J&J's single-dose coronavirus vaccine is 66% effective against COVID-19, raising worries that some variants may lower protection

• A Johnson & Johnson vaccine was found safe with 66% efficacy at preventing COVID-19.
• J&J expects to submit the late-stage trial data to the FDA in early February for authorization.
• If the vaccine gets the green light, it would boost supplies for the US vaccination campaign.

The US's effort to halt the coronavirus got some good news Friday as another vaccine succeeded in the final stages of clinical research.

The single-dose vaccine, developed by the healthcare giant Johnson & Johnson with US government funding, was 66% effective at preventing people from developing COVID-19, J&J said in a statement. That's significantly lower than the protection offered by shots from Pfizer and from Moderna, though both of those shots require two doses.

Still, J&J's vaccine is most likely poised to become the third authorized coronavirus vaccine in the US, further boosting available supply. The vaccine was tested in a trial of about 44,000 volunteers who randomly received either one shot or a placebo.

The success comes with a sobering caveat. J&J's vaccine was less effective in South Africa, suggesting a variant of the virus known as B.1.351, which was first identified in that country, has drifted far enough away from the original strain to diminish the benefits of the shot.

J&J's stock fell 4.1% in early trading, while Moderna's soared 29% and Pfizer's gained 4.5%. Novavax, which said Thursday that its coronavirus vaccine was effective, jumped 65%.

Overall, J&J's study found the shot to be 72% effective in the US, 66% effective in Latin America, and 57% effective in South Africa. J&J said 95% of the cases recorded in South Africa came from the B.1.351 variant. That raises the troubling prospect that B.1.351 and other variants could evade the vaccine, complicating a global immunization campaign.

J&J's results are the second piece of experimental evidence that coronavirus vaccines may be less effective against some new strains of the coronavirus. On Thursday, Novavax said its two-dose vaccine was 89% effective in a trial in the UK but reported efficacy of just 49% in South Africa.

Even with the variant concerns, J&J said its vaccine was 85% effective across all regions at preventing severe cases of COVID-19.

"The potential to significantly reduce the burden of severe disease, by providing an effective and well-tolerated vaccine with just one immunization, is a critical component of the global public-health response," Dr. Paul Stoffels, J&J's chief scientific officer, said in a statement.

The scientific success comes at a perilous moment in the pandemic, particularly in the US. The country is averaging about 155,000 cases a day, and more than 100,000 people are hospitalized, according to The COVID Tracking Project. About 3,300 deaths a day on average have been reported in the past week.

J&J said it would file in early February for emergency permission from US regulators to make the shot available more widely.

Single dose should simplify a difficult immunization campaign

A one-dose jab should simplify the task of immunizing hundreds of millions of people. J&J's vaccine can also be stored at typical refrigerator temperatures for several months, unlike the vaccine developed by Pfizer and BioNTech, which requires extremely cold storage.

These logistical challenges have led to a slower-than-expected rollout in the US, though President Joe Biden has set a goal of giving at least 100 million shots in his first 100 days in office.

Read more: What's coming next for COVID-19 vaccines? Here's the latest on 11 leading programs.

The US Food and Drug Administration is expected to follow the same regulatory process used for the Pfizer-BioNTech and Moderna vaccines, which includes convening an independent expert panel to vet the data. The FDA took 18 days to review Pfizer's and Moderna's applications and grant what are known as emergency-use authorizations. Typically, agency reviews for new medicines take six to 10 months.

J&J hopes to produce 1 billion doses of its vaccine in 2021. The US government and J&J agreed in August to a $1 billion deal for 100 million doses. The healthcare giant has pledged not to sell doses for a profit during the pandemic.

J&J's vaccine was codeveloped by a team led by the virologist Dr. Dan Barouch at Boston's Beth Israel Deaconess Medical Center.

J&J's vaccine uses a modified adenovirus, a virus that causes the common cold, to carry and deliver DNA-based instructions to cells on how to produce a part of the coronavirus known as the spike protein. The cells then produce the protein, generating an immune response in the body that helps fight off the actual coronavirus. J&J has engineered the adenovirus so it can't replicate in the body.

The Pfizer-BioNTech and Moderna vaccines rely on a new technology that uses genetic information called messenger RNA to instruct cells to make the coronavirus' spike protein.

J&J's shot compared with Moderna's and Pfizer's

The Moderna and Pfizer-BioNTech shots were overwhelmingly effective at preventing COVID-19. Large-scale studies for two-dose regimens found the vaccines were 94% and 95% effective at preventing symptomatic cases of COVID-19 after people got both doses.

Both shots also showed promising evidence that they're highly effective at preventing severe disease.

Neither Moderna's or Pfizer-BioNTech's vaccines raised any major safety concerns after being studied in tens of thousands of volunteers. But both had high rates of short-term, expected reactions to the shots.

The most common side effects for those two vaccines were injection-site pain, fatigue, and headache. Volunteers ages 18 to 64 recorded more side effects after getting Moderna's shot, with nearly one in five recording a severe reaction after the second dose.

The most common severe side effects for this age group were fatigue (11%), muscle pain (10%), joint pain (6%), fever (2%), and chills (2%). These side effects were significant enough to impede on daily activities but not life-threatening.

Pfizer and BioNTech's vaccine, by comparison, also had the highest rates of severe reactions in people under 55 years old after the second injection.

Among this group, 5% recorded severe fatigue, 3% had severe headaches, 2% had severe chills, and 2% had new or worsened muscle pain.

Both mRNA-based vaccines have also led to severe allergic reactions in a handful of people. While these cases have been quite rare, the US Centers for Disease Control and Prevention has said they are occurring at a rate about 10 times that of the flu shot. J&J said there were no cases of these severe allergic reactions, known as anaphylaxis, in its trial.

The company said its shot was generally tolerated, and fevers developed up in 9% of those who took part in the trial. More serious fevers were reported in 0.2% of trial participants, J&J said.

The company hasn't yet provided the detailed information on side effects and safety that Pfizer and Moderna have published.