Since the vaccines have only recently entered the scene it would be surprising to find masses of reports of any effect on breastfeeding.  FWIW I felt like crud the day after having my first Pandemrix injection last Friday.  I didn't stay home and in bed, wasn't that bad, but I didn't have much energy and I felt stiffer and achier than usual.  The very next day I felt fine and have continued to do so.  
I can't think of a physiologic reason for someone's milk production to cease immediately after getting immunized against anything and I would approach it as I would any case in which there is sudden drop in supply.

Rachel Myr, who is aware that Pandemrix contains 0.001% thimerosal and was still happy to be offered the vaccine, in Kristiansand, Norway

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