Three other vaccine candidates have a head start, with U.S. trials that began earlier in the summer, but the vaccine being developed by Janssen Pharmaceutical Companies, a division of J&J, has several advantages that could make it logistically easier to administer and distribute if it is proved safe and effective.

The company is initially testing a single dose, whereas the other vaccines being tested in the United States require a return visit and second shot three to four weeks after the first one to trigger a protective immune response. The J&J vaccine can also be stored in liquid form at refrigerator temperatures for three months, whereas two of the front-runner candidates must be frozen or kept at ultracold temperatures for long-term storage.

“A single-shot vaccine, if it’s safe and effective, will have substantial logistic advantages for global pandemic control,” said Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston, who partnered with J&J to develop the vaccine.