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Science’s COVID-19 reporting is supported by the Pulitzer Center and the Heising-Simons Foundation.

For the first time, a single-dose COVID-19 vaccine has been shown to deliver solid protection against severe disease and death. The vaccine, made by Johnson & Johnson (J&J), had 85% efficacy against COVID-19–related fatalities or severe symptoms in all age groups, regardless of a person’s other underlying medical conditions, the company announced today. Its efficacy against mild cases of the disease was lower, and dropped to 57% in South Africa, where a viral variant that can dodge immune responses now dominates. But when it came to severe disease, the vaccine appeared equally effective regardless of the viral strain, a result many vaccine experts hailed.

“If you can prevent severe disease in a high percentage of individuals, that will alleviate so much of the stress in human suffering and death in this epidemic,” said Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, at a telephone press conference this morning. Nahid Bhadelia, an infectious disease physician who is medical director of the special pathogens unit at Boston Medical Center, added that a single-dose vaccine could be a boon for people living in remote places and those who might have difficulty getting a second dose. Given the data against mild disease, “Someone is going to say that the [J&J] results are disappointing,” Bhadelia says, but they’re “a game changer.”

The 85% efficacy against severe disease everywhere in the multicountry trial rose to 100% protection when only considering deaths or COVID-19 cases that required hospitalization. To many vaccine experts, that is the key result. “Do you want a vaccine that prevents coughs or do you want a vaccine that prevents death?” asks Lawrence Corey of the University of Washington, Seattle, who co-leads a clinical trials network in the United States that is testing this vaccine and others that received support from the U.S. government’s Operation Warp Speed. The J&J results come less than 24 hours after another COVID-19 vaccine success reported by the small biotech Novavax. “What I take away from this week is that we have two more tools in our toolbox at a very precarious time,” Bhadelia says.

The J&J vaccine, which was developed by its Janssen Pharmaceuticals division, contains a gene for the SARS-CoV-2 spike protein stitched into adenovirus 26 (Ad26), which normally causes common colds but has been disabled so that it cannot replicate. This trial includes nearly 44,000 people in the United States, Latin America, and South Africa. The interim analysis announced today, described only in a press release so far, found 468 cases of symptomatic COVID-19 among the participants—far more than any other COVID-19 vaccine efficacy trial to date, which provides more confidence that the results are robust. Some protection was seen as early as 14 days after the dose, and by day 28, the overall efficacy against COVID-19 cases with any noticeable symptoms was 72% among trial participants in the United States, 66% in Latin America, and 57% in South Africa, the company said. No serious safety issues surfaced.

At the teleconference, company representatives would not give details about the various disease outcomes in the different groups. But the results appear to meet the threshold for emergency use authorization in many countries, and J&J says it will file for an EUA next week with the U.S. Food and Drug Administration. The company projects it can produce 1 billion doses of the vaccine this year for about $10 per dose, with 100 million doses going to the United States by the end of June.

In South Africa, the vaccine was up against a variant of the virus that lab studies have suggested escapes antibody responses triggered by multiple vaccines, including this one. The variant, which has now been found in the United States and other countries, can reinfect people who have recovered from COVID-19. And it accounts for virtually every new infection detected in South Africa today, which likely explains why the J&J vaccine had the lowest efficacy against symptomatic COVID-19 there.

Yet a co-chair of the J&J study, Glenda Gray, head of the Medical Research Council in South Africa, says she is greatly heartened that the vaccine worked just as well in her country against what matters most, severe disease. “This represents a dream vaccine for a doctor,” Gray says. “And a vaccine that can avert hospitalization and death is a public health vaccine.”

At the teleconference, Janssen scientist Mathai Mammen noted the Ad26 vector system used in the vaccine is especially good at triggering T cell responses, which help control production of antibodies and can themselves clear cells that become infected by viruses. “This is a particularly important aspect to our vaccine,” said Mammen, who hypothesizes that the South African variant that defies antibodies “has not been able to step around T cell activity.”

J&J’s single-dose option could dramatically boost the world’s fight against SARS-CoV-2. All of the other COVID-19 vaccines with proven protection to date require a second dose weeks later. A single dose vastly simplifies administering vaccines, and the J&J product can also be transported under normal refrigeration, not the subzero temperatures required for the messenger RNA (mRNA) vaccines. “Simple is beautiful,” says Matt Hepburn, who works on vaccines for the U.S. government’s COVID-19 Response—the new program that absorbed the Warp Speed program when President Joe Biden took office. If the J&J vaccine helps countries quickly ramp up vaccination, it could also help slow the emergence of other variants, Fauci and other scientists stressed.

Fauci concedes there will be a “messaging issue” about which COVID-19 vaccine people should get, because the first two to be approved in the United States, from the Pfizer-BioNTech collaboration and Moderna, showed even higher efficacy against all cases of the disease. If a person comes in for a vaccine and is told “go to the door on the left and you get 94% to 95%, go to the door on the right and you get 72%, what door do you want to go to? That obviously is something that needs to be messaged.” Fauci told Science “it would have made comparisons clearer” if the vaccine trials from the outset had made severe disease an additional primary endpoint, although that may have slowed trials because such cases occur less frequently.

Brazil has a similar variant to the one in South Africa that appears to have weakened the J&J vaccine’s overall efficacy. Gray says that is leading her to rethink the 50% efficacy data recently reported from the trial in Brazil of a vaccine made by China’s Sinovac Biotech. She expects these variants might also lead to “a diminishing effectiveness in other vaccines that are currently being rolled out at a global level.”

Mammen went further and cautioned against comparing the efficacy of the J&J vaccine with that of mRNA vaccines. “They ran their studies in a different time when the pandemic was less complex,” he says. “There were not the variants and there was not the same level of [COVID-19] incidence which puts pressure on vaccine efficacy.” He suggests that if the Pfizer-BioNTech collaboration and Moderna ran their efficacy trials today, “you’d likely see different results.”

Ongoing Pfizer-BioNTech and Moderna efficacy trials may answer that question.

With reporting from Meredith Wadman.