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Introduction

Pandemics and epidemics

There are many different influenza A virus subtypes, differing in the nature of the HA and NA glycoproteins on their surface. 12, x RA Lamb, RM Krug. Orthomyxoviridae: the viruses and their replication. DM Knipe, PM Howley, DE Griffin (Eds.) et al. Fields Virology 4th edn. (Lippincott Williams & Wilkins, 2001) (1487 - 1531) 13 x PF Wright, RG Webster. Orthomyxoviruses. DM Knipe, PM Howley, DE Griffin (Eds.) et al. Fields Virology 4th edn. (Lippincott Williams & Wilkins, 2001) (1533 - 1579) Sixteen HAs and nine NAs have been identified. Virus subtypes are distinguishable serologically, which means that antibodies to one subtype do not react with another subtype. Aquatic birds appear to serve as the reservoir of influenza A viruses; indeed, all virus subtypes presumably circulate in these wild birds. Only H1, H2 and H3 virus subtypes have been identified in humans, specifically the H1N1, H2N2 and H3N2 viruses corresponding to the three major pandemics of the last century. During recent outbreaks of avian influenza, there have been occasional transmissions of H5N1, H7N7 and H9N2 viruses to humans (see Chapter 3).

Antigenic shift

Influenza pandemics are the result of so-called antigenic shift of the virus (see Chapter 3). 13 x PF Wright, RG Webster. Orthomyxoviruses. DM Knipe, PM Howley, DE Griffin (Eds.) et al. Fields Virology 4th edn. (Lippincott Williams & Wilkins, 2001) (1533 - 1579) This means that a virus with a new HA (and NA) is introduced into the human population. Since everyone will be immunologically naive to that new virus subtype, the infection may spread rapidly and cause high morbidity and mortality among the entire population, including young healthy people. 3, x Y Ghendon. Introduction to pandemic influenza through history. Eur J Epidemiol 10 (1994) (451 - 453) Crossref. 14 x NP Johnson, J Mueller. Updating the accounts: global mortality of the 1918–1920 “Spanish” influenza pandemic. Bull Hist Med 76 (2002) (105 - 115) Crossref. Three major influenza pandemics struck the globe in the 20th century. 3, x Y Ghendon. Introduction to pandemic influenza through history. Eur J Epidemiol 10 (1994) (451 - 453) Crossref. 4 x WI Beveridge. The chronicle of influenza epidemics. Hist Phil Life Sci 13 (1991) (223 - 234) The Spanish flu (caused by an H1N1 influenza A virus) occurred in 1918–19, after the First World War, and resulted in the death of an estimated 50 million people, 14 x NP Johnson, J Mueller. Updating the accounts: global mortality of the 1918–1920 “Spanish” influenza pandemic. Bull Hist Med 76 (2002) (105 - 115) Crossref. more than the death toll of the war itself ( Figure 2 ). Other pandemics occurred in 1957 (Asian flu, H2N2) and 1968 (Hong Kong flu, H3N2), with a total number of deaths of approximately 2 million. 1 x World Health Organization. Avian influenza: assessing the pandemic threat. (www.who.int/csr/disease/influenza/WHO_CDS_2005_29/en/) (2005)

Figure 2 Emergency hospital during the Spanish flu pandemic in 1918, Camp Funston, Kansas, USA. source: Courtesy of the National Museum of Health and Medicine, Armed Forces Institute of Pathology, Washington, DC, USA (NCP1603).

f01-02-9780723434337

References in context

  • Since everyone will be immunologically naive to that new virus subtype, the infection may spread rapidly and cause high morbidity and mortality among the entire population, including young healthy people.3,14 Three major influenza pandemics struck the globe in the 20th century.3,4 The Spanish flu (caused by an H1N1 influenza A virus) occurred in 1918–19, after the First World War, and resulted in the death of an estimated 50 million people,14 more than the death toll of the war itself (Figure 2).
    Go to context

The current information supports the concept that new pandemic influenza viruses are derived from avian virus reservoirs. 13, x PF Wright, RG Webster. Orthomyxoviruses. DM Knipe, PM Howley, DE Griffin (Eds.) et al. Fields Virology 4th edn. (Lippincott Williams & Wilkins, 2001) (1533 - 1579) 15, 16, and 17 x K Subbarao, J Katz. Avian influenza viruses infecting humans. Cell Molec Life Sci 57 (2000) (1770 - 1784) Crossref. x RG Webster. Influenza virus: transmission between species and relevance to emergence of the next human pandemic. Arch Virol Suppl 13 (1997) (105 - 113) Crossref. x M Zambon. The pathogenesis of influenza in humans. Rev Med Virol 11 (2001) (227 - 241) Crossref. Avian influenza viruses may be directly transmitted to humans, which probably occurred in the case of the 1918 Spanish flu virus. 18, x JK Taubenberger, AH Reid, RM Lourens, et al.. Characterization of the 1918 influenza virus polymerase genes. Nature 437 (2005) (889 - 893) Crossref. 19 x JK Taubenberger, DM Morens. 1918 influenza: the mother of all pandemics. Emerg Inf Dis 12 (2006) (15 - 22) The possibility of direct transmission of an avian influenza virus to humans became evident for the first time during the H5N1 outbreak in Hong Kong in 1997. 20, x EC Claas, AD Osterhaus, et al.. Human influenza A H5N1 virus related to a highly pathogenic avian influenza virus. Lancet 351 (1998) (472 - 477) Crossref. 21 x K Subbarao, A Klimov, J Katz, et al.. Characterization of an avian influenza A (H5N1) virus isolated from a child with a fatal respiratory illness. Science 279 (1998) (393 - 396) Crossref. In this incident, 18 people were infected with the chicken flu virus, six of whom died. Similarly, in 2003, an outbreak of fowl plague (H7N7 subtype) in the Netherlands resulted in the direct transmission of the virus to poultry workers and veterinarians, killing one person. 22 x RAM Fouchier, PM Schneeberger, FW Rozendaal, et al.. Avian influenza A virus (H7N7) associated with human conjunctivitis and a fatal case of acute respiratory distress syndrome. Proc Natl Acad Sci USA 101 (2004) (1356 - 1361) Crossref. Furthermore, as of the end of 2003, continuing renewed outbreaks of highly pathogenic H5N1 avian influenza in Asia, and more recently also in Europe, Africa and the Middle East, have resulted in an increasing number of human infections with a very high case-fatality rate,23, 24, 25, 26, and 27 x JS Peiris, WC Yu, CW Leung, et al.. Re-emergence of fatal human influenza A subtype H5N1 disease. Lancet 363 (2004) (617 - 619) Crossref. x A Apisarnthanarak, R Kitphati, K Thongphubeth, et al.. Atypical avian influenza (H5N1). Emerg Infect Dis 10 (2004) (1321 - 1324) x MD de Jong, VC Bach, TQ Phan, et al.. Fatal avian influenza A (H5N1) in a child presenting with diarrhea followed by coma. New Engl J Med 352 (2005) (686 - 691) Crossref. x TH Tran, TL Nguyen, TD Nguyen, et al.. Avian influenza A (H5N1) in 10 patients in Vietnam. New Engl J Med 350 (2004) (1179 - 1188) x World Health Organization. Cumulative number of confirmed human cases of avian influenza A/(H5N1) reported to WHO. (www.who.int/csr/disease/avian_influenza/country/en/index.html) underscoring the disconcerting possibility that this virus may further adapt to the human host and thus cause a new deadly worldwide outbreak of influenza.

Antigenic shift may also be due to a genetic reassortment between an avian and a human influenza virus, with pigs possibly acting as an intermediate host. In this process, a human virus acquires a number of gene segments from an avian virus, at least including the gene segment encoding the HA, within a cell dually infected with two viruses from human and avian origin (see Chapter 3). It is well established that genetic reassortment has been the basis of the formation of the 1957 and 1968 pandemic viruses. 28 x T Ito, JN Couceiro, S Kelm, et al.. Molecular basis for the generation in pigs of influenza A viruses with pandemic potential. J Virol 72 (1998) (7367 - 7373)

Antigenic drift

In addition to the emergence of new human influenza virus subtypes with pandemic potential, established human virus subtypes undergo significant antigenic adaptation, referred to as antigenic drift (see Chapter 3). 13 x PF Wright, RG Webster. Orthomyxoviruses. DM Knipe, PM Howley, DE Griffin (Eds.) et al. Fields Virology 4th edn. (Lippincott Williams & Wilkins, 2001) (1533 - 1579) Antigenic drift involves minor changes in the HA, NA and possibly also other viral antigens, that occur due to mutations in the viral genome, resulting in amino acid substitutions in antigenic sites. These changes may render the new strain different enough to at least partially avoid the immunity induced by previous strains. Thus, new influenza epidemics may arise. In the temperate regions, these epidemics generally occur in the winter months (from October to April in the northern hemisphere, and from April to October in the southern hemisphere). Antigenic drift of established human virus subtypes necessitates regular update of the composition of the annual influenza vaccine (see Chapter 8).

 
x

Figure 2 Emergency hospital during the Spanish flu pandemic in 1918, Camp Funston, Kansas, USA. source: Courtesy of the National Museum of Health and Medicine, Armed Forces Institute of Pathology, Washington, DC, USA (NCP1603).

f01-02-9780723434337

References in context

  • Since everyone will be immunologically naive to that new virus subtype, the infection may spread rapidly and cause high morbidity and mortality among the entire population, including young healthy people.3,14 Three major influenza pandemics struck the globe in the 20th century.3,4 The Spanish flu (caused by an H1N1 influenza A virus) occurred in 1918–19, after the First World War, and resulted in the death of an estimated 50 million people,14 more than the death toll of the war itself (Figure 2).
    Go to context

References

Label Authors Title Source Year
1

References in context

  • The recent spread of bird flu across Asia and parts of Europe and the Middle East, as well as the occasional infections of humans with a disconcertingly high number of deaths, have caused worldwide concern about a potential new global epidemic of influenza.1 Are the ongoing outbreaks of avian flu indeed heralding the next pandemic?
    Go to context

  • Influenza pandemics are the result of so-called antigenic shift of the virus (see Chapter 3).13 This means that a virus with a new HA (and NA) is introduced into the human population.
    Go to context

World Health Organization. Avian influenza: assessing the pandemic threat. (www.who.int/csr/disease/influenza/WHO_CDS_2005_29/en/) (2005) 2005
3

References in context

  • It occurs not only in occasional major pandemic outbreaks, but also in epidemics of variable severity almost every winter.2–4 The term “influenza” has been derived from the Italian influentia in the mid-1300s, indicating that, at the time, the illness was believed to result from astrological influences.
    Go to context

  • Since everyone will be immunologically naive to that new virus subtype, the infection may spread rapidly and cause high morbidity and mortality among the entire population, including young healthy people.3,14 Three major influenza pandemics struck the globe in the 20th century.3,4 The Spanish flu (caused by an H1N1 influenza A virus) occurred in 1918–19, after the First World War, and resulted in the death of an estimated 50 million people,14 more than the death toll of the war itself (Figure 2).
    Go to context

  • Since everyone will be immunologically naive to that new virus subtype, the infection may spread rapidly and cause high morbidity and mortality among the entire population, including young healthy people.3,14 Three major influenza pandemics struck the globe in the 20th century.3,4 The Spanish flu (caused by an H1N1 influenza A virus) occurred in 1918–19, after the First World War, and resulted in the death of an estimated 50 million people,14 more than the death toll of the war itself (Figure 2).
    Go to context

Y Ghendon. Introduction to pandemic influenza through history. Crossref. Eur J Epidemiol 10 (1994) (451 - 453) 1994
4

References in context

  • It occurs not only in occasional major pandemic outbreaks, but also in epidemics of variable severity almost every winter.2–4 The term “influenza” has been derived from the Italian influentia in the mid-1300s, indicating that, at the time, the illness was believed to result from astrological influences.
    Go to context

  • Since everyone will be immunologically naive to that new virus subtype, the infection may spread rapidly and cause high morbidity and mortality among the entire population, including young healthy people.3,14 Three major influenza pandemics struck the globe in the 20th century.3,4 The Spanish flu (caused by an H1N1 influenza A virus) occurred in 1918–19, after the First World War, and resulted in the death of an estimated 50 million people,14 more than the death toll of the war itself (Figure 2).
    Go to context

WI Beveridge. The chronicle of influenza epidemics. Hist Phil Life Sci 13 (1991) (223 - 234) 1991
12

References in context

  • Influenza virions are enveloped particles of spherical or elongated shape, measuring 80–120 nm in diameter and containing a segmented, single-stranded RNA genome (see Chapter 2).12,13 The influenza virus belongs to the family of the Orthomyxoviridae.
    Go to context

  • There are many different influenza A virus subtypes, differing in the nature of the HA and NA glycoproteins on their surface.12,13 Sixteen HAs and nine NAs have been identified.
    Go to context

RA Lamb, RM Krug. Orthomyxoviridae: the viruses and their replication. DM Knipe, PM Howley, DE Griffin (Eds.) et al. Fields Virology 4th edn. (Lippincott Williams & Wilkins, 2001) (1487 - 1531) 2001
13

References in context

  • Influenza virions are enveloped particles of spherical or elongated shape, measuring 80–120 nm in diameter and containing a segmented, single-stranded RNA genome (see Chapter 2).12,13 The influenza virus belongs to the family of the Orthomyxoviridae.
    Go to context

  • There are many different influenza A virus subtypes, differing in the nature of the HA and NA glycoproteins on their surface.12,13 Sixteen HAs and nine NAs have been identified.
    Go to context

  • Influenza pandemics are the result of so-called antigenic shift of the virus (see Chapter 3).13 This means that a virus with a new HA (and NA) is introduced into the human population.
    Go to context

  • The current information supports the concept that new pandemic influenza viruses are derived from avian virus reservoirs.13,15–17 Avian influenza viruses may be directly transmitted to humans, which probably occurred in the case of the 1918 Spanish flu virus.18,19 The possibility of direct transmission of an avian influenza virus to humans became evident for the first time during the H5N1 outbreak in Hong Kong in 1997.20,21 In this incident, 18 people were infected with the chicken flu virus, six of whom died.
    Go to context

  • In addition to the emergence of new human influenza virus subtypes with pandemic potential, established human virus subtypes undergo significant antigenic adaptation, referred to as antigenic drift (see Chapter 3).13 Antigenic drift involves minor changes in the HA, NA and possibly also other viral antigens, that occur due to mutations in the viral genome, resulting in amino acid substitutions in antigenic sites.
    Go to context

PF Wright, RG Webster. Orthomyxoviruses. DM Knipe, PM Howley, DE Griffin (Eds.) et al. Fields Virology 4th edn. (Lippincott Williams & Wilkins, 2001) (1533 - 1579) 2001
14

References in context

  • Since everyone will be immunologically naive to that new virus subtype, the infection may spread rapidly and cause high morbidity and mortality among the entire population, including young healthy people.3,14 Three major influenza pandemics struck the globe in the 20th century.3,4 The Spanish flu (caused by an H1N1 influenza A virus) occurred in 1918–19, after the First World War, and resulted in the death of an estimated 50 million people,14 more than the death toll of the war itself (Figure 2).
    Go to context

  • Since everyone will be immunologically naive to that new virus subtype, the infection may spread rapidly and cause high morbidity and mortality among the entire population, including young healthy people.3,14 Three major influenza pandemics struck the globe in the 20th century.3,4 The Spanish flu (caused by an H1N1 influenza A virus) occurred in 1918–19, after the First World War, and resulted in the death of an estimated 50 million people,14 more than the death toll of the war itself (Figure 2).
    Go to context

NP Johnson, J Mueller. Updating the accounts: global mortality of the 1918–1920 “Spanish” influenza pandemic. Crossref. Bull Hist Med 76 (2002) (105 - 115) 2002
15

References in context

  • The current information supports the concept that new pandemic influenza viruses are derived from avian virus reservoirs.13,15–17 Avian influenza viruses may be directly transmitted to humans, which probably occurred in the case of the 1918 Spanish flu virus.18,19 The possibility of direct transmission of an avian influenza virus to humans became evident for the first time during the H5N1 outbreak in Hong Kong in 1997.20,21 In this incident, 18 people were infected with the chicken flu virus, six of whom died.
    Go to context

K Subbarao, J Katz. Avian influenza viruses infecting humans. Crossref. Cell Molec Life Sci 57 (2000) (1770 - 1784) 2000
16

References in context

  • The current information supports the concept that new pandemic influenza viruses are derived from avian virus reservoirs.13,15–17 Avian influenza viruses may be directly transmitted to humans, which probably occurred in the case of the 1918 Spanish flu virus.18,19 The possibility of direct transmission of an avian influenza virus to humans became evident for the first time during the H5N1 outbreak in Hong Kong in 1997.20,21 In this incident, 18 people were infected with the chicken flu virus, six of whom died.
    Go to context

RG Webster. Influenza virus: transmission between species and relevance to emergence of the next human pandemic. Crossref. Arch Virol Suppl 13 (1997) (105 - 113) 1997
17

References in context

  • The current information supports the concept that new pandemic influenza viruses are derived from avian virus reservoirs.13,15–17 Avian influenza viruses may be directly transmitted to humans, which probably occurred in the case of the 1918 Spanish flu virus.18,19 The possibility of direct transmission of an avian influenza virus to humans became evident for the first time during the H5N1 outbreak in Hong Kong in 1997.20,21 In this incident, 18 people were infected with the chicken flu virus, six of whom died.
    Go to context

M Zambon. The pathogenesis of influenza in humans. Crossref. Rev Med Virol 11 (2001) (227 - 241) 2001
18

References in context

  • The current information supports the concept that new pandemic influenza viruses are derived from avian virus reservoirs.13,15–17 Avian influenza viruses may be directly transmitted to humans, which probably occurred in the case of the 1918 Spanish flu virus.18,19 The possibility of direct transmission of an avian influenza virus to humans became evident for the first time during the H5N1 outbreak in Hong Kong in 1997.20,21 In this incident, 18 people were infected with the chicken flu virus, six of whom died.
    Go to context

JK Taubenberger, AH Reid, RM Lourens, et al.. Characterization of the 1918 influenza virus polymerase genes. Crossref. Nature 437 (2005) (889 - 893) 2005
19

References in context

  • The current information supports the concept that new pandemic influenza viruses are derived from avian virus reservoirs.13,15–17 Avian influenza viruses may be directly transmitted to humans, which probably occurred in the case of the 1918 Spanish flu virus.18,19 The possibility of direct transmission of an avian influenza virus to humans became evident for the first time during the H5N1 outbreak in Hong Kong in 1997.20,21 In this incident, 18 people were infected with the chicken flu virus, six of whom died.
    Go to context

JK Taubenberger, DM Morens. 1918 influenza: the mother of all pandemics. Emerg Inf Dis 12 (2006) (15 - 22) 2006
20

References in context

  • The current information supports the concept that new pandemic influenza viruses are derived from avian virus reservoirs.13,15–17 Avian influenza viruses may be directly transmitted to humans, which probably occurred in the case of the 1918 Spanish flu virus.18,19 The possibility of direct transmission of an avian influenza virus to humans became evident for the first time during the H5N1 outbreak in Hong Kong in 1997.20,21 In this incident, 18 people were infected with the chicken flu virus, six of whom died.
    Go to context

EC Claas, AD Osterhaus, et al.. Human influenza A H5N1 virus related to a highly pathogenic avian influenza virus. Crossref. Lancet 351 (1998) (472 - 477) 1998
21

References in context

  • The current information supports the concept that new pandemic influenza viruses are derived from avian virus reservoirs.13,15–17 Avian influenza viruses may be directly transmitted to humans, which probably occurred in the case of the 1918 Spanish flu virus.18,19 The possibility of direct transmission of an avian influenza virus to humans became evident for the first time during the H5N1 outbreak in Hong Kong in 1997.20,21 In this incident, 18 people were infected with the chicken flu virus, six of whom died.
    Go to context

K Subbarao, A Klimov, J Katz, et al.. Characterization of an avian influenza A (H5N1) virus isolated from a child with a fatal respiratory illness. Crossref. Science 279 (1998) (393 - 396) 1998
22

References in context

  • Similarly, in 2003, an outbreak of fowl plague (H7N7 subtype) in the Netherlands resulted in the direct transmission of the virus to poultry workers and veterinarians, killing one person.22 Furthermore, as of the end of 2003, continuing renewed outbreaks of highly pathogenic H5N1 avian influenza in Asia, and more recently also in Europe, Africa and the Middle East, have resulted in an increasing number of human infections with a very high case-fatality rate,23–27 underscoring the disconcerting possibility that this virus may further adapt to the human host and thus cause a new deadly worldwide outbreak of influenza.
    Go to context

RAM Fouchier, PM Schneeberger, FW Rozendaal, et al.. Avian influenza A virus (H7N7) associated with human conjunctivitis and a fatal case of acute respiratory distress syndrome. Crossref. Proc Natl Acad Sci USA 101 (2004) (1356 - 1361) 2004
23

References in context

  • Similarly, in 2003, an outbreak of fowl plague (H7N7 subtype) in the Netherlands resulted in the direct transmission of the virus to poultry workers and veterinarians, killing one person.22 Furthermore, as of the end of 2003, continuing renewed outbreaks of highly pathogenic H5N1 avian influenza in Asia, and more recently also in Europe, Africa and the Middle East, have resulted in an increasing number of human infections with a very high case-fatality rate,23–27 underscoring the disconcerting possibility that this virus may further adapt to the human host and thus cause a new deadly worldwide outbreak of influenza.
    Go to context

JS Peiris, WC Yu, CW Leung, et al.. Re-emergence of fatal human influenza A subtype H5N1 disease. Crossref. Lancet 363 (2004) (617 - 619) 2004
24

References in context

  • Similarly, in 2003, an outbreak of fowl plague (H7N7 subtype) in the Netherlands resulted in the direct transmission of the virus to poultry workers and veterinarians, killing one person.22 Furthermore, as of the end of 2003, continuing renewed outbreaks of highly pathogenic H5N1 avian influenza in Asia, and more recently also in Europe, Africa and the Middle East, have resulted in an increasing number of human infections with a very high case-fatality rate,23–27 underscoring the disconcerting possibility that this virus may further adapt to the human host and thus cause a new deadly worldwide outbreak of influenza.
    Go to context

A Apisarnthanarak, R Kitphati, K Thongphubeth, et al.. Atypical avian influenza (H5N1). Emerg Infect Dis 10 (2004) (1321 - 1324) 2004
25

References in context

  • Similarly, in 2003, an outbreak of fowl plague (H7N7 subtype) in the Netherlands resulted in the direct transmission of the virus to poultry workers and veterinarians, killing one person.22 Furthermore, as of the end of 2003, continuing renewed outbreaks of highly pathogenic H5N1 avian influenza in Asia, and more recently also in Europe, Africa and the Middle East, have resulted in an increasing number of human infections with a very high case-fatality rate,23–27 underscoring the disconcerting possibility that this virus may further adapt to the human host and thus cause a new deadly worldwide outbreak of influenza.
    Go to context

MD de Jong, VC Bach, TQ Phan, et al.. Fatal avian influenza A (H5N1) in a child presenting with diarrhea followed by coma. Crossref. New Engl J Med 352 (2005) (686 - 691) 2005
26

References in context

  • Similarly, in 2003, an outbreak of fowl plague (H7N7 subtype) in the Netherlands resulted in the direct transmission of the virus to poultry workers and veterinarians, killing one person.22 Furthermore, as of the end of 2003, continuing renewed outbreaks of highly pathogenic H5N1 avian influenza in Asia, and more recently also in Europe, Africa and the Middle East, have resulted in an increasing number of human infections with a very high case-fatality rate,23–27 underscoring the disconcerting possibility that this virus may further adapt to the human host and thus cause a new deadly worldwide outbreak of influenza.
    Go to context

TH Tran, TL Nguyen, TD Nguyen, et al.. Avian influenza A (H5N1) in 10 patients in Vietnam. New Engl J Med 350 (2004) (1179 - 1188) 2004
27

References in context

  • Similarly, in 2003, an outbreak of fowl plague (H7N7 subtype) in the Netherlands resulted in the direct transmission of the virus to poultry workers and veterinarians, killing one person.22 Furthermore, as of the end of 2003, continuing renewed outbreaks of highly pathogenic H5N1 avian influenza in Asia, and more recently also in Europe, Africa and the Middle East, have resulted in an increasing number of human infections with a very high case-fatality rate,23–27 underscoring the disconcerting possibility that this virus may further adapt to the human host and thus cause a new deadly worldwide outbreak of influenza.
    Go to context

  • During these ongoing H5N1 outbreaks, there have been 179 confirmed human infections (March 1, 2006), with a case-fatality rate of over 50%.27 Clearly, it is only a matter of time before another pandemic influenza virus will emerge.
    Go to context

World Health Organization. Cumulative number of confirmed human cases of avian influenza A/(H5N1) reported to WHO. (www.who.int/csr/disease/avian_influenza/country/en/index.html)
28

References in context

  • Antigenic shift may also be due to a genetic reassortment between an avian and a human influenza virus, with pigs possibly acting as an intermediate host.
    Go to context

T Ito, JN Couceiro, S Kelm, et al.. Molecular basis for the generation in pigs of influenza A viruses with pandemic potential. J Virol 72 (1998) (7367 - 7373) 1998

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