3.1 EA H1N1 SIVs form two distinct antigenic groups.
Preexisting immunity can protect hosts from related influenza viruses, but antigenic drift can decrease the protective efficacy. Yang et al. previously evaluated the antigenic relationship of EA H1N1 SIVs isolated from 2010 to 2013(Yang et al., 2016), and we quantitatively analyzed their cross-reactivity with ferret antisera using the methods described by Smith et al .(Smith et al., 2004). The 10 EA H1N1 SIVs could be classified into two different antigenic groups A and B (Figure 1). Most of them were in group A, while two viruses, A/swine/Guangdong/104/2013 and A/swine/Guangdong/306/2013, belonged to group B. These results demonstrated that the isolated EA H1N1 SIVs form two distinct antigenic groups, but the underlying genetic determinants for the antigenic variation remains unclear.