The Centers for Disease Control and Prevention closely monitors Covid-19 cases in people who have been fully vaccinated, said Dr. Rochelle Walensky, agency director, said Thursday during a hearing before the House Select subcommittee on the coronavirus crisis.
The CDC said Thursday morning that it has reports of about 5,800 so-called breakthrough infections, among the nearly 77 million individuals in the United States who have been fully vaccinated.
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Such cases are not unexpected; no vaccine is 100 percent effective against coronavirus infection. Yet it is rare, and experts tell NBC News that the data is largely reassuring.
“This is a very good scenario, even with nearly 6,000 breakthrough infections,” says Tara Smith, a professor of epidemiology at Kent State University College of Public Health in Ohio. “Most of it was slightly symptomatic or asymptomatic. That’s exactly what we were hoping for.”
In a statement by email, a CDC spokesman said the majority of people with breakthrough infections were not hospitalized, and that 29 percent had asymptomatic infections.
In addition, 65 percent of the cases were in women and just over 40 percent in people 60 years and older. Seven percent of the people were admitted to the hospital, and 74 people died.
The data is not yet publicly available, but the CDC expects to regularly publish information on breakthrough infections on its website from Monday. The agency also plans to collect additional data on the cases, including demographics, type of vaccination, type of variant, location and the length of time since the individuals completed their vaccinations.
“We reach out to all our state health officials, as well as to our hospitals,” Walensky said.
Demographics of patients who may explain an increased risk of breakthrough infections remain largely unclear on Thursday. The CDC did not want to provide further clarification on the cases, including whether the patients had other risk factors, such as the immune system that was affected, and whether they continued the mitigation measures after vaccinations, such as wearing masks.
“I would like to receive information about their clinical symptoms, whether these people are getting sick from Covid and whether it is actually the cause of their hospitalization or their death,” said Dr. Colleen Kelley, associate professor of infectious diseases at the Emory, said University School of Medicine.
It is also unclear why asymptomatic individuals were tested on Covid-19 after being fully vaccinated. It may be that some employers, such as healthcare systems, need to be tested regularly. What’s more, many hospital systems still have a protocol to test for Covid-19 for any hospitalization, such as heart attacks or routine inpatient procedures.
“We’ve seen some breakthroughs ourselves,” Kelley said. “Often it does not have many symptoms, and they are admitted to the hospital for other reasons.”
“If you are asymptomatic and vaccinated, you are unlikely to be present for testing,” said Bill Hanage, associate professor of epidemiology at the Harvard TH Chan School of Public Health. As such, the number of breakthrough infections reported to the CDC is “probably an underestimation.”
People are considered fully vaccinated two weeks after receiving their second dose of dual-dose vaccine, such as the Pfizer-BioNTech or Modern vaccines, or two weeks after a single dose of Johnson & Johnson vaccine. (The use of the Johnson & Johnson vaccine is currently being halted in the U.S. as federal health officials investigate cases of rare blood clots linked to the shots.)
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In clinical trials, the Pfizer and Moderna vaccines were found to be approximately 95 percent effective against Covid-19, and the Johnson & Johnson vaccine was 72 percent effective against moderate to severe diseases in the US trial.
How well the vaccines work in the real world can still differ from the clinical trial results. A CDC study published in March found that the Pfizer and Moderna vaccines were about 90 percent effective against infection, based on actual data.
Vaccination remains critical to ending the pandemic.
“It’s a way to take the pandemic and turn it from a horrible, horrible, unmanageable problem into a manageable problem,” Hanage said.
“It does not reduce the risk to zero, but it does reduce the risk to something we can handle.”
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