welcome to healthcare magic
i think the feeling of heaviness in the head is due to the illness itself because of running nose and nasal congestion
are congested making the head feel heavy,
i dont think there is anything serious going on once the illness will be cured the symptoms will disappear.
doesnt cover all the strains so you can catch influenza
but effects will be less compared to one who isnt vaccinated at all
hope to have answered your query