The big COVID-19 news from Monday is Pfizer s announcement that its experimental vaccine looks to be highly effective at preventing the disease, with no apparent safety problems although, to be clear, thats all based on the companys own preliminary data.

The announcement got attention around the world, sending financial markets up on hopes that a proven vaccine would soon be available. And in the U.S., it set off an immediate political controversy as the Trump administration and its critics argued over how much credit, if any, the administrations Warp Speed initiative deserves for the apparent advance.

Experts, meanwhile, were quick to sound cautionary notes, including one from Pfizer itself. The findings, which come from phase 3 trials, have not yet gotten an outside peer review.

Even if the vaccine really works as well as the preliminary Pfizer data suggests, and even if similar vaccines also in development soon show similar results, manufacturing and then distributing them will require a massive, unprecedented effort.

Still, infectious disease specialists and other experts expressed palpable excitement at the news that, so far, the vaccine has proven 90% effective. That is a lot higher than most had expected from the first-generation COVID-19 vaccines. If that number holds up and if the vaccine proves safe two very big ifs then it could be a major step toward controlling the pandemic.

90% vaccine effectiveness is outstanding, Preet Malani , an infectious disease specialist at the University of Michigan, told HuffPost. We will want to see data, but this is a best-case scenario.

Todays announcement makes me excited for the promise of a vaccine, said Joshua Sharfstein , vice dean at the Johns Hopkins Bloomberg School of Public Health.

But Sharfstein, a former deputy commissioner of the U.S. Food and Drug Administration, also cautioned that there are a number of steps still to go to realize this promise, starting with collecting more safety data and the independent review of the study results.

Heres some background on what the announcement says and what, maybe, it means.