“It’s really hard to even sequence the virus sometimes because there’s very little virus, and it’s there for a short period of time,” he said.
Still, most of the data has been gathered on the Pfizer-BioNTech and Moderna vaccines, Dr. Krammer cautioned. Because Johnson Johnson’s vaccine was authorized later, there are fewer studies assessing its effectiveness.
In clinical trials, the Johnson Johnson vaccine had 72 percent efficacy — lower than the figure for the Pfizer and Moderna vaccines. And effectiveness was measured in terms of moderate and severe disease, rather than mild disease.
“It’s a very good vaccine, and I’m sure it will save many, many, many lives,” Dr. Krammer said. “But we need more data on how well the J.J. vaccine prevents infection, and how well it prevents transmission.”
Variants of the virus have been a particular worry for scientists. While Dr. Walensky cited evidence showing that the mRNA vaccines like those from Pfizer and Moderna are effective against the variants circulating in the United States, there is little data about variants and the Johnson Johnson vaccine. And new variants are emerging constantly.
“I’m not at all saying that this is now a big problem,” Dr. Krammer said. But before lifting the masking requirements, “I might have waited a little bit longer to look at the numbers.”
In a statement on Friday, a C.D.C. spokesman said, “All of the authorized vaccines provide strong protection against serious illness, hospitalization, and death, and we are accumulating data that our authorized vaccines are effective against the variants that are circulating in this country.”