With reports of a thousand new COVID-19 cases every hour, there's growing anxiety about how well protected some vaccinated Americans are against the highly contagious delta variant.
A new lab study posted online Tuesday has raised some concerns that the one-dose Johnson & Johnson vaccine isn't as robust in fighting off illness from coronavirus variants, including the delta variant, as the two-dose mRNA shots.
Since the beginning of the month, the U.S. has averaged 10,000 to 40,000 new cases a day, according to NBC News data. At least 13 million people in the U.S. have received the single Johnson & Johnson dose — but experts say people shouldn't be rushing out to boost their Johnson & Johnson vaccinations with any other shots.
In the study, which hasn't been peer-reviewed or published in a medical journal, researchers at the NYU Grossman School of Medicine compared how well two doses of the mRNA vaccines — from Pfizer-BioNTech and Moderna — stood up against the variants compared to the one-dose Johnson & Johnson vaccine. In the lab experiment, researchers compared a small number of blood samples from 10 people who had received the Johnson & Johnson vaccine and 17 people who had received either the Pfizer or Moderna shots. The results suggested that the Johnson & Johnson vaccine may be less effective against some of the variants of concern.
The study looked only at the antibody response in the blood samples, according to the researchers. Other crucial components of the immune response, such as T cells that can protect the body against the virus, weren't examined.
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Early this month, researchers from South Africa reported on their real world data about health care workers who had received the one-dose vaccine. The research, which has yet to be published, showed that, overall, over 90 percent of breakthrough infections were mild.
Last week, researchers from Beth Israel Deaconess Medical Center in Boston found in a laboratory setting that the antibody response from the Johnson & Johnson vaccine worked well against the delta variant and that the immune response lasted eight months. The research, published in the New England Journal of Medicine, was reassuring to the millions of people who had received the Johnson & Johnson vaccine.
In a statement Tuesday, Johnson & Johnson responded that the new research doesn't show "the full nature of immune protection."
On "The Beat with Ari Melber" on MSNBC on Wednesday, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said that the new study's lab samples showed a lower antibody level from the one-dose Johnson & Johnson shot compared to the two-dose vaccines but that it's important to wait for clinical data to determine the impact on people who got the Johnson & Johnson vaccine and whether they're getting sick.
"We don't have the clinical data that matched one [vaccine] against the other," Fauci said. "What we need to do is wait for the clinical data. If the clinical data reflect the lab data, then you have to re-evaluate."
I got the Johnson & Johnson vaccine. What should I do?
"I would not be worried," said Dr. Carlos del Rio, an infectious disease specialist and chair of the department of global health at Emory University in Atlanta. "If I'm starting to see people getting hospitalized who all received J&J, I think that's going to tell me something. At this point in time, I see no evidence that the J&J protection is less than with the other vaccines.
A spokesperson for the Centers for Disease Control and Prevention noted that the NYU research is just one study of vaccine effectiveness. The agency's position on boosters still stands: "Americans who have been fully vaccinated do not need a booster shot at this time."
Will COVID-19 boosters be necessary?
Fauci said Tuesday at a Senate hearing on the COVID response in the U.S. that studies are being conducted to determine whether or not boosters — third doses of the mRNA vaccines or second doses of the Johnson & Johnson vaccine — will be needed to increase protection against variants or the possibility of waning immunity.
"We don't want people to believe that when you're talking about boosters that means that the vaccines are not effective," he said. "They are highly effective."
On Thursday, the CDC's vaccine advisory committee will discuss booster shots, particularly for people with weakened or compromised immune systems. They are the people who would be likely to be first in line for booster shots when they are available.
This story first appeared on NBCNews.com.