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 US.
"We've been sitting on the sidelines passively attempting to wear our masks and social distance and not go out when it's not necessary. This is the first step of becoming active against this," researcher Dr Frank Eder said at the trial site in Binghamton, New York. "There's really no other way to get past this."
As if to underline how high the stakes are, there were more setbacks in efforts to contain the coronavirus.
In Washington, the Trump Administration disclosed that national security adviser Robert O'Brien has the virus — the highest-ranking US official to test positive so far. The White House said he has mild symptoms and "has been self-isolating and working from a secure location off site."
As for relief from the economic damage done by the virus, Republicans on Capitol Hill planned to roll out a US$1 trillion package that could include a new round of $1200 stimulus money but reduce the extra US$600 a week in federal unemployment benefits that are expiring for millions of Americans this week.
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 travellers 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.
Scientists set speed records getting a made-from-scratch vaccine into such massive testing just months after the coronavirus emerged. But they stressed that the public shouldn't fear that anyone is cutting corners.
"This is a significant milestone," NIH Director Francis Collins said after the very first test injection was given in Savannah, Georgia. "Yes, we're going fast, but no, we are not going to compromise" on proving whether the vaccine is safe and effective.
"We are focusing on speed because every day matters," added Stephane Bancel, CEO of Massachusetts-based Moderna.
After volunteers get two doses a month apart, scientists will closely track which group experiences more infections as they go about their daily routines, especially in areas where the virus is spreading unchecked.
The answer probably won't come until November or December, cautioned Dr Anthony Fauci, NIH's infectious-diseases chief.
Among many questions the study may answer: How much protection does just one dose offer compared with the two scientists think are needed? If it works, will it protect against severe disease or block infection entirely?
Don't expect a vaccine as strong as the measles vaccine, which prevents about 97 per cent of measles infections, Fauci said, adding he would be happy with a Covid-19 vaccine that's 60 per cent effective.
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 US requires its own tests of any vaccine that might be used in the country.
Every month through the northern autumn, the government-funded Covid-19 Prevention Network will roll out a new study of a leading candidate, each with 30,000 volunteers.
The final US 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.
That's a stunning number of people needed to roll up their sleeves for science.
- AP
