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In Japan, the Ministry of Health, Labour and Welfare (MHLW) designates one specific virus strain for each component of the quadrivalent seasonal influenza vaccine, and four domestic manufacturers produce egg-based influenza vaccines with the same formulation (inactivated, split-virus) using uniform vaccine strains. Thus, discussions of the development of effective seasonal influenza vaccines so far has focused solely on the antigenic match between the vaccine strains and epidemic viruses. However, in 2017, the Japanese selection system of vaccine viruses demonstrated that even a candidate vaccine virus that is antigenically similar to the predicted circulating viruses is not necessarily suitable for vaccine production, given lower productivity of the vaccine. Taking this experience into account, the MHLW reformed the scheme of vaccine strain selection in 2018, and instructed the Vaccine Epidemiology Research Group created by the MHLW to probe how the virus strains for the seasonal influenza vaccine should be selected in Japan. In this context, a symposium, entitled "Issues of the Present Seasonal Influenza Vaccines and Future Prospects", was held as part of the 22nd Annual Meeting of the Japanese Society for Vaccinology in 2018, and subjects related to the influenza vaccine viruses were discussed among relevant administrators, manufacturers, and researchers. This report summarizes the presentations given at that symposium in order to convey the present scheme of vaccine virus selection, the evaluation of the resulting vaccines, and the efforts at new vaccine formulation in Japan. Notably, from March 2022, the MHLW has launched a discussion of the merits of the seasonal influenza vaccines produced by foreign manufacturers., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: ‘Takao Hozawa is an employee of Denka Co., Ltd. in Tokyo, Japan. All other authors declare that they are free of competing interests.’, (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Manufactured influenza vaccines have to contain a defined amount of hemagglutinin (HA) antigen. Therefore, vaccine viruses with a high HA antigen yield (HAY) are preferable for manufacturing vaccines, particularly vaccines in response to a pandemic, when vaccines need to be rapidly produced. However, the viral properties associated with a high HAY have not yet been fully clarified. To identify the HAY-associated traits, we first propagated 26 H5 candidate vaccine viruses (CVVs) in eggs, which were previously developed based on genetic reassortment methods using master viruses, to determine their total protein yield (TPY), ratio of HA to total viral protein (%-HA content) and HAY. The results revealed that the HAY was correlated with the TPY but not with the %-HA content. We further found that altering the sequences of the 3' noncoding region of HA vRNA or replacing the master virus improved the HAYs and TPYs of the low-HAY CVVs to approximately double the values of the original CVVs but did not change the %-HA content, which a previous study suggested was associated with the HAY. Analyses based on real-time PCR assays and scanning electron microscopy revealed that the virus samples with an improved HAY contained more copies of the virus genome and viral particles than the original samples. The results suggest that an improvement in virus growth (i.e., an increase in the amount of viral particles) leads to an increase in the TPY and thus in the HAY, regardless of the %-HA content. The approximately twofold increase in the HAY shown in this study may not appear to represent a large improvement, but the impact will be significant given the millions of chicken eggs used to produce vaccines. These findings will be informative for developing high-HAY vaccine viruses., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2023 Arita et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

The practical use of cell-based seasonal influenza vaccines is currently being considered in Japan. From the perspective of adventitious virus contamination, we assessed the suitability of NIID-MDCK cells (NIID-MDCK-Cs) as a safe substrate for the isolation of influenza viruses from clinical specimens. We first established a sensitive multiplex real-time PCR system to screen for 27 respiratory viruses and used it on 34 virus samples that were isolated by passaging influenza-positive clinical specimens in NIID-MDCK-Cs. Incidentally, the limit of detection (LOD) of the system was 100 or fewer genome copies per reaction. In addition to influenza viruses, human enterovirus 68 (HEV-D68) genomes were detected in two samples after two or three passages in NIID-MDCK-Cs. To further investigate the susceptibility of NIID-MDCK-Cs to adventitious viruses, eight common respiratory viruses were subjected to passages in NIID-MDCK-Cs. The genome copy numbers of seven viruses other than parainfluenza 3 decreased below the LOD by passage 4. By passaging in NIID-MDCK-Cs, the genome numbers of the input HEV-D68, 1 × 10 8 copies, declined to 10 2 at passage 3 and to under the LOD at passage 4, whereas those of the other six viruses were under the LOD by passage 3. These results implied that during the process of isolating influenza viruses with NIID-MDCK-Cs, contaminating viruses other than parainfluenza 3 can be efficiently removed by passages in NIID-MDCK-Cs. NIID-MDCK-Cs could be a safe substrate for isolating influenza viruses that can be used to develop cell-based influenza vaccine candidate viruses., (© 2022 The Societies and John Wiley & Sons Australia, Ltd.)

Two phenotypes of human influenza A(H1N1) virus are currently circulating in Japan. One (group 1) agglutinates both chicken and goose red blood cells (CRBC and GRBC), the other (group 2) agglutinates GRBC but not CRBC. In the 1995/6 season, group 2 viruses accounted for 70% of the H1N1 viruses isolated in MDCK cells. The 1995/6 viruses were located on two branches of the genetic tree. One branch contained both group 1 and group 2 viruses and the other branch contained only group 2 viruses. Group 2 viruses had aspartic acid at residue 225 in the haemagglutinin (HA) protein, the key amino acid residue for group 2 phenotype. The HA protein of group 1 viruses had a change from aspartic acid to asparagine at residue 225 and the expressed HA protein of these viruses adsorbed CRBC. Serial passage of group 2 viruses in MDCK cells or embryonated chicken eggs caused these viruses to gain the ability to agglutinate CRBC. MDCK-adapted viruses had the same amino acid sequences of HA polypeptide as the original ones, but egg-adapted viruses had changed amino acid sequences. The expressed HA protein from one egg-adapted virus that originally belonged to group 2 adsorbed CRBC.