Johnson & Johnson’s one-shot vaccine generated strong protection against Covid-19 in a large, late-stage trial, raising hopes that it can rapidly reshape a stumbling immunization campaign.

In the more than 44,000-person study, the vaccine prevented 66% of moderate to severe cases of Covid-19, according to a company statement on Friday. And it was particularly effective at stopping severe disease, preventing 85% of severe infections and 100% of hospitalizations and deaths.

“If you can prevent severe disease in a high percentage of individuals, that will alleviate so much of the stress and human suffering” of the pandemic, said Anthony Fauci, the top U.S. infectious-disease official, at a briefing on the results with company and government officials.

Based on the result, J&J plans to file for an emergency-use authorization in the U.S. next week. The drug giant’s top scientist said this month that he expects a clearance in March, and that it would have product ready to ship then. The company didn’t specify how much of the vaccine would be available immediately, though it reaffirmed that the U.S. would receive 100 million doses in the first half of the year.

Shares of J&J declined 4% to $162.52 in premarket trading on Friday.

If cleared, J&J’s vaccine could go a long way toward ending the pandemic. Competing vaccines from Moderna Inc. and Pfizer Inc. have generated stronger overall efficacy rates, but require two shots to be given before their full benefits take hold. J&J’s shot also can be kept in a refrigerator for three months, while those from Pfizer and Moderna must be kept frozen.

“This is a single shot that can be given easily, it protects completely from that which we fear, having to go to the emergency room or a hospital,” said Mathai Mammen, head of global research and development for J&J’s pharmaceutical division, in an interview. “It’s going to change the nature of the disease.”

Concerns about keeping second doses of the current shots on hand have complicated the push to get as many people as possible inoculated. Some countries have chosen to spread out the time between doses to address the problem, at the risk of diminishing their effectiveness.

The rise of new coronavirus variants has added to the pressure to get immunizations moving faster. The J&J trial was conducted around the globe, including at dozens of clinical-trial sites in hot spots such as South Africa and Brazil where new variants have caused infections to rise.

J&J’s results produced more evidence that the variants will be harder to ward off. In the U.S., where mutations aren’t thought to be as widespread, the vaccine was 72% effective. But in South Africa, where a variant called B.1.351 is circulating widely, it was only 57% effective. And the shot was 66% effective in Latin America.

Nonetheless, J&J’s vaccine is likely to give countries around the world a powerful new tool to fight a virus that has infected more than 101 million people and killed 2.2 million worldwide.

If confirmed, the results suggest people could get one dose of the vaccine to provide initial protection against severe consequences, allowing them to return to their pre-pandemic lives. Then, if needed, they could later take a booster shot J&J is testing in other large, late-stage trials that could produce results before year-end.

J&J said a review by a monitoring board identified no significant safety concerns. While 9% of people who took the shot developed a fever, there were no severe allergic reactions.

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