A cohort of 3000 adults is followed to evaluate the effectiveness of the monovalent 2009 H1N1 vaccines used 2009-2010 and seasonal influenza vaccines used 2010-2011 in preventing the first laboratory-confirmed 2009 H1N1 influenza during the influenza season 2010-11.

In the beginning of the study, changes in the background information collected in the original study (conducted 2009-2010) are asked with a questionnaire. Information on influenza vaccinations is collected with vaccination cards and monthly short message service (SMS), and verified in registers, if needed. If an epidemic caused by the 2009 H1N1 virus starts in Finland, the participants are asked to actively report symptoms of influenza like illnesses (ILI). The symptoms are also monitored by weekly SMS. In case of ILI, nasal and oral swabs are collected to verify the 2009 H1N1 influenza cases. Paired serum samples are collected at the acute on convalescence phase of the disease. The occurrence of laboratory-confirmed 2009 H1N1 influenza cases is compared between vaccinated and not (yet) vaccinated persons.

Inclusion criteria for the interview follow-up:

• Participating in the study "Evaluation of a vaccination campaign with A(H1N1)v pandemic vaccines: a prospective cohort study" (AH1N1-483-09THL)
• Assigned to use the services of Tampere health care centre;
• Written consent for the interview follow-up phase obtained by mail;
• Able to adhere to all protocol-required study procedures without any special burden or risk, as judged by the participant himself/herself

Inclusion criteria for the confirmation of 2009 H1N1 influenza cases:

• Participating in the study interview follow-up of the current study 'Evaluation the effectiveness of vaccination with 2009 H1N1 vaccines during influenza season 2010-11 (H1N1-495-10THL)
• Assigned to use the services of Tampere health care centre
• Written informed consent for the confirmation phase obtained at the first study visit
• Able to adhere to all protocol required study procedures without any special burden or risk, as judged by the investigator or designate

Exclusion criteria:

• No specific exclusion criteria will be applied

Participation in the study will be offered to all participants of the cohort study "Evaluation of a vaccination campaign with A(H1N1)v pandemic vaccines: a prospective cohort study" AH1N1-483-09THL, who still live in the study area (city of Tampere) and who have complied to the SMS or phone follow-up. The eligibility criteria of the original study AH1N1-483-09THL were: full legal competence; community-dwelling; age 18 to 75 years, written informed consent obtained; able to communicate fluently in Finnish or Swedish and able to adhere to all protocol required study procedures without any special burden or risk. Originally, 3500 persons participated in the study