Public health officials across the United States are preparing for the biggest vaccination effort in U.S. history. That is because they are likely to approve within weeks a vaccine against the new coronavirus, which causes COVID-19. As a result, they will need to give out hundreds of millions of doses of vaccine and decide who gets the vaccine first. The effort will involve making sure people who get the first vaccine shot return for the second one, which is required. Drug maker Pfizer recently raised hopes by saying early trials suggest its vaccine is 90 percent effective. The biotechnology company Moderna reported success Monday with a second vaccine in a major study. The company said the vaccine appears to be 95 percent effective, based on early data. The push could begin as early as next month, when federal officials say the first vaccine may be approved for emergency use. Federal officials have stated that, once that happens, they will immediately provide doses to high-risk groups, such as health care workers.