BioNTech's /zigman2/quotes/214419716/composite BNTX -4.91% stock was halted and shares of Pfizer Inc. /zigman2/quotes/202877789/composite PFE +1.77% gained 4.7% in trading on Wednesday morning after the companies said that all 24 participants in the Phase 1/2 clinical trial who received one of two lower doses of a COVID-19 vaccine candidate developed neutralizing antibodies. This data was shared by the companies and was also published in a preprint study , which means it has not been peer-reviewed. The study has 45 participants between the ages of 18 and 55 years old who were randomized and vaccinated; 24 of which received two rounds of two lower doses of the vaccine candidate. (Twelve participants received one high dose, and nine received two doses of the placebo.) The antibody findings are important because they indicate that the experimental vaccine can generate antibodies at the same level or higher than those seen in convalescent sera, which is collected from people who have recovered from COVID-19. There were no serious adverse events reported in the trial, although some reported pain in the area of the body where the investigational vaccine was administered. Pfizer said in a news release that additional data from this trial will inform the selection of the candidate - the companies are evaluating four COVID-19 vaccine candidates - and dosing level for the Phase 2b/3 study, which could begin this month, with up to 30,000 participants. Since the start of the year, BioNTech's stock is up 97.0%, shares of Pfizer are down 16.5%, and the S&P 500 /zigman2/quotes/210599714/realtime SPX -1.18% is down 4.0%.