Pneumonia can be a life threatening situation. Many people die each year due to pneumonia from the Influenza virus or to secondary bacterial pneumonia. A yearly flu shot is required because the Influenza viruses change so frequently. The common Influenza viruses of last year will not be the common ones we see this year. Therefore every year vaccine manufacturers must make a new vaccine.
Influenza viruses can be quite different from each other.
Others grow well but do not make many viruses per egg. As a result some years flu vaccines can be very difficult to make in mass quantities. Last year the United States alone used 76 million doses of the flu vaccine. This year has been a problem year for the manufacture of the flu vaccine. This has caused a delay in supplying the vaccine to health care providers.