On February 17–18, 2010, the World Health Organization (WHO) convened the 6th meeting on the “Evaluation of pandemic influenza vaccines in clinical trials” to review the progress made on new A (H1N1) 2009 vaccines and prototype H5N1 vaccines and their evaluation in clinical trials. A number of vaccine types were reviewed, including classical egg-derived and cell culture-derived inactivated vaccines, such as split virus or whole-virion vaccines, and live-attenuated vaccines (LAIV), as well as vaccines developed using new technologies. The amount of antigen needed, the effect of adjuvants and the number of doses required to induce adequate antibody responses in various populations, together with the issue of safety of the vaccines, were major topics of the meeting. The effectiveness of H1N1 vaccines and the need for standardization of vaccine potency tests were also discussed. Independent of the vaccine type and the presence or absence of an adjuvant, all A (H1N1) 2009 vaccines were well tolerated, eliciting only mild to moderate local or systemic reactions. For most vaccines tested, a single dose was sufficient to elicit a potentially protective antibody response in the majority of vaccinees >10 years of age. However, a second dose of vaccine was needed to boost immune responses in infants and toddlers 6 months to 3 years of age and, with some vaccines, in children aged 3–9 years.