WHY DO PEOPLE RECEIVED INFLUENZA VACCINE LAST YEAR STILL NEED TO GET VACCINATED THIS WHEN THE VIRUSES HAVEN'T CHANGED?

Although the strains may sometimes be the same as in the previous year's vaccine, you should NOT use the previous season's vaccine you might still have in your refrigerator. Influenza vaccine distributed in the northern hemisphere expires on June 30 after each season; expired vaccine should NEVER be administered. Secondly, antibody titers that persons might have achieved from the previous year's vaccination will have waned and need to be boosted with a dose of the current year's vaccine.