The team compared traditional needle-and-syringe methods with the use of sticky patches made from sucrose and polyvinyl alcohol, each containing an array of 100 micro-needles - tiny protrusions less than a millimetre in height.
The patch punctures the uppermost layers of the skin, whereas regular flu injections go all the way through and into muscle.
Rouphael added the participants who used the patch liked it. They provided a mostly accurate and balanced overview of the study, but only BBC News mentioned the "mild" side effects of redness, soreness and itchiness reported by people using the patch.
"A "painless" sticking plaster flu jab that delivers vaccine into the skin has passed important safety tests in the first trial in people", BBC News reports. Neither had received the flu vaccine in the 2014 to 2015 season.
The researchers found that, immediately after vaccination, 96% of adults who received the patch reported in a questionnaire that they felt no pain, whereas 82% of those who received the traditional flu shot reported no pain.
According to emerging reports, researchers in the United States are now developing technology which may see a small pain-free patch act as an alternative to the traditional jab.
"A product like this would boost not only flu vaccinations but it (could) apply to other diseases - could be a real boom", he said. The people in the fourth group put the microneedle patch on themselves after watching a short, instructional video.
Associate Professor Nadine Rouphael at Emory University School of Medicine and co-author of the study, said: "Despite the recommendation of universal flu vaccination, influenza continues to be a major cause of illness leading to significant morbidity and mortality".
Prof Mary Horgan, a consultant in infectious diseases at Cork University Hospital, said that while the patch was at very early clinical trial stage, the big benefit would be that it is self-applied, and eliminates "needle phobia".
"This bandage-strip sized patch of painless and dissolvable needles can transform how we get vaccinated", said Roderic I. Pettigrew, Ph.D., M.D., director of the National Institute of Biomedical Imaging and Bioengineering (NIBIB), which funded the study.
The patch could also save money because it is easily self-administered and could be transported and stored without refrigeration, and is easily dropped off after use without sharps waste.
The patch also could reduce the need for bins to dispose of syringes and for a cold chain to store vaccines, she said.
Further testing in larger trials needs to be done to be sure these initial results hold true and that the vaccine patch is safe and effective.
Four weeks after receiving the microneedle vaccine patch, 70 percent of the participants said that they'd prefer getting their flu vaccine this way, according to the study. Health officials recommend flu vaccines to prevent such infections each year.