The composition of U.S. flu vaccines is reviewed annually and updated as needed to match circulating flu viruses. Flu vaccines protect ..... below. CDC's final estimates for the 2017-2018 season will not become available until after the season is over and may be lower than the interim estimates. read more
Second, because flu viruses are constantly changing, the formulation of the flu vaccine is reviewed each year and sometimes updated to keep up with changing flu viruses. For the best protection, everyone 6 months and older should get vaccinated annually. read more
You are correct that you need a flu vaccine once a year. The influenza virus is highly variable and mutates so frequently that our immune systems simply cannot keep up with it. Generally speaking (from a USA standpoint), the flu vaccine becomes available in late summer/early fall, and is available until the following spring. read more
In a 2007 report, the global capacity of approximately 826 million seasonal influenza vaccine doses (inactivated and live) was double the production of 413 million doses. In an aggressive scenario of producing pandemic influenza vaccines by 2013, only 2.8 billion courses could be produced in a six-month time frame. read more