Manufacturing
Research continues into the idea of a "universal" influenza vaccine that would not require tailoring to a particular strain, but would be effective against a broad variety of influenza viruses. No vaccine candidates had been announced by November 2007, but as of 2021, there are several universal vaccines candidates, in pre-clinical development and in clinical trials.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. If all high- and upper-middle-income countries sought vaccines for their entire populations in a pandemic, nearly two billion courses would be required. If China pursued this goal as well, more than three billion courses would be required to serve these populations. Vaccine research and development is ongoing to identify novel vaccine approaches that could produce much greater quantities of vaccine at a price that is affordable to the global population. Egg-based
Most flu vaccines are grown by vaccine manufacturers in fertilized chicken eggs. In the Northern hemisphere, the manufacturing process begins following the announcement (typically in February) of the WHO recommended strains for the winter flu season. Three strains (representing an H1N1, an H3N2, and a B strain) of flu are selected and chicken eggs are inoculated separately. These monovalent harvests are then combined to make the trivalent vaccine.
Notes
See also
H5N1 vaccine
Seasonal influenza vaccine brands
Universal influenza vaccine

References
Further reading
World Health Organization (October 2017). The immunological basis for immunization series: module 23: influenza vaccines. World Health Organization (WHO). hdl:10665/259211. ISBN 978-9241513050. Ramsay M (ed.). "Chapter 19: Influenza". Immunisation against infectious disease. Public Health England. Hamborsky J, Kroger A, Wolfe S, eds. (2015). "Chapter 12: Influenza". Epidemiology and Prevention of Vaccine-Preventable Diseases (13th ed.). Washington D.C.: U.S. Centers for Disease Control and Prevention (CDC). ISBN 978-0990449119. Budd A, Blanton L, Grohskopf L, Campbell A, Dugan V, Wentworth DE,  et al. (March 29, 2019). "Chapter 6: Influenza". In Roush SW, Baldy LM, Hall MA (eds.). Manual for the surveillance of vaccine-preventable diseases. Atlanta GA: U.S. Centers for Disease Control and Prevention (CDC). National Advisory Committee on Immunization (May 2020). "Canadian Immunization Guide Chapter on Influenza and Statement on Seasonal Influenza Vaccine for 2020–2021" (PDF). Public Health Agency of Canada. Cat. : HP37-25F-PDF; Pub. : 200003. Lay summary. {{cite web}}: Cite uses deprecated parameter |lay-url= (help)
National Advisory Committee on Immunization (NACI) (May 2018). NACI literature review on the comparative effectiveness and immunogenicity of subunit and split virus inactivated influenza vaccines in adults 65 years of age and older (PDF). Government of Canada. ISBN 9780660264387. Cat. : HP40-213/2018E-PDF; Pub. : 180039. Lay summary. {{cite book}}: Cite uses deprecated parameter |lay-url= (help)
Rajaram S, Wojcik R, Moore C, Ortiz de Lejarazu R, de Lusignan S, Montomoli E,  et al. (August 2020). "The impact of candidate influenza virus and egg-based manufacture on vaccine effectiveness: Literature review and expert consensus". Vaccine.