It will be months before results trickle in, and there is no guarantee the vaccine will ultimately work against the scourge that has killed about 650,000 people around the world, including almost 150,000 in the U.S.
In Washington, the White House disclosed national security adviser Robert O’Brien has the coronavirus, the highest-ranking U.S. official to test positive so far.
On virus relief, Republicans on Capitol Hill planned to roll out a $1 trillion package that could include a new round of $,1200 stimulus checks but reduce the extra $600 a week in federal unemployment benefits that are expiring for millions of Americans this week.
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In Europe, rising infections in Spain and other countries caused alarm only weeks after nations reopened their borders in hopes of reviving tourism. Over the weekend, Britain imposed a 14-day quarantine on travelers arriving from Spain, Norway ordered a 10-day quarantine for people returning from the entire Iberian peninsula, and France urged its citizens not to visit Spain's Catalonia region.
After two doses, scientists will closely track which participants — those getting real shots, or a dummy — experience more infections as they go about their daily routines, especially in hard-hit areas where the virus still is spreading. Testing is planned at close to 90 sites, officials said.
“Unfortunately for the United States of America, we have plenty of infections right now” to get that answer, the government’s top infectious-disease expert, Dr. Anthony Fauci, said recently.
Several other vaccines made by China and by Britain's Oxford University began smaller final-stage tests in Brazil and other hard-hit countries earlier this month. But the U.S. requires its own tests of any vaccine that might be used in the country.
Every month through the fall, the government-funded COVID-19 Prevention Network will roll out a new study of a leading candidate, each with 30,000 volunteers, to test not only whether the shots work but whether they are safe.
The final U.S. study of the Oxford shot is set to begin in August, followed by a candidate from Johnson & Johnson in September and one from Novavax in October. Pfizer Inc. plans its own 30,000-person study this summer.
It normally takes years to create a accine from scratch, but scientists are setting speed records this time, spurred by knowledge that vaccination is the world's best hope against the pandemic.
Governments around the world are already trying to stockpile millions of doses of the leading candidates so that immunizations can begin immediately if the vaccines win approval. But the first available doses will most likely be reserved for people at highest risk.
“We’re optimistic, cautiously optimistic” that the vaccine will work and that “toward the end of the year” there will be data to prove it, Dr. Stephen Hoge, president of Massachusetts-based Moderna, told a House subcommittee last week.