WASHINGTON: The efficacy of third doses of the Pfizer and Moderna mRNA vaccines wanes substantially by the fourth month after administration, a new study by the US Centers of Disease Control and Prevention (CDC) said Friday. Though it’s now well documented that vaccine efficacy goes down after two doses, relatively little has been published on the duration of protection after a booster. The new study was based on more than 241,204 visits to the emergency department or an urgent care clinic, and 93,408 hospitalisations, which are more serious, among adults with Covid-19–like illness during Aug …