growing evidence of the need for this third shot, a booster shot, that they say will now be available starting september 20th, of course, pending fda approval. tonight, a new preliminary study in the uk finding that vaccinated people who get breakthrough cases with the delta variant have a higher viral load. we've heard this from u.s. authorities before. this study says they are more likely to transmit the virus to others. and just today, the news coming in that three senators announcing separately that they have now tested positive. all of them were fully vaccinated, so, these are breakthrough cases. senator roger wicker of mississippi, a republican, senator angus king, an independent from maine and senator john hickenlooper, a colorado democrat who tweeted, "i'm grateful for the vaccine for limiting my symptoms," urging others to still get the shot. abc's victor oquendo on all of this from miami tonight. >> reporter: tonight, health officials stressing just how critical that third covid booster shot will be to prevent more breakthrough infections,