Johnson & Johnson's single-dose coronavirus vaccine is much less effective against the highly contagious Delta variant than it is against the original virus, new research claims.

"The message that we wanted to give was not that people shouldn't get the J&J vaccine, but we hope that in the future, it will be boosted with either another dose of J&J or a boost with Pfizer or Moderna," study author Nathaniel Landau, a virologist at NYU's Grossman School of Medicine, told The New York Times.

The troubling news came the same day that Dr. Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention, told lawmakers that the Delta variant now accounts for 83% of new coronavirus cases in this country.

"This is a dramatic increase, up from 50% for the week of July 3," Walensky said during a Senate committee hearing on Tuesday.

As worrying as the J&J vaccine findings are, they came from experiments on blood samples in a laboratory, and may not reflect the vaccine's performance in the real world, theTimes reported. But mounting evidence is suggesting that the 13 million Americans who have gotten the J&J vaccine may need a second dose.

Experts noted that all of the vaccines seem to work better when given in two doses.

"I have always thought, and often said, that the J&J vaccine is a two-dose vaccine," John Moore, a virologist at Weill Cornell Medicine in New York City, told the Times.

Moore added that several studies in monkeys and people have shown greater efficacy with two doses of the J&J vaccine. He noted the new study was particularly credible because it was published by a team with no ties to any of the vaccine makers.

The latest conclusions do run counter to those from smaller studies published by the company in early July that suggested a single dose of the vaccine works against the variant up to eight months after the shot is administered, the Times reported.

The new study was published online on the preprint server BioRxiv and has not yet been peer-reviewed or published in a medical journal. But it jibes with observations that a single dose of the AstraZeneca vaccine—which is similar to J&J's shot—shows only about 33 percent efficacy against symptomatic disease caused by the Delta variant.

In the study, Landau and his colleagues looked at blood samples taken from 17 people who had been immunized with two doses of an mRNA vaccine and 10 people with one dose of the J&J vaccine.

The J&J vaccine started out with a lower efficacy than the mRNA vaccines (Pfizer and Moderna) and showed a bigger drop in efficacy against the Delta variant. "The lower baseline means that what's left to counter Delta is very weak," Moore said. "That is a substantial concern."

While using the J&J shot in a second dose may be enough to counter the variant, there's evidence that using an mRNA vaccine for the second shot, rather than another J&J shot, may work even better. According to the Times, a number of studies have shown that combining one dose of the AstraZeneca vaccine with a dose of the Pfizer-BioNTech or Moderna vaccines raises the immune response more effectively than two doses of AstraZeneca.

Even as some vaccines falter against the Delta variant, U.S. vaccination rates have stalled. Less than half of the country's population is fully vaccinated, according to CDC data, and the majority of those who are not vaccinated are not likely to get vaccinated, according to a poll published Tuesday by Axios-Ipsos.

If many of those who are holding out do not get a vaccine, Dr. Anthony Fauci said, the country can expect a "smoldering" outbreak for "a considerable period of time."

—Ernie Mundell and Robin Foster