CARL O’DONNELL Pfizer Inc and BioNTech have revealed that their COVID-19 vaccine is around 91% effective at preventing the disease, citing updated trial data that included participants inoculated for up to six months. The shot was also 100% effective in preventing illness among trial participants in South Africa, where a new variant called B1351 is dominant, although that rate was derived from a relatively small number of nine infections observed there, which were all in the placebo group, Pfizer said. While the new overall efficacy rate of 91.3% is lower than the 95% originally reported in November for its 44,000-person…