The School of Public Health is inviting Hong Kong residents aged between 18-45 to join a study to examine to what extent repeated influenza vaccination can stimulate immunity against the strains in the current vaccine, compared to older strains from previous years. Participants will be randomly assigned by computer to different study groups and receive new-generation influenza vaccine or placebo (saline) every year. We will collect blood, nasal strip and other respiratory samples before and after the vaccination, and ask you some questions about the vaccine and administer study questionnaires at corresponding time points. The collected samples will be tested for influenza antibody levels and to assess immune responses to the seasonal flu shot. Participant will be provided complimentary basic blood test and will receive supermarket coupons at each scheduled visit to provide blood or respiratory specimens.

The vaccine used in our study is a recombinant quadrivalent influenza vaccine (Flublok®, Sanofi Pasteur). It has been approved by the USFDA and is expected to be introduced to the market in Hong Kong in 2021. Compared with the current standard dose influenza vaccine widely used in Hong Kong which uses egg-grown virus to match vaccine, the Flublok® uses a cell culture approach which provides better matched virus for flu vaccine. On the one hand, the proportion of adverse reactions is relatively low, and the mutation produced during the production is also slightly less which makes the vaccine hemagglutinin (HA) closer to the circulating strains of the season. On the other hand, since eggs are not used in the production process, the production time can be reduced by several weeks, and the production is not limited to the number of available eggs. This method can therefore produce more vaccines faster and respond more effectively to flu outbreaks and even pandemics.