Transcript for COVID-19 delta variant surges in areas with low vaccine rates
Gio, excellent coverage all day. History made today. And a momentary break from reality. But the surge in coronavirus cases, 83% of new cases are from the delta variant. Sweeping through the unvaccinated, and now affecting the vaccinated, too, with the so-called breakthrough cases. Dr. Fauci saying mild breakthrough cases are now to be expected. Dr. Rochelle walensky talking about the highly transmissible variant behind 4 of 5 new cases in this country. Tonight, we're inside a Memphis hospital with large numbers of unvaccinated Americans. And new questions after a new study that suggests the Johnson & Johnson one-shot vaccine may not be as effective as the other vaccines against this variant. Not peer reviewed, more study is but here's Trevor Ault. Reporter: Tonight, the delta variant spreading furiously among the unvaccinated, now estimated to account for 4 out of every 5 cases of coronavirus. The delta variant now represents 83% of sequenced cases. This is a dramatic increase up from 50% for the week of July 3rd. Reporter: And tonight, adding to the concern, the new headline involving the Johnson & Johnson one-shot vaccine. A new study raising questions about whether it's less effective against the delta variant than the original virus. Halth experts caution, though, these findings are from blood samples in a lab, and other studies found the j&j vaccine seemed to work well against the delta variant. But it does fuel the debate over whether those who got the one shot at some point might be encouraged to get a second of one of the other vaccines. The new study has not been peer reviewed. And it comes as the number of new cases skyrockets in the U.S. In Tennessee, only 38% of the state is fully vaccinated. Frustrating health care workers like Memphis nurse Matt Robinson, who took us inside methodist LE bonheur hospital. Now we're seeing someone that's my age coming through the door, and it's sad because we have that vaccine to help reduce this, and a majority of the patients are coming through the door unvaccinated. Reporter: This new surge forced them to re-open and re-expand their covid unit, because right now it's already full. How long do you and your team think you can keep this up? Not too much longer. Reporter: Missouri mother Melanie hall sent her 12-year-old daughter Jillian to summer camp unvaccinated, thinking she could wait to get her the shot. But within days of coming home with a stuffy nose, she was seriously sick with covid. I've never seen my daughter gasp for air. She was literally panting to keep air in her lungs. Reporter: Jillian was rushed to the hospital and is now recovering, but her mom is urging parents to take covid seriously. Why play Russian roulette with it? Why? I wouldn't wait another minute to get my children vaccinated. Reporter: And today on capitol hill, Dr. Anthony Fauci acknowledging the virus can break through, but insisting the vaccines still protect against serious illness. When you hear about a breakthrough infection, that doesn't necessarily mean the vaccine is failing. Reporter: And in a heated exchange, senator Rand Paul accusing Dr. Fauci of lying to congress. Senator Paul, you do not know what you are talking about, quite frankly, and I want to say that officially. Reporter: Dr. Fauci flatly denying allegations the nih funded so-called gain of function research in Wuhan, which includes experiments to make a virus more transmissible. All the evidence is that it came from the lab and there will be responsibility for those who funded the lab, including yourself. I totally resent the lie that you are now propagating, senator. And you are implying that what we did was responsible for the deaths of individuals. I totally resent that. But it could have been. It could have been. And if anybody's lying here, senator, it is you. Trevor, Dr. Fauci was also asked today about the notion whether millions of Americans may one day need a booster shot, an additional shot. He seemed to offer a new threshold that may trigger the need for a booster? Reporter: He did. The pfizer and modern vaccines are more than 90% effective. He said if it dropped to the 80s or 70s, they would need a booster. But he said it's too soon right now.
This transcript has been automatically generated and may not be 100% accurate.