Adjuvants are substances that can be added to human vaccines. They stimulate the immune system and increase the response to a vaccine, without having a specific antigenic effect themselves. The advantages, certainly in a pandemic situation, are obvious: they improve the immune response whilst the dose size (the amount of virus material or antigen in the vaccine) can be decreased. The latter is an excellent way to increase global vaccine manufacturing capacity. Compared to the normal antigen dose of 15 μg in seasonal influenza vaccines, an adjuvanted vaccine would only need to contain as little as 4 μg of antigen or less to be effective. To date, several adjuvants have met regulatory standards for safety. These include aluminium salts (which have been used safely in seasonal vaccines in the past) and complex oil in water emulsions. Active research and development programmes are ongoing to develop more new generations of adjuvants that are safe and antigen-sparing and broaden the immune response against influenza when incorporated in seasonal, avian and pandemic vaccines.