Why don’t we have data on breakthrough cases?
Up until May 1, 2021, the CDC recorded all breakthrough cases, whether mild, moderate, or severe. But starting on May 1, they began recording only breakthrough cases that resulted in hospitalization or death. The CDC explains that they made this change to “help maximize the quality of data collected on cases of greatest clinical and public health importance.”
But this means that in the U.S. we do not know how often breakthrough cases occur or which variants are causing breakthroughs (though given that by August 21, nearly 99% of COVID-19 cases in the U.S. were from the Delta variant, it is probable that many of the breakthrough cases, at least recently, are from Delta).
So, should you worry that a fully vaccinated person who may have an asymptomatic breakthrough case can unknowingly transmit the virus to someone who is unvaccinated?
I would say it’s not likely, but we don’t yet know with certainty, says Dr. Meyer. “Theoretically, it may be true that people are being infected by their asymptomatic vaccinated contacts, but we’re just not seeing that clinically,” she says. “And the verdict is out on this in terms of the epidemiologic data.”