The Centers for Disease Control and Prevention has stopped tracking every case that occurs when a COVID-19 vaccine fails to protect someone. Instead, the agency is focusing on people who get very sick or die.
The decision is controversial. Critics argue the strategy could miss important information that could leave the U.S. vulnerable, including early signs of new variants that are better at outsmarting the vaccines.
The CDC and outside infectious disease experts, however, say it makes sense to prioritize efforts on those who get hospitalized or die.
"These vaccines were studied to prevent severe illness, hospitalization and deaths. And as we look at these breakthrough infections, these are the ones we're most concerned about," CDC Director Rochelle Walensky said at a White House COVID-19 briefing on May 18.
The COVID-19 vaccines are very effective, but people can get infected in what appear to be rare cases. More than 10,000 of the 101 million people who were fully vaccinated as of April 30 caught the virus, according to a recent CDC report.
Most of those who get infected despite being vaccinated experience either no symptoms or very mild illness. But at least 2,298 fully vaccinated people have been hospitalized, and at least 439 people have died from COVID-19, according to the CDC.
The CDC had been reporting all breakthrough infections but decided to focus on hospitalized cases and deaths beginning May 1 because those are the most serious and appear to be most likely to yield useful information, Walensky said.
"What we were starting to find is a large portion of them were fully asymptomatic, and in fact when we went to study them and sequence them, there was inadequate virus to even do so," Walensky said.
If scientists can't sequence genes from the virus, there's not much chance these people are contagious, and there's not much scientists can learn about the virus by studying them, the CDC and other researchers say.
But critics say the CDC should be casting a much wider net.
"Just looking at hospitalizations or cases from people who die is really keeping, I believe, blindfolds on your eyes and not fully understanding what's happening with this virus," says Rick Bright, a former federal health official who's now with the Rockefeller Foundation. "It puts us at a disadvantage of better understanding this virus and how to end the pandemic."
Investigating the full spectrum of breakthrough infections could provide crucial information, including clues about whether some vaccines are working better than others and whether breakthroughs are happening more in some people than others, Bright and others say.
In addition, carefully studying all breakthrough infections is critical to determine whether the variants are breaking through the vaccines more than expected, and whether new mutants have emerged that can outsmart the vaccines, the critics say.
"These variants are spreading, and if you're just looking at the small percentage, then you're really missing the big picture," Bright says. "You're missing the big story of where the virus is and how it's changing."
At the very least, the CDC should be regularly sequencing the genetic code of a random sample of virus from all types of breakthrough infections, according to Saad Omer, a vaccine researcher at Yale University.
"If there is a new variant or there is a change in frequency of a variant, you might want to find out earlier rather than wait for it to appear in severe and hospitalized cases," Omer says. "That gives you the ability to be ahead of the outbreak rather than follow it."
Careful study of breakthrough infections also could provide useful information for improving the vaccines as well as revelations about possible long-term health effects of these infections on people who don't initially develop symptoms or only become mildly ill.
"To say that a breakthrough infection has no clinical consequence, it feels too early to say that," says Kavita Patel, a primary care physician at the Brookings Institution. "I just feel like we're just way too early in this pandemic to kind of write off the value of information at this point"
CDC officials say the agency isn't ignoring other breakthroughs entirely. The agency is involved in a range of research projects aimed at determining how effective the vaccines are, including in specific groups such as health care workers and at nursing homes.
"CDC will continue to look at vaccine effectiveness in all populations including people with mild infections, and that's being done through special studies, vaccine effectiveness studies and other monitoring in different populations and settings," Dr. Marc Fischer, who's leading the CDC's vaccine breakthrough team, told NPR.
At least some of those studies include examining the role of variants, according to Dr. Jennifer Verani, who's helping lead the CDC's vaccine effectiveness team.
"We recognize the importance of variants of concern in terms of assessing vaccine effectiveness," Verani said in an interview. "So we are wherever possible aiming to access vaccine effectiveness against specific variants."
In addition, some individual states and independent scientists are investigating all breakthrough infections more closely. "I just think from a strategy and prioritizing standpoint they are doing the right thing," says Dr. Carlos del Rio, an infectious disease researcher at Emory University. "They're really focusing on what matters."
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