The first coronavirus vaccine that aims to protect people with a single shot has entered the final stages of testing in the United States in an international trial that will recruit up to 60,000 participants.

The experimental vaccine being developed by pharmaceutical giant Johnson & Johnson is the fourth vaccine to enter the large, Phase 3 trials in the United States that will determine whether they are effective and safe. Paul Stoffels, chief scientific officer of J&J, predicted that there may be enough data to have results by the end of the year and said the company plans to manufacture 1 billion doses next year.

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 will be shipped frozen, but can 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 until shortly before use.

“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.

The United States began Phase 3, one of the last steps in developing a vaccine, on July 27, 2020. We take you inside the process. (Jhaan Elker/The Washington Post)

The United States has invested billions of dollars in an array of vaccine technologies, including close to $1.5 billion to support the development of the J&J vaccine and an advance purchase of 100 million doses. The J&J vaccine is the second to use a viral-vector approach, taking a harmless virus and inserting into it a gene that contains the blueprint for a distinctive part of the novel coronavirus.

“It is a really good thing that we have this diversity of platforms, because this is a critical crisis in terms of our global circumstance,” said Francis Collins, director of the National Institutes of Health. “Now, here in the U.S. with 200,000 deaths, we want to do everything we can without sacrificing safety or efficacy.”

In a monkey study published in Nature in July, Barouch showed his approach successfully taught the immune system to protect against a real infection. Data from early-stage human trials that included 400 participants in the United States and Belgium was scheduled to be submitted to a preprint server Wednesday, but Stoffels said that overall, the vaccine showed that it triggered a promising immune response and that side effects of the vaccine were tolerable, including some fevers that resolved within one to two days.

J&J, like other vaccine companies, promised to publish the full protocol for its trial, which includes detailed information on how researchers will determine whether it is safe and effective.

Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said that once there were 154 cases of covid-19 in the trial, it would be possible to tell whether the vaccine was effective. But there are also predetermined intervals in which a data and safety monitoring board evaluates the progress to see whether the vaccine shows early signs that require stopping the trial for one of several reasons — the vaccine is very successful, it is causing harm or it is futile, that is, unlikely to yield a result.

The trial is designed to be twice as large as the initial design of other Phase 3 trials in the United States, although the Pfizer trial has also expanded to include 44,000 participants. Half the participants will receive a vaccine, and half will receive a placebo.

As the other coronavirus trials in the United States have struggled to recruit diverse populations, the size and international scope of the J&J trial could provide an advantage.

“The vaccine trials need to have participation that reflect the diversity of our nation,” said Michelle Andrasik, director of community engagement for the Covid-19 Prevention Network, the federal network partnering with companies to run trials. “We need everyone involved to ensure that we find a vaccine that is effective for everyone.”

In addition, at a time when the safety of the vaccine is very much part of the public debate because of worries that bringing one to market so quickly could cut corners, it could provide more data to assure people of the vaccine’s track record.

“With a larger trial, it also increases the safety data set,” Barouch said. “Safety has been a lot in the public eye, and increasing the size of trials increases the safety data set, as well.”