Healthcare personnel (HCP) are believed to be at increased risk of influenza infection and often work while ill, which increases the risk of secondary exposure to vulnerable patients. Vaccination of HCP against influenza is an important component of infection control in healthcare settings, but persistently low rates of vaccine uptake among HCP remains a topic of international concern and debate. Although recent reviews confirm that the seasonal influenza vaccine is moderately effective in reducing the risk of influenza illness \[10\], multiple gaps in knowledge remain regarding the preventive value of vaccine among HCP.

During at least two consecutive influenza seasons or years, this prospective cohort study of healthcare personnel in middle-income countries has four primary objectives: (1) Describe the frequency and impact of acute respiratory illnesses (ARI) among HCP during wintertime; (2) Estimate the effectiveness of influenza vaccine in preventing symptomatic influenza illness, missed work due to influenza illness, and hours of direct patient care provided by HCP with symptomatic influenza infections; (3) Examine the association between repeated influenza vaccination and HCP's baseline immune landscape, their response to influenza vaccines, and their subsequent protection against infection; (4) Examine if influenza vaccine modifies symptom severity and duration among HCP with breakthrough influenza infections despite vaccination.

Approximately 2,400-2,800 HCP providing direct patient care will be enrolled in a prospective cohort to be followed for at least two years (or two influenza seasons). In Israel, approximately 1,200 HCPs will be enrolled in Year 1 of the study, with an expansion to full enrollment in Year 2. Thus, the investigators anticipate a 3 year cohort study in which \~1,200 participants will contribute to years 1 and 2 of the study and the remaining sample will contribute to years 2 and 3 of the study. Information on socio-demographic characteristics, current medical conditions, medical history, medical care utilization, and influenza vaccination history will be extracted from medical and employee records with the participant's permission. Other information on socio-demographics, occupational responsibilities, health status, and knowledge and attitudes about viruses and vaccines will be collected by self-report through an enrollment survey. Collection of blood specimens will occur prior to and after influenza seasons during each study year; in addition, HCP who receive the influenza vaccine will provide an additional blood draw 21-42 days after vaccination. Active surveillance to identify acute illnesses (associated with cough, runny nose, body aches, or feverishness) will occur twice-weekly during weeks of local influenza circulation. When an acute illness is identified, participants will provide a respiratory specimen (nasal self-swab) for influenza testing and complete two brief surveys during and after resolution of their illness. For a sub-sample of consented participants, additional information on attitudes toward morbidity with influenza and respiratory illness will be collected through in-depth qualitative interviews. Additional brief surveys will be completed at the end of each season/year and at the start of the second influenza season to update information on participant health, work responsibilities, and attitudes and practices associated with vaccination and other inflection control and prevention measures.