ABC's Lisa Stark reports from Washington: As government health officials encourage people to get vaccinated against the H1N1 vaccine, they are running into opposition from an unusual source: doctors. Some doctors have decided they will not give the vaccine, and are telling patients not to get it. Those doctors have been quoted as saying they are concerned that the vaccine is too new. Today, the head of the Food and Drug Administration sent a letter to physicians to explain how the vaccine was developed and how its safety is being monitored. The letter, from FDA Commissioner Dr. Margaret Hamburg, tells physicians that “understanding more about the manufacturing and approval process for these vaccines should help you to answer (patient) questions." It points out that the H1N1 vaccine was developed in exactly the same way as the seasonal flu vaccine. The process, writes Dr. Hamburg, has “produced effective and very safe vaccines time and time again.” She says that the companies that are manufacturing the H1N1 vaccine are the using “same factories where they are licensed to manufacture seasonal influenza vaccines – using the same equipment and the same testing procedures.” The FDA also reminds doctors that in the clinical trials involving 3600 patients, there have been no adverse effects. Finally, the agency says it is looking for any rare and serious side effects, and encourages doctors to report any adverse effects they see. “We are not cutting any corners,” insists Hamburg, and see adds, “The benefits of preventing serious consequences from infection with the 2009 H1N1 influenza virus far outweigh the risks associated with vaccination.” It’s a message that has made sense to many patients who have waited hours in line for the vaccine, which is still in limited supply. It’s a message the government is hoping all doctors will understand as well.