摘要：We reviewed the epidemiology, clinical characteristics, disease severity, and economic burden of influenza B as reported in the peer-reviewed published literature. We used MEDLINE to perform a systematic literature review of peer-reviewed, English-language literature published between 1995 and 2010. Widely variable frequency data were reported. Clinical presentation of influenza B was similar to that of influenza A, although we observed conflicting reports. Influenza B–specific data on hospitalization rates, length of stay, and economic outcomes were limited but demonstrated that the burden of influenza B can be significant. The medical literature demonstrates that influenza B can pose a significant burden to the global population. The comprehensiveness and quality of reporting on influenza B, however, could be substantially improved. Few articles described complications. Additional data regarding the incidence, clinical burden, and economic impact of influenza B would augment our understanding of the disease and assist in vaccine development. THERE ARE 3 TYPES OF INFLUenza, A and B being most common in humans, each with unique characteristics. Influenza C is less common and produces milder disease. 1,2 Influenza A virus subtypes are based on 2 surface proteins: hemagglutinin (H) and neuraminidase (N). Current influenza A subtypes found in people are H1N1 and H3N2. Influenza B is not divided into subtypes; however, 2 antigenically and genetically distinct lineages, B/Victoria/2/87–like (Victoria lineage) and B/Yamagata/16/88–like (Yamagata lineage), have circulated worldwide since 1983. 3 Two influenza A subtypes and 1 influenza B lineage are included in current trivalent seasonal influenza vaccines. The first influenza virus—A (H1N1)—was recovered in 1933; influenza B was first identified in 1940 by Francis. 4 In early years, influenza B epidemics were noted to occur at intervals of 2 to 4 years and were generally well-defined and discrete; medically attended illnesses, including clinic visits and hospitalizations, were common in all age groups. 5 The emergence of a second lineage of influenza B in 1983, 3 along with changing demographics and rapid movement of human populations, has changed the epidemiology of influenza B. 1,6 New variants of influenza B arise less frequently than for influenza A 2 ; therefore, in some years, adults with previous exposure to influenza B may have less severe illness than similarly exposed children who invariably have higher attack rates. Since 2001, both influenza B lineages have been cocirculating each influenza season, in contrast to the pattern of multiyear dominance by a single lineage that occurred between 1985 and 2000 7–32 Although both influenza A and B have an impact on human health, multiple differences exist between them, including molecular differences. 33 Although just 2 subtypes of influenza A have cocirculated in recent years, a total of 16 subtypes are found in animal species, especially birds and pigs, providing an opportunity for pandemics through mutation or reassortment. 33 By contrast, with no natural animal host (other than seals) and a slower rate of mutation, influenza B has little potential for such impact. 3,33 Given these differences, it is reasonable to hypothesize that there are also differences in the presentation, symptoms, risk factors, and total burden of influenza A and B viruses which may, in turn, inform vaccination and prevention efforts. To date, more research has focused on influenza A than on influenza B. 33 Current opinion of influenza B remains influenced by early studies that concluded that influenza B posed less of a disease burden than influenza A. 1,2 Furthermore, because of the ability of influenza A to cause severe pandemics, it is more frequently a topic of press coverage than influenza B, reinforcing the perception that influenza B does not pose a serious threat to public health. In contrast to the popular view that influenza B has minimal impact, there are indications that the impact of influenza B is substantial. Before initiating a formal review of the literature, we examined recent surveillance data. Among US pediatric influenza deaths between 2004 and 2011, excluding the 2009–2010 pandemic, 22% to 44% of deaths each season were confirmed to be influenza B–related; the remainder were related to influenza A. 34 Similar multiseason mortality data are not available in the European Union; however, in the United Kingdom, influenza B dominated the 2010–2011 season with both influenza B lineages cocirculating. 35,36 Of 607 UK fatalities associated with influenza during that season, 40 were associated with influenza B (through June 30, 2011). 35,36 Surveillance data from the United States and Europe suggest a potentially increasing burden of influenza B in recent years ( Figure 1 ). This high variability in influenza B circulation may be attributable to variable population immunity and competition between the 2 cocirculating lineages of influenza B. Furthermore, behavioral trends, such as increasing urbanization and travel, facilitate the spread of influenza viruses. 37 In 2002, 52 million persons embarked on international flights demonstrating how respiratory viruses can be spread rapidly. 38,39 It is notable that the B lineage selected for the seasonal influenza vaccine and the dominant circulating B strain have matched only 5 times in the 10 seasons between 2001–2002 and 2010–2011. 40 Open in a separate window FIGURE 1— Influenza B activity, as indicated by proportion of samples testing positive for influenza B in Europe and the United States, 1994–2011. Note . Data on influenza B activity in Europe from the European Influenza Surveillance Network are unavailable before 2000. Source . Data were obtained from the Centers for Disease Control and Prevention and the European Influenza Surveillance Network. 7–32 As evidence of influenza B burden accumulates and vaccine technology advances, it is increasingly important to understand and quantify the impact of influenza B on the worldwide population. We designed this review to comprehensively examine the epidemiology, clinical characteristics, disease severity, and economic burden of influenza B as reported in the recent peer-reviewed literature.
