Major Flu Vaccine Antigen (image) University of Pennsylvania School of Medicine Share Print E-Mail Caption The major flu vaccine antigen, hemagglutinin, is shown. Colored in dark blue is a region of the vaccine that differs between the current egg-grown H3N2 vaccine strain and circulating H3N2 viral strains. Credit Zost et al, PNAS, 2017 Usage Restrictions None Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.