Factors Affecting Intention to Receive and Self-Reported Receipt of 2009 Pandemic (H1N1) Vaccine in Hong Kong: A Longitudinal Study

Background: Vaccination was a core component for mitigating the 2009 influenza pandemic (pH1N1). However, a vaccination program's efficacy largely depends on population compliance. We examined general population decision-making for pH1N1 vaccination using a modified Theory of Planned Behaviour (TBP). Methodology: We conducted a longitudinal study, collecting data before and after the introduction of pH1N1 vaccine in Hong Kong. Structural equation modeling (SEM) tested if a modified TPB had explanatory utility for vaccine uptake among adults. Principal Findings: Among 896 subjects who completed both the baseline and the follow-up surveys, 7% (67/896) reported being "likely/very likely/certain'' to be vaccinated (intent) but two months later only 0.8% (7/896) reported having received pH1N1 vaccination. Perception of low risk from pH1N1 (60%) and concerns regarding adverse effects of the vaccine (37%) were primary justifications for avoiding pH1N1 vaccination. Greater perceived vaccine benefits (beta = 0.15), less concerns regarding vaccine side-effects (beta = 20.20), greater adherence to social norms of vaccination (beta = 0.39), anticipated higher regret if not vaccinated (beta = 0.47), perceived higher self-efficacy for vaccination (beta = 0.12) and history of seasonal influenza vaccination (beta = 0.12) were associated with higher intention to receive the pH1N1 vaccine, which in turn predicted self-reported vaccination uptake (beta = 0.30). Social norm (beta = 0.70), anticipated regret (beta = 0.19) and vaccination intention (beta = 0.31) were positively associated with, and accounted for 70% of variance in vaccination planning, which, in turn subsequently predicted self-reported vaccination uptake (beta = 0.36) accounting for 36% of variance in reported vaccination behaviour. Conclusions/Significance: Perceived low risk from pH1N1 and perceived high risk from pH1N1 vaccine inhibited pH1N1 vaccine uptake. Both the TPB and the additional components contributed to intended vaccination uptake but social norms and anticipated regret predominantly associated with vaccination intention and planning. Vaccination planning is a more significant proximal determinant of uptake of pH1N1 vaccine than is intention. Intention alone is an unreliable predictor of future vaccine uptake.