Genetic diversity of influenza B virus: the frequent reassortment and cocirculation of the genetically distinct reassortant viruses in a community

To characterize the genetic diversity of influenza B viruses isolated during one influenza season, the antigenic and genetic relationships among 20 strains of influenza B virus isolated in February and March 2001 at one pediatric clinic in Yamagata City, Japan, were investigated. The HA gene and seven other gene segments were phylogenetically divided into three distinct sublineages (Harbin/7/94-, Tokyo/6/98-, and Shiga/T30/98-related lineage) of the Yamagata/16/88-like lineage. The NS genes of the viruses belonging to the Harbin/7/94-related lineage have additional three nucleotides at positions 439-447, and were phylogenetically distinguishable from those of the currently circulating Yamagata/16/88- and Victoria/2/87-like lineages, but were closely related to that of the Yamagata/16/88-like lineage isolated before 1994. Moreover, four strains of influenza B virus isolated in the same community between 2002 and 2003 were further examined. Phylogenetic analysis revealed that a virus of Victoria/2/87-like lineage isolated in 2003 had acquired the NA, NS, M, and PA gene segments from a Shiga/T30/98-like virus, and two strains of Harbin/7/94-related lineage had acquired the various gene segments from Shiga/T30/98-like virus through a reassortment event. These results indicate that genetically distinct multiple viruses can combine to cause an influenza B epidemic in a community and that the frequent reassortment among these viruses plays a role in generating the genetic diversity of influenza B viruses.     

Detection and characterization of new influenza B virus variants in 2002

One-hundred five influenza B-positive specimens obtained from southeast Asia in 2002 were categorized on the basis of DNA sequencing of HA1 gene as well as real-time PCR analysis of the NA gene. Phylogenetic analysis of the HA1 gene sequences showed that the majority of the viruses (96.2%) belonged to the B/Victoria/2/87 lineage, while a smaller percentage of the viruses (3.8%) belonged to the B/Yamagata/16/88 lineage. The B/Yamagata/16/88 viruses displayed significant antigenic drift in the deduced amino acid sequences of the HA1 protein, and the B/Victoria/2/87-like viruses consisted of B/Hong Kong/1351/02-like (72.3%) and B/Hong Kong/330/01-like (27.7%) viruses. The B/Hong Kong/1351/02-like viruses were reassortants with the HA gene belonging to the B/Victoria/2/87 lineage and the NA gene belonging to the B/Yamagata/16/88 lineage, whereas both the HA and NA genes of B/Hong Kong/330/01 virus belonged to the B/Victoria/2/87 lineage. In this study, however, all the B/Hong Kong/330/01-like isolates exhibited the B/Yamagata/16/88-like NA gene, which likely resulted from reassortment of B/Hong Kong/330/01 and B/Hong Kong/1351/02 viruses during coinfection. Additional molecular characterization of the six internal genes showed that the M, NS, PA, and PB2 genes of the new variants were B/Hong Kong/1351/02 in origin, whereas the NP and PA genes retained the B/Hong Kong/330/01 origin. Interestingly, these new variants all appeared late in the year 2002. These results support the notion that influenza B viruses continued to evolve through antigenic drift and shift.     

Evolution of Surface and Nonstructural-1 Genes of Influenza B Viruses Isolated in the Province of Québec,Canada, during the 1998–2001 Period

After 2 minor winter seasons, influenza B viruses were predominantly isolated in the Province of Quebec, Canada, during the 2000–2001 season representing 74% of laboratory-confirmed influenza viruses. We performed an antigenic study of the hemagglutinin (HA) protein and a molecular characterization of the HA1 region, nonstructural-1 (NS1) and neuraminidase (NA)/NB genes of 20 influenza B strains isolated in the Province of Quebec during the 1998–2001 period. Our isolates were compared to recent vaccine strains (B/Harbin/7/94 in 1998–1999, B/Yamanashi/166/98 in 1999–2000 and 2000–2001, and B/Sichuan/379/99 in 2001–2002). The hemagglutination inhibition (HI) test revealed that all isolates were different from B/Harbin/7/94 and were more related to the 2 other vaccine strains although precise identification was often impossible. Molecular analysis of the HA1 gene revealed that both B/Yamanashi/166/98-like and B/Sichuan/379/99-like isolates co-circulated during the 1998–1999 season whereas isolates from the 2 subsequent years were more related to B/Sichuan/379/99. Most isolates (8/9) of the 2000–2001 season contained a N126D substitution recently associated with altered antigenicity in recent influenza B/Yamagata/16/88-related viruses. Although the HA1 and NS1 protein sequences of viruses isolated during the 1998–1999 season were clearly different from those of the respective vaccine strain (B/Harbin/7/94), the NA protein sequence of those isolates was slightly more related to B/Harbin/7/94 than B/Yamanashi/166/98 suggesting distinct patterns of evolution for these genes. This study confirms the importance of a detailed molecular analysis for understanding the evolution of influenza B viruses.     

Cocirculation and evolution of two lineages of influenza B viruses in europe and Israel in the 2001-2002 season

Forty-nine influenza B virus isolates collected in Belgium, Finland, Spain, and Israel during the 2001-2002 winter season were categorized into either of two lineages, B/Yamagata/16/88 or B/Victoria/2/87, based on the phylogenetic studies of HA1 sequences. The data trace the geographic spread of B/Victoria/2/87-like viruses and support the emergence of B/Hong Kong/1351/02-like viruses, possibly due to selective advantages of reassortment.     

Genetic analysis of an influenza B virus isolated from a patient with encephalopathy in Japan

An influenza B virus, B/Saga/S172/99 (SAG99), was isolated from the nasopharynx of a patient with encephalopathy/encephalitis in Japan in 1999. To clarify the molecular characteristics of this virus, detailed analysis of the gene segments coding for the hemagglutinin (HA), neuraminidase (NA), nucleoprotein (NP), matrix protein (M) and non-structural protein (NS) was undertaken. All five genes of SAG99 showed high nucleotide and predicted amino acid similarities with those of recent non-encephalopathic strains isolated in the same epidemic season. Subsequent phylogenetic analysis revealed that all five gene segments of SAG99 analyzed in the present study were most similar to those of the recent Yamagata/16/88-like viruses. The hemagglutinin and neuraminidase proteins of SAG99 were each distinguished from those of recent epidemic strains by one characteristic amino acid substitution. These substitutions were not found in the previously reported encephalopathy/encephalitis-derived influenza B viruses, and we could not find any common characteristic amino acid changes in SAG99 and these viruses. Similarly, among the internal proteins studied, only the M2 protein of SAG99 was found to contain a single novel amino acid change when compared with other recent isolates. Thus, it was apparent that SAG99 contained very few amino acid differences when compared with other epidemic viruses. The association of recent B/Yamagata/16/88-like viruses with encephalitis/encephalopathy observed in the present study and previously suggest that these viruses may have a higher potential for causing neurological complications in certain individuals.     

Molecular characterization of influenza B viruses circulating in northern Italy during the 2001-2002 epidemic season

During the 2001-2002 influenza season, virological surveillance highlighted the predominant circulation of B viruses (86% of isolates) in Italy, in contrast to many other countries in Europe and North America where AH3N2 viruses were isolated most frequently, and in contrast to the infrequent isolation of B viruses in Italy during the previous two years. Associated with this predominance of influenza B was the re-emergence of B/Victoria/2/87-lineage viruses, closely related to B viruses prevalent during the 1980s, which are distinct antigenically and genetically from circulating B/Sichuan/379/99-like viruses of the B/Yamagata/16/88 lineage, which predominated in most parts of the world during the last 10 years. Ninety-four viruses isolated in two regions of northern Italy were characterized, 50 by direct sequencing of haemagglutinin (HA). Viruses of both Victoria and Yamagata lineages co-circulated throughout the 12 weeks of the influenza season. The HAs of the Yamagata-lineage viruses were heterogeneous and comprised two sublineages, represented by B/Sichuan/379/99 and B/Harbin/7/94, whereas the Victoria-lineage viruses were more homogeneous and closely related to B/Hong Kong/330/01, the current prototype vaccine strain. The antigenic and genetic characteristics of the viruses correlated with certain epidemiological features. In particular, the low age (<14 years) of individuals infected with B/Hong Kong/330/01-like viruses is likely to reflect the greater susceptibility of the youngest cohort, due to lack of previous exposure to Victoria-lineage viruses, and is consistent with the conclusion that vaccination with a B/Sichuan/379/99-like virus would give poor protection against infection with B/Hong Kong/330/01-like (Victoria-lineage) viruses.     

Phylogenetic analysis of influenza B virus in Taiwan, 1997 to 2001.

Seventeen strains of influenza B virus were isolated and identified from 1997 to 2001. Throat swabs were collected in children who presented in medical centers in both central and northern parts of Taiwan. To clarify the molecular characteristics of these isolates, both partial hemagglutinin (HA) gene and nonstructural (NS) gene nucleotide sequences were cloned and subjected to nucleotide sequence analysis. The phylogenetic analysis of the HA gene revealed that 16 out of 17 strains were similar to B/Yamagata/16/88-like virus, but grouped together to form an independent cluster. Only one strain, B/Taiwan/21706/97, was similar to the B/Victoria/2/87-like lineage. In addition, all isolates, except for B/Taiwan/21706/97, were similar to B/Beijing/184/93 and B/Yamanashi/166/98, which were chosen as the recommended vaccine strains in 1999 and 2001. In contrast, the NS gene of these isolates was evolved from B/Guangdong/8/93. Based on the accumulation of antigenic drift in our isolates, we conclude that influenza B virus is still prevalent in Taiwan and the accumulation of nucleotide mutations indicated that our isolates form a new cluster that evolved from the YA88 lineage.     

Cocirculation of two distinct evolutionary lineages of influenza type B virus since 1983

During 1988-1989 two highly distinct antigenic variants of influenza type B were recognized in hemagglutination-inhibition tests with postinfection ferret serum. These viruses were antigenically related to either B/Victoria/2/87, the most recent reference strain, or B/Yamagata/16/88, a variant that was isolated in Japan in May 1988. All influenza B viruses isolated in the United States during an epidemic in the winter of 1988-1989 were antigenically related to B/Victoria/2/87. However, in several countries in Asia, both B/Victoria/2/87-like viruses and B/Yamagata/16/88-like viruses were isolated. Sequence analysis of the hemagglutinin (HA) genes of several influenza B isolates from 1987 to 1988 indicated that the HA1 domains of the B/Yamagata/16/88-like viruses and B/VI/87-like viruses isolated in 1988 differed by 27 amino acids. Evolutionary relationships based on this sequence data indicated that the B/Yamagata/16/88-like viruses were more closely related to epidemic viruses from 1983 (B/USSR/100/83-like viruses) than to more recent reference strains such as B/Victoria/2/87. All other Asian strains, as well as selected isolates from the United States in 1988, were confirmed by sequence analysis as being genetically related to B/Victoria/2/87. These data provide clear evidence that two parallel evolutionary pathways of influenza type B have existed since at least 1983 and that viruses from each of the separate lineages were isolated from cases of influenza B in 1988. This finding is similar to earlier observations for type A H1N1 and H3N2 influenza viruses.     

Increasing appearance of reassortant influenza B virus in Taiwan from 2002 to 2005

Genetic and antigenic analyses of influenza B virus field strains isolated in Taiwan from 1998 to 2005 were performed. To investigate the molecular evolution of influenza B viruses, sequence analysis of the hemagglutinin (HA1 subunit) and neuraminidase genes was performed. All influenza B viruses isolated between 1998 and 2000 belonged to the B/Yamagata/16/88 lineage. The B/Victoria/2/87 lineage, which was cocirculating with the Yamagata lineage, was identified in Taiwan in March 2001. Concurrently, there was an increasing prevalence of this lineage in many parts of the world, including North America and Europe, during the 2001-2002 season. Since 2002, genetic reassortants of influenza B virus with the Victoria lineage of hemagglutinin and the Yamagata lineage of neuraminidase have been found at a rate of 46%. Therefore, in 2002, at least three sublineages of influenza B virus strains, the B/Shanghai/361/2002-like strain (Yamagata lineage), the B/Hong Kong/330/01-like strain (Victoria lineage), and the B/Hong Kong/1351/02-like strain (B reassortant lineage), were identified in Taiwan. The results showed that genetically distinct lineages can cocirculate in the population and that the reassortment among these strains plays a role in generating the genetic diversity of influenza B viruses. Interestingly, from January to April 2005, B reassortant viruses became dominant (73%) in Taiwan, which indicated that a mismatch had occurred between the influenza B vaccine strain recommended for the 2004-2005 season in the Northern hemisphere by the World Health Organization and the epidemic strain.     

Phylogenetic studies of influenza B viruses isolated in southern Africa: 1998-2001

Influenza B viruses isolated in southern Africa during the period from 1998 to 2001 were analysed by sequence analysis of the viral haemagglutinin HA1 subunit and the phylogenetic relationships were determined. Influenza B activity varied considerably in South Africa during the 4-year study period with no activity detected in 2000. Phylogenetic analysis revealed that viruses isolated in 1998 from a localised outbreak in Durban belonged to two distinct sub-lineages. Some of the influenza B viruses isolated throughout South Africa in 1999 as well as several viruses obtained from Mozambique in the same year were closely related to the B/Yamanashi/166/98-like viruses. In contrast, the majority of the 1999 isolates, represented by B/Johannesburg/5/99, exhibited considerable drift from the B/Yamanashi/166/98 stain. The viruses isolated during the 2001 season fell into two sub-lineages, one of which had evolved from the B/Johannesburg/5/99-like viruses and the other which had evolved from the group of viruses that included one of the 1998 Durban isolates. These molecular epidemiological studies reveal a diverse and complex pattern of influenza B virus strains circulating in southern Africa.     

Clinical and genetic characterization of severe influenza B-associated diseases during an outbreak in Taiwan.

BACKGROUND: Mismatches between circulating and vaccine strains of influenza virus had been observed in Taiwan. A comprehensive clinical and genetic analysis of influenza B viruses-associated important diseases was lacking. OBJECTIVES: Clinical and phylogenetic analysis of influenza B viruses during an outbreak in Taiwan. STUDY DESIGNS: Clinical manifestations of hospitalized, culture-confirmed patients were analyzed from July 2004 to June 2005. Partial genome sequence analysis of hemagglutinin (HA), neuraminidase (NA), and nonstructural (NS) genes were performed in 54 influenza B isolates during the study period, and nine srandomly chosen isolates during 2000 and 2003. RESULTS: Three specific diseases were found in these patients, including 13 of encephalitis/encephalopathy, 28 of influenza-associated myositis (IAM), and one of acute respiratory distress syndrome (ARDS). Three phylogenetic groups were identified, including reassortant strains-group 1 (Victoria lineage of HA, Yamagata lineage of NA, clade A of NS), group 2 (Yamagata lineage of HA, Yamagata lineage of NA, clade A of NS), and group 3 (Yamagata lineage of HA, Yamagata lineage of NA, clade B of NS). CONCLUSIONS: Severe influenza B-associated disease in children was not rare and might be fatal. We offered the evidence of co-circulation of the two HA lineages in the same outbreak.     

Genetic evolution of swine influenza A (H3N2) viruses in China from 1970 to 2006

Pigs are susceptible to both human and avian influenza viruses and have been proposed to be intermediate hosts, or mixing vessels, for the generation of pandemic influenza viruses through reassortment or adaptation to the mammalian host. In this study, we summarize and report for the first time the coexistence of wholly human-like H3N2 viruses, double-reassortant H3N2 viruses, and triple-reassortant H3N2 viruses in pigs in China by analyzing the eight genes of swine influenza A (H3N2) viruses found in China from 1970 to 2006. In 1970, the first wholly human-like H3N2 (Hong Kong/68-like) viruses were isolated from pigs in Taiwan, and then in the next years Victoria/75-like, Sydney/97-like, New York/99-like, and Moscow/99-like swine H3N2 viruses were regularly isolated in China. In the 1980s, two triple-reassortant viruses were isolated from pigs. Recently, the double-reassortant viruses containing genes from the human (HA and NA) and avian (PB2, PB1, PA, NP, M, and NS) lineages and the triple-reassortant viruses containing genes from the human (HA and NA), classical swine (NP), and avian (PB2, PB1, PA, M, and NS) lineages emerged in pigs in China. The coexistence of wholly human-like and reassortant viruses provides further evidence that pigs serve as intermediate hosts, or mixing vessels, and emphasizes the importance of reinforcing swine influenza virus surveillance in China.     

Reassortment and evolution of current human influenza A and B viruses

During the 2001-2002 influenza season, human influenza A (H1N2) reassortant viruses were detected globally. The hemagglutinin (HA) of these H1N2 viruses was similar to that of the A/New Caledonia/20/99 (H1N1) vaccine strain both antigenically and genetically, while their neuraminidase (NA) was antigenically and genetically related to that of recent human influenza H3N2 reference viruses such as A/Moscow/10/99. All six internal genes of the H1N2 reassortants originated from an H3N2 virus. After being detected only in eastern Asia during the past 10 years, Influenza B/Victoria/2/87 lineage viruses reappeared in many countries outside of Asia in 2001. Additionally, reassortant influenza B viruses possessing an HA similar to that of B/Shandong/7/97, a recent B/Victoria/2/87 lineage reference strain, and an NA closely related to that of B/Sichuan/379/99, a recent B/Yamagata/16/88 lineage reference strain, were isolated globally and became the predominant influenza B epidemic strain. The current influenza vaccine is expected to provide good protection against H1N2 viruses because it contains A/New Caledonia/20/99 (H1N1) and A/Panama/2007/99 (H3N2) like viruses whose H1 HA or N2 NA are antigenically similar to those of recent circulating H1N2 viruses. On the other hand, widespread circulation of influenza B Victoria lineage viruses required inclusion of a strain from this lineage in influenza vaccines for the 2002-2003 season.     

Phylogenetic analysis of H1N2 isolates of influenza A virus from pigs in the United States

Twenty-four H1N2 influenza A viruses were newly isolated from pigs in the United States. These isolates originated from 19 farms in 9 different swine producing states between 1999 and 2001. All farms had clinical histories of respiratory problem and/or abortion. The viral isolates were characterized genetically to determine the origin of all eight gene segments. The results showed that all H1N2 isolates were reassortants of classical swine H1N1 and triple reassortant H3N2 viruses. The neuraminidase (NA) and PB1 genes of the H1N2 isolates were of human origin, while the hemagglutinin (HA), nucleoprotein (NP), matrix (M), non-structural (NS), PA and PB2 polymerase genes were of avian or swine origin. Fifteen of the 24 H1N2 isolates were shown to have a close phylogenic relationship and high amino acid homology with the first US isolate of H1N2 (A/SW/IN/9K035/99). The remaining nine isolates had a close phylogenic relationship with classical swine influenza H1N1 in the HA gene. All other genes including NA, M, NP, NS, PA, PB1 and PB2 showed a close phylogenic relationship with the H1N2 (A/SW/IN/9K035/99) strain and triple reassortant H3N2 viruses. However, PB1 genes of two isolates (A/SW/KS/13481-S/00, A/SW/KS/13481-T/00) were originated from avian influenza A virus lineage. These results suggest that although there are some variations in the HA genes, the H1N2 viruses prevalent in the US swine population are of a similar genetic lineage.     

Molecular analysis of isolates from influenza B outbreaks in the U.S. and Nepal, 2005

Summary. Currently circulating influenza B viruses can be divided into two antigenically and genetically distinct lineages referred to by their respective prototype strains, B/Yamagata/16/88 and B/Victoria/2/87, based on amino acid differences in the hemagglutinin surface glycoprotein. During May and July 2005, clinical specimens from two early season influenza B outbreaks in Arizona and southeastern Nepal were subjected to antigenic (hemagglutinin inhibition) and nucleotide sequence analysis of hemagglutinin (HA1), neuraminidase (NA), and NB genes. All isolates exhibited little reactivity with the B/Shanghai/361/2002 (B/Yamagata-like) vaccine strain and significantly reduced reactivity with the previous 2003/04 B/Hong Kong/330/2001 (B/Victoria-like) vaccine strain. The majority of isolates were antigenically similar to B/Hawaii/33/2004, a B/Victoria-like reference strain. Sequence analysis indicated that 33 of 34 isolates contained B/Victoria-like HA and B/Yamagata-like NA and NB proteins. Thus, these outbreak isolates are both antigenically and genetically distinct from the current Northern Hemisphere vaccine virus strain as well as the previous 2003–04 B/Hong Kong/330/2001 (B/Victoria lineage) vaccine virus strain but are genetically similar to B/Malaysia/2506/2004, the vaccine strain proposed for the coming seasons in the Northern and Southern Hemispheres. Since these influenza B outbreaks occurred in two very distant geographical locations, these viruses may continue to circulate during the 2006 season, underscoring the importance of rapid molecular monitoring of HA, NA and NB for drift and reassortment.     

Genetic characterization of the hemagglutinin of two strains of influenza B virus co-circulated in Taiwan.

Two isolates of influenza B virus were obtained in the spring of 1997. One strain, B/Taiwan/21706/97, was isolated from a patient who had acute tonsillitis. The other, B/Taiwan/3143/97, was isolated from a patient who was diagnosed with meningoencephalitis. This implies that the influenza B viruses not only cause respiratory symptoms but may also cause inflammation of the nervous system. Sequence analysis of the hemagglutinin (HA) gene, HA1 domain, indicated that there were remarkable amino acid changes in the strain B/Taiwan/3143/97 compared to B/Victoria/2/87, B/Yamagata/16/88, and B/Taiwan/7/88. The changes in the positions 116, 200, 238, 242, and 271 were correlated with receptor binding. Furthermore, a potential glycosylation site at position 233 was lost. In total, 30 amino acid changes were noted at positions ranging from 116 to 295. These changes may affect the antigenicity of the virus. Phylogenetic analyses also showed that the B/Taiwan/3143/97 was located in an independent lineage, when compared to the reference strains belonging to B/Victoria/2/87 and B/Yamagata/16/88 lineages. This supports the hypothesis that influenza B viruses with distinct genetic characteristic were co-circulated in Taiwan.     

Influenza and respiratory syncytial virus in infants and children: relationship with attendance at a paediatric emergency unit and characteristics of the circulating strains

A study was carried out on 2,696 Italian children, aged 0-14 years. The goals were: (1) to define the age-related impact of acute respiratory infections (ARI), measured as the risk of attendance at the Paediatric Emergency Room, (2) to better define the importance and proportion of influenza and respiratory syncytial virus (RSV) infections and (3) to acquire deeper knowledge of the influenza strains circulating in infants and children. A standardised emergency unit attendance risk (EUAR) was calculated, by age group for ARI. Specific EUARs were also calculated for the two pathogens. Pharyngeal swabs were tested by polymerase chain reaction (PCR) for influenza and RSVs. Isolation in Madine-Darby canine kidney cells (MDCK) and Hep cells, haemagglutination inhibition (HI) testing and HA1 gene sequence analysis were performed for influenza viruses. Most of the patients enrolled were aged 0-5 years, 1,139 (84.6%) and 1,061 (78.5%) in the two seasons, respectively. The most represented age class was that of 1 year olds (331 cases in 2001-2002 and 301 in 2002-2003). The highest EUAR for ARI was in patients aged 0-3 years (16.8 and 12.9 during the two seasons). The same was observed on calculating this risk by specific pathogens: 17.4 and 5.5 for influenza and 13.0 and 12.7 for RSV. Virological analysis was performed on 2,696 samples, 595 of which proved positive (22%). The highest number of isolates (326) came from patients aged 1-3 years. RSVs were more often identified than influenza viruses in infants aged up to 1 year (32 vs. 20 isolates). Of 265 strains isolated in 2001-2002, 103 were RSVs (87 type A, 16 B) and 162 were influenza (90 type A, 72 B). HI showed that influenza B viruses were related to two lineages, B/Victoria/2/87 (32%) and B/Yamagata/16/88 (68%). Of 330 strains isolated in 2002-2003, 102 were RSVs (91 type A, 11 B) and 228 were influenza viruses (220 type A, 8 B). A/H3N2 strains belonged to two clusters, A/Panama/2007/99-like and A/Fujian/411/02-like, a new variant. This paper discusses the possible role of the identified flu strains in determining EUARs among the population by age class.     

Laboratory-based surveillance and molecular epidemiology of influenza virus in Taiwan

A laboratory-based surveillance network of 11 clinical virological laboratories for influenza viruses was established in Taiwan under the coordination of the Center for Disease Control and Prevention (CDC), Taiwan. From October 2000 to March 2004, 3,244 influenza viruses were isolated, including 1,969 influenza A and 1,275 influenza B viruses. The influenza infections usually occurred frequently in winter in the northern hemisphere. However, the influenza seasonality in Taiwan was not clear during the four seasons under investigation. For example, the influenza A viruses peaked during the winters of 2001, 2002, and 2003. However, some isolated peaks were also found in the summer and fall (June to November) of 2001 and 2002. An unusual peak of influenza B also occurred in the summer of 2002 (June to August). Phylogenetic analysis shows that influenza A isolates from the same year were often grouped together. However, influenza B isolates from the year 2002 clustered into different groups, and the data indicate that both B/Victoria/2/87-like and B/Yamagata/16/88-like lineages of influenza B viruses were cocirculating. Sequence comparison of epidemic strains versus vaccine strains shows that many vaccine-like Taiwanese strains were circulating at least 2 years before the vaccine strains were introduced. No clear seasonality of influenza reports in Taiwan occurred in contrast to other more continental regions.     

Genetic analysis and evaluation of the reassortment of influenza B viruses isolated in Taiwan during the 2004-2005 and 2006-2007 epidemics

Influenza B viruses were predominant in Taiwan during the 2004-2005 epidemic and both Victoria and Yamagata lineage viruses co-circulated. A reassortant influenza B virus that contained a Victoria lineage hemagglutinin (HA) gene and Yamagata lineage neuraminidase (NA) gene appeared first in 2002 and became predominant during the 2004-2005 epidemic. During the 2006-2007 epidemic, an influenza B outbreak occurred in Taiwan and only Victoria lineage viruses circulated. We characterized the viruses isolated in the 2006-2007 epidemic and found that the HA genes of influenza B viruses from that epidemic were highly similar to those from the 2004-2005 epidemic. We also analyzed the NA genes of isolates from the 2006-2007 epidemic and found that they all belonged to the Yamagata lineage and formed a new genetic subclade. Comparison of isolates from the 2004-2005 and 2006-2007 epidemics revealed four substitutions, N220K, E320D, K343R and E404K in NA genes. Although the HA sequences from the 2006-2007 epidemic were similar to those from the 2004-2005 epidemic, the NA sequences differed, suggesting distinct patterns of evolution of the HA and NA genes from 2004-2007 in Taiwan. This study emphasizes that the evolution of the NA genes may contribute to reemergence of influenza B viruses.     

Antigenic and genetic characterization of influenza C viruses which caused two outbreaks in Yamagata City, Japan, in 1996 and 1998

During the 3 years from January 1996 to December 1998, a total of 33 strains of influenza C virus were isolated from 10,726 throat swab specimens collected from children with acute respiratory illness who visited two pediatric clinics in Yamagata City, Japan. These 33 strains were isolated in clusters during two different periods, 20 strains in May to August 1996 and the remaining 13 in March to June 1998. Antigenic analysis with monoclonal antibodies to the hemagglutinin-esterase (HE) glycoprotein and phylogenetic analysis of seven RNA segments showed that the 33 influenza C viruses isolated were antigenically and genetically similar and that they were reassortant viruses which had obtained PB2, PB1, HE, M, and NS genes from a C/pig/Beijing/115/81-like virus and P3 and NP genes from a C/Mississippi/80-like virus. These observations suggest strongly that during the survey period of 3 years, two outbreaks of influenza C occurred in Yamagata City, both of which were caused by a reassortant virus having the genome composition described above.