The onset of the COVID-19 pandemic was accompanied with a pandemic of fake news spreading over social media (SM). Fact checking might help combat fake news and a plethora of fact-checking platforms exist, yet few people actually use them. Moreover, whether fact checking is effective in preventing citizens from falling for fake news, particularly COVID-19 related, is unclear. Against this backdrop, we examine potential antecedents to fact checking that can be a target for interventions and establish that fact checking is actually effective for preventing the public from falling for harmful COVID-19 fake news. We use a representative U.S. sample collected in April of 2020 and find that awareness of fake news and patterns of active SM use (e.g., commenting on content instead of reading it) increases the fact checking of COVID-19 fake news, whereas SM homophily reduces fact checking and the effects of SM use as users are trapped in “echo chambers”. We also find that fact checking helps users identify accurate information on how to protect themselves against COVID-19 instead of false and often harmful claims propagated on SM. These findings highlight the importance of fact checking for combating COVID-19 fake news and help identify potential interventions.