HI assay is accepted as a standard laboratory procedure for the classification or subtyping of hemagglutinating viruses. For influenza virus, HI assay is adapted to identify the hemagglutinin subtype specificity of antibodies to influenza virus (Pedersen et al. 2014). In the context that there were an abrupt spread of influenza B/Y lineage virus in 2017–2018 winter, we performed HI antibody screening using cryopreserved sera samples, aimed to evaluate the HI antibody transition after the outbreak of B/Y lineage. By setting 1:80 as cut-off value, the positivity of HI titer and GMT in both B/Y and B/V lineages were compared between the year of 2017–2018 to previous year of 2016–2017. Results indicated that the positivity of total HI titer in 2017–2018 was significantly higher than that in 2016–2017, and the GMT was also significantly increased in 2017–2018 than in the previous year. However, we did not find any difference in HI titer between B/Y and B/V in the two consecutive years. In 2016–2017, positivity and GMT of HI titer for both B/Y and B/V were at a low level. Conversely, in 2017–2018, the two indicators increased simultaneously. It was indicated in many reports that B/Y prevalence account for severe spreading of influenza during that time (Dayan. 2018). The HI antibody to B/Y lineage with percentage of 66.1%, far exceeds reported positivity of antigen tests to Flu B at around 10–20%, with research proved it that positivity of RT-PCR assay to Flu B at 38% in ILI cases during 2017–2018 winter (Zhu et al. 2019), suggesting recessive infection would be a primary reason for acquiring the HI antibodies under Flu B epidemic.

Throughout the years, influenza A viruses have attracted a great deal of attention and have caused several global pandemics due to their strong transmission capacity and great variability. In contrast, there is less literature focused on the epidemiology and societal burdens of influenza B (Caini et al. 2015). Sudden outbreak of Flu B/Y virus during the 2017–2018 winter may reflect changes of the viral fitness (Virk et al. 2019), as well as low herd immunity against this leading strain. The B/Y variant become to dominate might reflect the vaccine effectiveness (VE) or natural situation cannot provide adequate protective immunity against this strain. In this study, low GMT of HI titer level of B/Y and B/V in previous epidemic year (2016–2017) add value to this assumption and provide a platform to the emergence of the B/Y epidemic. HI titer could not only serve as an indicator for the protective immunity after immunization, but also hint infection/transmission status in a well-defined population (Bodewes et al. 2011). Flu B HI titer was detected at very low incidence in ILI patients during 2018–2019 seasonal flu epidemic (unpublished data by our group), which could be attributed to previous year’s high incidence of HI antibodies to B/Y and B/V lineages.

In China, the public has gradually developed a strong “vaccine hesitancy”. Individuals even those with high risk are reluctant or refuse to be vaccinated despite the availability of influenza vaccine. Influenza vaccination coverage rate in China was at 1.5% of the total population between 2004 and 2014 (Yang et al. 2016). It can be presumed the establishment of herd immunity against influenza virus mainly relies on the passive immunization by natural infection. Low immunization coverage and lack of B/Y antigen in current trivalent influenza vaccine (TIV) in China (before 2018–2019 flu season) may offer access to the dominance and prevalence of B/Y virus. Long-term co-circulating with viruses of Flu A H1/H3 subtypes, accompanied by decreased HI antibody to both Flu B virus strains may serve as an immunological niche for the B/Y lineage virus spreading in China during the 2017–2018 winter. Our findings have implications that quadrivalent influenza vaccines (QIV) should be timely updated for better prevention against seasonal influenza epidemics.

It has been well documented that antisera raised against Victoria lineage of Flu B virus shared low cross reaction with that against Yamagata lineage (Rota et al. 1992). However, whether the distinct immunodominance of the two lineages of Flu B virus could affect or elicit more potent cross react or protective antibodies based on previous research of infection needs in-depth investigations. Same situation had been occurred with Flu A as previous seasonal H1N1 strain was replaced by pdm/H1N1/2009 virus, and inducing cross protection antibody response in infections (Victora and Wilson, 2015; Andrews et al. 2015). On the background, HI assay could be used as evaluation of infection and herd immunity but not suitable for diagnosis in clinical for cross reaction between B/Y and B/V lineages from previous infection could not be excluded.

In summary, our study indicates that there was a significantly elevation of HI antibodies to influenza viruses B in children with ARI in 2017–2018, compared with children in 2016–2017. On the other hand, the low level of HI antibodies to both B/Y and B/V in 2016–2017 could contribute to the severe B/Y epidemic in 2017–2018 to some extent. We aimed to offer an insight into the dynamics of FluB viruses epidemiology. Serology results provide important context for B/Yamagata virus may act as the predominate strain attacking children during 2017–2018 flu season. This results also were convinced from phylogenetic analyses of HA genes (Yan et al. 2020). Limitations exist in the study, First of all, we used preserved clinical samples from children with ARI to perform the cross-sectional study, the sera were not so typical and previous infection or vaccination might affect production of HI titers. Furthermore, the sera samples preserved in 2016–2017 were not sufficient enough, we had to make a composition using samples from children with or without ARI together, after excluding difference between the two groups, and to compare to those from 2017–2018. At last, since we did not perform HI assay to Flu A and other strain of Flu B/Y, we are not sure the influence of HI reaction to Flu B.