Last year, about 300 Dunedin volunteers were part of the Commonwealth Serum Laboratories trial involving a total of 7500 people in 19 Australian centres and four centres in New Zealand.
A further 7500 are being sought this year, including 500 people in Dunedin.
Principal investigator for Dunedin Associate Prof Jim Reid said last year was such a mild flu season few people in the trial got the flu.
The trial, which will use this year's standard flu vaccine, is designed to both check the effectiveness of the vaccine and the belief some people have that they get flu symptoms after receiving the vaccine.
Two-thirds of those who participate will be given the vaccine and one-third a placebo.
Neither the participants nor those delivering the vaccine or placebo will know which is which.
Prof Reid said those who took part last year were not eligible for this year's trial.
Because the trial is continuing, all of the people who participated last year cannot yet be told whether they received placebo or vaccine.
Prof Reid said they had been advised of this.
Results of the trial are expected to be published in about two years and will include data from both years.
Participants are paid $300.
They are required to be healthy people aged between 18 and 64 who do not qualify for a free flu vaccination and who have not received a flu vaccination in the past six months.
Women who are pregnant, breast-feeding or planning to become pregnant are ineligible.
Anybody interested in participating can contact the Caversham Medical Centre or phone 0800 262-385.
Prof Reid said it was hoped the vaccinations could be completed by the end of May.
This year's flu vaccine is now available from general practitioners.
Free immunisation is available before the end of June for those 65 and older, those who regularly use asthma preventers, have heart disease, cancer or another serious medical condition.