With more than 40 million Americans having received at least the first dose of the Pfizer or Moderna vaccine, a third dose may be needed to prevent the spread of new variants of the disease, Bill Gates told CBS News Tuesday. Gates’ comments come amid growing concern that the current vaccines are not effective against the more contagious Brazilian and South African variants.Pfizer and Moderna have stated that their vaccines are 95% and 99% effective, respectively, against the initial strain of COVID. However, some scientists have questioned those statements. Additionally, the World Health Organization and vaccine companies have conceded that...