurses, we confirmed that the willingness of community nurses to accept influenza A (H1N1) vaccination is low. Future studies that evaluate interventions to address nurses' specific concerns or interventions that aim to raise the awareness among nurses on the importance of influenza A (H1N1) vaccination to protect vulnerable patient populations is needed. The 2009 pandemic of influenza A (H1N1) infection has alerted many governments to make preparedness plan to control the spread of influenza A (H1N1) infection. With evidence on the effectiveness of vaccination in the control and prevention of seasonal influenza [1, 2] , vaccination for pandemic influenza is one of the most important primary preventative measures to reduce the disease burden associated with influenza A (H1N1) infection [3] .