Johnson & Johnson begins Phase 3 COVID-19 vaccine trial in U.S.

A fourth Covid-19 vaccine candidate has gone into the final stage of clinical trials in the U.S., with Johnson & Johnson announcing the start of its Phase 3 trial Wednesday.

The drugmaker follows Pfizer and Moderna, whose Phase 3 trials began in late July. AstraZeneca also started its Phase 3 vaccine trial this month, but it remains on pause in the U.S. after a participant in the U.K. was reported to have developed a spinal cord injury.

Full coverage of the coronavirus outbreak

The Johnson & Johnson trial is not expected to yield preliminary results for at least two months.

President Donald Trump has repeatedly pressed for faster work on vaccine research — even suggesting that there could be some kind of approval before the Nov. 3 election. That timeline would be impossible, at least for Johnson & Johnson, whose interim results are not expected until late November at the earliest.

Johnson & Johnson's vaccine candidate, which is being developed in partnership with Janssen Pharmaceuticals, differs in several ways from the others that have reached the final stage of trials.

Unlike Pfizer's and Moderna's vaccines — which require two doses about a month apart — the Johnson & Johnson vaccine can be administered fully in one dose, avoiding the complicated coordination to require that people return in time for the second dose.

In addition, the Johnson & Johnson vaccine requires basic refrigeration for storage. Pfizer's vaccine must be stored at minus 94 degrees Fahrenheit. (For comparison, that's colder than the South Pole's average winter temperature: minus 76 degrees.)

Moderna has aimed to enroll 30,000 participants, and Pfizer expects to sign up 44,000 in U.S. Phase 3 trials. Johnson & Johnson's enrollment will be much larger, aiming for 60,000 people.

Related: Two vaccine candidates are in phase 3 trials to determine whether they work, with more on the way.

The larger study is "notable and commendable," said Dr. Gregory Poland, an infectious diseases expert who directs the Mayo Clinic's Vaccine Research Group in Rochester, Minnesota. He said a single dose of vaccine in a pandemic provides an opportunity to fully vaccinate more people more quickly.

The Johnson & Johnson vaccine was made using a technique used in previous vaccines, including the one the company developed for the Ebola virus. It combines genetic material from SARS-CoV-2 (the virus that causes Covid-19) with an adenovirus, which causes the common cold but has been genetically altered to be harmless in this case.

Using the genetic material from the coronavirus, the disarmed adenovirus is able to teach the immune system to generate antibodies to attack SARS-CoV-2's spike protein, which is what allows the virus to enter human cells.

The accelerated efforts to find a safe and effective Covid-19 vaccine are part of Operation Warp Speed under the departments of Defense and Health and Human Services. The Johnson & Johnson vaccine trials are sponsored in part by the National Institute of Allergy and Infectious Diseases.

The agency's director, Dr. Anthony Fauci, called the latest vaccine trial launch a "very important advance" during a call with journalists Tuesday. He attributed the speed of the vaccine's research and development to longtime working partnerships among NIAID, Johnson & Johnson and Janssen.

"These things don't just happen overnight. What you're seeing now is the result of the fruitful relationships that have been developed over literally several years," Fauci said.

Download the NBC News app for full coverage of the coronavirus outbreak

The Food and Drug Administration has said an effective vaccine needs to protect at least 50 percent of people to be considered effective. Companies are working toward a higher percentage, at 60 percent.

That is key for scientists to determine whether a vaccine is working. Investigators need to log 154 infections, or "events," among study participants to see whether more of the infections were in the groups getting the actual vaccine or a placebo shot.

"From the statistical construction of the trial, in order to get to an efficiency target of 60 percent, you would need 154 events. Now, it could be that you have a much more effective vaccine" than 60 percent, Fauci said. "We're hoping that that's actually the case."

The first shots are expected to be administered Wednesday. Johnson & Johnson's chief scientific officer, Dr. Paul Stoffels, said the company is targeting "high-risk areas and specific populations" that are at greater risk for infection and complications of the virus.

A diverse study group is key for safety and efficacy, said Poland, who is not involved with the Johnson & Johnson trial.

"It's fine to tell me that in healthy 18- to 55-year-old that the vaccine was safe and 70 percent effective, for example," he said. "But also tell me what it would be in grade school kids. Tell me what it will be in pregnant women, in people 75 or 80 and older with diabetes and obesity."

A varied study population is necessary to figure out who should or should not get a vaccine. "You don't just complete the trial and say, 'Well, nobody died,'" Poland said. "You want to carefully analyze and parse the data to say, 'Are there any surprises here?'"

Follow NBC HEALTH on Twitter & Facebook.