As Moderna Inc.‘s experimental coronavirus vaccine sprinted into the final phase of human testing on Monday, company and U.S. officials described rosy prospects for winning regulatory approval by the end of the year and producing hundreds of millions of doses in 2021.

Under “Operation Warp Speed,” the U.S. government is helping to underwrite the development and production of several promising vaccine candidates. But Moderna is the first to launch the pivotal 30,000-subject study required to assess safety and effectiveness by comparing the immunization to a placebo.

Even if the optimistic timetable is realistic — some experts are skeptical — Moderna’s novel vaccine is not expected, or required, to be highly effective at preventing COVID-19. To win the U.S. Food and Drug Administration’s blessing, it has to be at least 50% effective — on par with the seasonal flu vaccine. The trial is aiming for 60%.

“We’d love to see higher, but 60% is standard,” Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said during a Monday teleconference. “Even though we won’t be seeing the 98% effectiveness we see with the measles vaccine, it would be a major step toward controlling” the pandemic.

The final-phase trial will also evaluate whether the severity of illness is lessened in those who get the vaccine and still get infected. How long protection might last is another unknown, but studies of other coronaviruses suggest a year or two.

Moderna, based in Cambridge, Mass., has never brought a vaccine to market, and it is using messenger RNA technology that no one has managed to turn into a successful vaccine, despite two decades of research.

Preliminary results from the first 45 subjects, published in New England Journal of Medicine this month, were encouraging. All the volunteers developed “neutralizing” antibodies against the virus, and there were no serious side effects.

The first volunteer in the pivotal study rolled up a sleeve in Savannah, Ga., shortly before 7 a.m. Monday.

“This is a historic day,” Moderna CEO Stéphane Bancel said.

Across the country, 89 sites will be recruiting and vaccinating the participants, who must commit to two shots, 28 days apart, and two years of monitoring. Nearly 200,000 people have already expressed a willingness to volunteer by joining a national registry, the Coronavirus Prevention Network.

The Trump administration has projected an effective vaccine will be ready as early as October, a feat that would be a boon to President Donald Trump’s reelection bid.

Both Fauci and Bancel, however, projected results from Moderna’s trial would not be available until November or December at best. “It could be earlier, but I would be reluctant to say that,” Fauci said.

The unprecedented speed of coronavirus vaccine development reflects the public health crisis, huge investments by governments around the world, and simultaneous work on phases that would usually be done step by step. Normally, vaccine design, animal testing, clinical trials, and construction of manufacturing facilities take a decade or more.

Operation Warp Speed leader Matt Hepburn boasted during the briefing that the government has laid the groundwork for producing and distributing millions of doses of vaccine. Bancel estimated Moderna could make 500 million doses next year, then added, “I think a billion doses is possible.”

Critics say such optimism belies the federal pandemic response to date.

A vaccine distribution system “is exactly the sort of challenge that the U.S. government has proved unprepared for in this pandemic,” Sara Zhang wrote in the Atlantic.