Shares of AstraZeneca /zigman2/quotes/200304487/composite AZN -0.03% /zigman2/quotes/203048482/delayed UK:AZN -0.60% were down 3.4% in trading on Monday after a medical journal published a study that found the company's COVID-19 vaccine candidate produced antibody and T-cell responses in participants in a Phase 1/2 clinical trial. AstraZeneca is developing the experimental vaccine with the University of Oxford; the study was published in The Lancet. The single-blind, randomized, controlled trial found that neutralizing antibody responses were reported in 91% of 35 patients when utilizing one test and 100% of 35 patients when using a different test. It also found that T-cell responses peaked on day 14. "A single dose elicited both humoral and cellular responses against SARS-CoV-2, with a booster immunization augmenting neutralizing antibody titers," the study's authors wrote. BioNTech /zigman2/quotes/214419716/composite BNTX +5.96% and Pfizer Inc. /zigman2/quotes/202877789/composite PFE -0.02% , which are also developing a COVID-19 vaccine, said earlier on Monday that their vaccine candidate produced a T-cell response in participants in an early-stage trial in Germany. AstraZeneca's stock has gained 21.3% year-to-date, in comparison with the S&P 500 /zigman2/quotes/210599714/realtime SPX +0.47% , which is down 0.2%.