BIDEN: "I made sure we had 600 million doses, not of either the Johnson and Johnson and or AstraZeneca. So there's enough vaccine that is basically 100% unquestionable for every single solitary American."President Joe Biden sought to reassure the American public Tuesday, after the U.S. Food and Drug Administration announced a pause for administering Johnson & Johnson's single-shot vaccine after six women developed rare blood clots after getting the shot. U.S. COVID-19 RESPONSE COORDINATOR JEFF ZIENTS: "Let me start by saying that this announcement will not have a significant impact on our vaccination program."Biden's COVID coordinator Jeff Zients said the U.S. has more than enough supply from Pfizer and Moderna to keep up with the country's aggressive goals.Top infectious disease official Anthony Fauci said the J&J pause is likely to last days or weeks, after six women between the ages of 18 and 48 developed rare blood clots after getting the shot. One person died and one is in critical condition.FAUCI: "This is a really rare event. If you look at what we know so far, there have been six out of the 6.85 million doses, which is less than one in a million."During a Tuesday morning phone conference with federal health officials, Dr. Peter Marks of the FDA said a big reason for the pause was to warn doctors that administering the standard treatments for clots for those who received the J&J shot could be dangerous or even fatal. MARKS: "The issue here with these types of blood clots is that if one administers the standard treatment that we as doctors have learned to give for blood clots, one can actually cause tremendous harm."Marks said it was "plainly obvious" that the J&J cases were "very similar" to the AstraZeneca cases. European regulators said earlier this month they had found a possible link between AstraZeneca's vaccine and a similar rare blood clotting problem that led to a small number of deaths.And while Marks, Fauci and several other health officials stressed that the risk posed by the J&J vaccine appeared extremely low - and that there had been no reported blood clot cases related the Moderna or Pfizer shots - some health experts expressed concern that the FDA's announcement could contribute to vaccine hesitancy.FAUCI: "You know, when you want to talk about safety, this is an extraordinary safety record that the others have. And the fact that a pause was done, I think, just is a testimony to how seriously we take safety."Some appointments across the country were rescheduled. Others, like at New York City's Javits Center, appeared to proceed with some adjustments.VACCINE SEEKER MATTHEW REGAN: "So I had a J&J vaccine appointment today and I saw the news about it getting kind of revoked, so I called and I asked and they said, 'you can get the Pfizer vaccine instead.'"