New data presented to a Centers for Disease Control and Prevention committee provided more evidence that the Covid-19 vaccines provided robust protection against severe disease through July, after the Delta variant of the coronavirus had spread widely through the United States.
Scientists also confirmed that the Pfizer-BioNTech and Moderna shots confer a small risk of heart problems in younger men, but that the benefits still outweighed the risks.
At the committee’s meeting on Monday, Dr. Sara Oliver, a C.D.C. scientist, presented unpublished data from Covid-Net, a hospital surveillance system. All three vaccines used in the United States remained highly effective at preventing hospitalizations from April through July, when Delta became dominant, the data suggested.
For adults under the age of 75, the shots were at least 94 percent effective at preventing hospitalizations, a rate that has remained steady for months, Dr. Oliver said. Protection against hospitalization did decline in July for adults 75 or older, but still remained above 80 percent.
“Covid vaccines continue to maintain high protection against severe disease, hospitalization and death,” Dr. Oliver said.
Protection against infection or mild disease does appear to have declined somewhat in recent months, however. “These reasons for lower effectiveness likely include both waning over time and the Delta variant,” she said.
The data comes in the midst of an ongoing debate about the necessity and timing of booster doses. On Aug. 18, health officials recommended that adults who received either the Pfizer or Moderna vaccines get a third shot eight months after their second dose. If the F.D.A. clears the booster shots, they will be available beginning Sept. 20, top federal health officials have said.
The recommendation was based on data suggesting that the vaccines may become less effective at protecting against infection and mild disease over time. But the shots still work well against severe disease and death, and many scientists have criticized the plan for booster shots, saying that it’s not yet clear that they’re needed.
The C.D.C. advisory committee will review additional data on the safety, effectiveness and potential need for booster doses at a meeting in September.
However, getting shots to unvaccinated people should continue to be the top priority, Dr. Oliver said: “Planning for delivery of booster doses to vaccinated individuals should not deter outreach for delivery of primary series to unvaccinated individuals.”
The committee unanimously voted to recommend the Pfizer-BioNTech vaccine, which was approved by the Food and Drug Administration last week, for Americans 16 or older.
Scientists also presented to the committee new data on the risks of two heart conditions following vaccination: myocarditis, an inflammation of the heart muscle, and pericarditis, an inflammation of the membrane that surrounds the heart.
The side effects tend to be mild, temporary, and uncommon, the data confirmed. For every million doses of the second shot given to 12- to 39-year-olds, there were 14 to 20 extra cases of the heart problems, according to the new data, which was presented Monday at a meeting of an independent advisory committee to the C.D.C.
“The data suggest an association of myocarditis with mRNA vaccination in adolescents and young adults,” Dr. Grace Lee, a pediatrician at Stanford and chair of the committee, said at the meeting on Monday. “Further data are being compiled to understand potential risk factors, optimal management strategies and long-term outcomes.”
But the benefits of the vaccines are substantial, even for those in the highest risk groups. According to an analysis presented by a C.D.C. scientist on Monday, every million doses of the Pfizer vaccine administered to 16- and 17-year-old boys would be expected to cause 73 cases of the heart problems, while preventing more than 56,000 Covid-19 cases and 500 related hospitalizations.
A study published in the New England Journal of Medicine last week reported that the risk of myocarditis was substantially higher after infection with the virus than after vaccination.