Eric Topol, a professor of molecular medicine at Scripps Research in San Diego, agreed with Walensky that asymptomatic breakthrough cases are not as important to follow. Nonetheless, he argued, the CDC should monitor those people, along with hospitalized and fatal cases, to determine whether and how virus variants might evade vaccine protection, help discover new variants and track how well certain vulnerable groups, such as the immunocompromised, are shielded by vaccines.