At least 10,000 vaccinated people were infected with the coronavirus through the end of April. Now the agency has stopped pursuing the mildest cases.
Julie Cohn was fully vaccinated when she went to cheer at her 12-year-old son’s basketball game in March. Midway through the match, he asked to remove his mask because he was getting so hot. She thought little of it.
Three days later, he had a sore throat. He tested positive for the coronavirus, as did his younger brother a few days later. Ms. Cohn cared for them, thinking she was protected, but she woke up with what seemed like a head cold on March 28. The next day, she, too, tested positive.
No vaccine provides perfect protection, and so-called breakthrough infections after coronavirus vaccination are rare and unlikely to lead to serious illness. Federal health officials have told fully vaccinated people they no longer need to wear masks or maintain social distance because they are protected, nor do they need to be tested or quarantine after an exposure, unless they develop symptoms.
Now, the Centers for Disease Control and Prevention has stopped investigating breakthrough infections among fully vaccinated people unless they become so sick that they are hospitalized or die.
Earlier this year, the agency was monitoring all cases. Through the end of April, when some 101 million Americans had been vaccinated, the C.D.C. had received 10,262 reports of breakthrough infections from 46 states and territories, a number that was very likely “a substantial undercount,” according to a C.D.C. report issued on Tuesday.
Genomic sequencing could be done on only 555, or about 5 percent, of the reported breakthrough cases. Over half of them involved so-called variants of concern, including the B.1.1.7 and B.1.351 variants.