J&J said it has begun shipping the vaccine to the U.S. government, which is managing allocation and distribution. That indicates first doses could be administered during the coming week.

“There is light at the end of the tunnel, but we cannot let our guard down now or assume that victory is inevitable. We must continue to remain vigilant, act fast and aggressively, and look out for one another,” President Biden said.

The shot’s addition to the country’s vaccination drive comes at a precarious moment in the pandemic in the U.S. Newly reported cases, hospitalizations and deaths have been dropping in recent weeks, but remain higher than peaks seen last summer and spring.

Also, new variants are spreading. More than 2,100 cases linked to variants first identified in the U.K., South Africa and Brazil have now been detected in the U.S., according to the Centers for Disease Control and Prevention.

J&J’s shot wasn’t as effective in studies as two Covid-19 vaccines, from

Pfizer Inc.

PFE -0.98%

and

Moderna Inc.,

MRNA 4.33%

that have been previously authorized. It also didn’t work as well against at least one new strain of the virus—the one first identified in South Africa—as against the original strain of the virus.

Vaccines’ Effectiveness

Efficacy against

severe disease

Efficacy in people

65 and older

Health authorities say J&J’s vaccine still works well, and are encouraging eligible people to get whatever shot they can.

Also, the vaccine showed preliminary signs, according to J&J, of curbing spread of the virus among people who don’t show symptoms. They have been a significant source of transmission.

J&J’s single-dose shot could greatly simplify the vaccination drive, which has been complicated by the need to give two doses, three or four weeks apart, as required by the two vaccines cleared earlier.

The J&J vaccine also has more favorable temperature requirements for distribution and storage, making it easier for sites, including many in rural areas, that lack expensive freezers to give the shots.

Some studies have shown that the Pfizer and Moderna vaccines provide good protection after just one shot, but the research is ongoing and regulators haven’t signed off on any change.

Likewise, the Pfizer and Moderna vaccines can be stored for longer at higher temperatures than was initially thought, but storage and handling of J&J’s shot will still be easier. Pfizer’s vaccine can be stored in a regular refrigerator for five days, while J&J’s can stay there for at least three months.

Limited initial supplies, however, will probably blunt any early impact from the addition of the newly authorized shot. J&J expects to have just 4 million doses ready to ship right away. The company has said it would deliver 20 million doses by the end of March.

J&J expects output will quickly increase, so that the company can deliver a total of 100 million doses for use in the U.S. by the end of June.

“This milestone follows a year of incredible work by our dedicated teams and unprecedented collaboration with health leaders around the world—all of whom shared a goal of bringing a single-shot vaccine to the public,” J&J Chief Executive

Alex Gorsky

said in a statement. “We will do everything we can to help bring this pandemic to an end, in the United States and throughout the world.”

The FDA authorized the vaccine’s use in adults. Like other leading Covid-19 vaccine developers, J&J compressed a process that normally takes years into months. The FDA, likewise, moved relatively quickly to authorize the shot’s use.

The agency turned to an emergency-use authorization process created by federal law to deal with situations like the current global pandemic.

The FDA’s standard review, resulting in an approval, usually takes months. Yet the agency said it still held J&J’s vaccine to virtually the same kinds of standards for safety and effectiveness it would normally require.

The FDA based its decision on a study of about 44,000 volunteers 18 years old and older.

The study found J&J’s vaccine was 66.1% effective at protecting people from developing moderate and severe cases of Covid-19 at least 28 days after vaccination, and was generally safe.

Its overall effectiveness, however, wasn’t as high as that from the first two vaccines to be authorized in the U.S. Shots from Pfizer and partner

BioNTech

BNTX -2.94%

SE and from Moderna were more than 94% effective in their late-stage studies.

And the J&J vaccine appeared to be less effective against the South Africa variant. Other Covid-19 vaccines also have shown less potency against virus variants. South Africa began rolling out the J&J vaccine in mid-February.

The effectiveness of J&J’s vaccine was mostly consistent across various demographic subgroups, though it was lower in people 60 years and older with underlying medical conditions like diabetes and high blood pressure—and among American Indian and Alaska Natives.

Yet J&J’s vaccine had relatively high efficacy against severe Covid-19 disease, even in places where the variants have spread. The vaccine was 85.4% effective against severe Covid-19 at least 28 days after vaccination.

The most common side effects among people who received the vaccine were injection-site pain, headache and fatigue. Most were mild to moderate. J&J has received preliminary reports of two cases of severe allergic reactions in vaccine recipients, one of which was anaphylaxis.

“With today’s authorization, we are adding another vaccine in our medical toolbox to fight this virus. At the same time, the American people can be assured of the FDA’s unwavering commitment to public health through our comprehensive and rigorous evaluation of the data submitted for vaccines to prevent COVID-19,” said Peter Marks, director of the FDA division that reviews vaccines.

J&J’s is the first Covid-19 vaccine in the U.S. to trigger immune protection by using another virus modified so it doesn’t cause disease, in this case a version of a virus that causes the common cold.

The modified cold virus carries DNA that, once injected, instructs the body’s cells to make a protein resembling the spike protein found on the surface of the new coronavirus.

This, in turn, causes an immune response that can later protect people if they are exposed to the real coronavirus.

AstraZeneca

PLC and the University of Oxford have used a similar approach for their Covid-19 vaccine, which is authorized in the U.K. and other countries but not in the U.S. Vaccines developed in Russia and China also use such viral vectors.

Viral-vector vaccines have been in development for many years, but only recently has the technology yielded vaccines, targeting the deadly Ebola disease, that gained approval from government regulators. J&J developed one of the Ebola vaccines.

The Pfizer and Moderna Covid-19 vaccines use a different design called messenger RNA, but share a similar goal of inducing the body’s own cells to make the coronavirus spike protein that mobilizes immune defenses.

How Viral Vector Vaccines Work

Johnson & Johnson’s vaccine relies on a different mechanism for conferring immunity than traditional vaccines.

Traditional Vaccines

1. In classic vaccines, such as those against measles and polio, the patient is inoculated with weakened or inactivated versions of the virus. This triggers the immune system to produce specialized antibodies that are adapted to recognize the virus.

2. After vaccination, the antibodies remain in the body. If the patient later becomes infected with the actual virus, the antibodies can identify and help neutralize it.

Johnson & Johnson’s Vaccine

Scientists have isolated the genes in coronavirus responsible for producing these spike proteins. The genes are spliced into weakened, harmless versions of other viruses.

Instead of using the whole virus to generate an immune response, these vaccines use only coronavirus’s outer spike proteins, which are what antibodies use to recognize the virus.

Weakened virus with

spike protein genes

When injected into a patient, the genetically engineered viruses enter healthy cells where they produce coronavirus spike proteins.

The spike proteins produced by the cells prompt the immune system to mount a defense, just as with traditional vaccines.

Vaccine-generated antibody response

1. In classic vaccines, such as those against measles and polio, the patient is inoculated with weakened or inactivated versions of the virus. This triggers the immune system to produce specialized antibodies that are adapted to recognize the virus.

2. After vaccination, the antibodies remain in the body. If the patient later becomes infected with the actual virus, the antibodies can identify and help neutralize it.

Johnson & Johnson’s Vaccine

Scientists have isolated the genes in coronavirus responsible for producing these spike proteins. The genes are spliced into weakened, harmless versions of other viruses.

Instead of using the whole virus to generate an immune response, these vaccines use only coronavirus’s outer spike proteins, which are what antibodies use to recognize the virus.

Weakened virus with

spike protein genes

When injected into a patient, the genetically engineered viruses enter healthy cells where they produce coronavirus spike proteins.

The spike proteins produced by the cells prompt the immune system to mount a defense, just as with traditional vaccines.

Vaccine-generated antibody response

1. In classic vaccines, such as those against measles and polio, the patient is inoculated with weakened or inactivated versions of the virus. This triggers the immune system to produce specialized antibodies that are adapted to recognize the virus.

2. After vaccination, the antibodies remain in the body. If the patient later becomes infected with the actual virus, the antibodies can identify and help neutralize it.

Johnson & Johnson’s Vaccine

Scientists have isolated the genes in coronavirus responsible for producing these spike proteins. The genes are spliced into weakened, harmless versions of other viruses.

Instead of using the whole virus to generate an immune response, these vaccines use only coronavirus’s outer spike proteins, which are what antibodies use to recognize the virus.

Weakened virus with

spike protein genes

When injected into a patient, the genetically engineered viruses enter healthy cells where they produce coronavirus spike proteins.

The spike proteins produced by the cells prompt the immune system to mount a defense, just as with traditional vaccines.

Vaccine-generated antibody response

1. In classic vaccines, such as those against measles and polio, the patient is inoculated with weakened or inactivated versions of the virus. This triggers the immune system to produce specialized antibodies that are adapted to recognize the virus.

2. After vaccination, the antibodies remain in the body. If the patient later becomes infected with the actual virus, the antibodies can identify and help neutralize it.

Johnson & Johnson’s Vaccine

Instead of using the whole virus to generate an immune response, these vaccines use only coronavirus’s outer spike proteins, which are what antibodies use to recognize the virus.

Scientists have isolated the genes in coronavirus responsible for producing these

spike proteins. The genes are spliced into weakened, harmless versions of other viruses.

Weakened virus with

spike protein genes

When injected into a patient, the genetically engineered viruses enter healthy cells where they produce coronavirus spike proteins.

The spike proteins produced by the cells prompt the immune system to mount a defense, just as with traditional vaccines.

Vaccine-generated antibody response

J&J, based in New Brunswick, N.J., has said it is making the vaccine available on a not-for-profit basis during the pandemic emergency. The shot could generate about $4.7 billion in sales for J&J this year, Bernstein analysts estimated.

The U.S. government agreed last year to buy 100 million doses of J&J’s vaccine for $1 billion, or $10 a dose, with options to buy more, and provided additional funding for clinical trials and other vaccine development activities at J&J.

The CDC told state officials Friday that they can begin ordering doses of J&J’s vaccine on Sunday and that shipments are expected to arrive within two days of order placement, a state health official said.

The company also has signed vaccine supply contracts with the European Commission and other countries.

It is manufacturing the drug substance for the vaccine at plants in Baltimore, the Netherlands and India. Additional steps in the production process and filling vials with the vaccine are taking place at facilities in Indiana, Michigan, Pennsylvania, Spain and other countries.

Each vial of the vaccine contains five doses.

McKesson Corp.

MCK -2.64%

, a drug wholesaler, is serving as the centralized distributor for the J&J vaccine, and will work with logistics partners such as

FedEx Corp.

, to send the doses to hospitals, pharmacies and other vaccination sites around the U.S.

Because of the limited supply of Covid-19 vaccine doses, states continue to give priority to certain groups for vaccination, such as healthcare workers, the elderly and adults with high-risk health conditions. Some states also have reserved initial supplies for certain occupations like police and educators.

Government officials have said that the general population could start to get access to the vaccines in the spring or summer.

J&J is testing its vaccine in 12- to 17-year-olds in a separate study, and is also studying whether two doses of the vaccine, given about eight weeks apart, are more effective against Covid-19.

The company expects results of a large study testing the two-dose regimen later in the year.

J&J also has begun work on a potential new vaccine that would better target the virus variant first identified in South Africa, and has said it would monitor additional variants that may warrant a modified vaccine.

Write to Peter Loftus at peter.loftus@wsj.com and Thomas M. Burton at tom.burton@wsj.com