Repeated flu shots may blunt effectiveness

"In the September 2014 issue of Clinical Infectious Diseases, US investigators documented a lower vaccine effectiveness in those who’d had flu vaccine over the previous five years, compared to those who’d had only the previous year’s vaccine or no vaccine previously.

 

The findings are also consistent with a 1999 observation of variable flu vaccine efficacy depending on immunization history, by Derek J. Smith, the current director, WHO Collaborating Centre for Modelling, Evolution and Control of Emerging Infectious Diseases."

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"..“old” antibodies may mop up the current vaccine’s antigen before the newly vaccinated person can mount an immune response to it (this is called antibody interference)."

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4387051/

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Intraseason Waning of Influenza Vaccine Effectiveness

"The odds ratio (OR) increased linearly by approximately 16% for each additional 28 days since vaccination."

"Our results suggest that effectiveness of inactivated influenza vaccine wanes during the course of a single season."

https://academic.oup.com/cid/article/68/10/1623/5094689

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Seasonal Flu Vaccine Effectiveness 

https://www.cdc.gov/flu/vaccines-work/effectiveness-studies.htm