Background:The new flu virus pandemic of 2009/2010 occurred while much of the population did not have antibody against the virus at the time. Due to the pandemic vaccine supply shortage, many failed to receive a pandemic flu vaccine, and many others received only the seasonal flu vaccine containing virus isolated before 2007.The main objective of this research was to verify the effectiveness of the flu vaccine based on observations of family members in households where a person was infected with the virus.Methods: Period of investigation: August -October 2010 after the first outbreak of the new influenza (2009/2010).Place: Baba Clinic, Osaka, Japan. Method: Interviewed the parents (Also used surveys). Participants: 562 households and their 2193 family members. Contents: Verified if the subjects were vaccinated or not (Seasonal or pandemic flu). Examined the circumstances of the infection (who was infected first, or in which order the members were infected).Results: Of the 2193 subjects, 366 (16.7%) were infected; and among the 562 households, at least one member in 226 (40.2%) households were infected with the virus. By investigating the infected subjects in different age groups, the age group to first show signs of the virus within households was mostly children between the ages of 7 to 15 and then followed by ages 1 to 6 then ages above 16. Out of all the 562 families, the infection rates of the households with at least one (pandemic) vaccinated family member were 28%; compared to 40% for non-vaccinated households. Furthermore, none of the family members in 54 (72%) of 75 households that received the pandemic vaccine showed signs of the flu during the outbreak.Conclusion: 1. Children infected with the disease at school transmitted the virus to other family members and contributed to the spread of the virus at a regional level.2.There were fewer cases of household transmission in families with at least one child vaccinated with the pandemic flu vaccine, in comparison to households with unvaccinated children. It is strongly encouraged to provide the latest vaccine for children between 7 and 15, which would benefit society by preventing further outbreaks. http://dx.