virology blog
About viruses and viral disease
From the European Congress of Virology in Rotterdam, Vincent and local co-host Ben Berkhout speak with Ron Fouchier, Rosina Girones, and Marie-Paule Kieny about their careers and their work on influenza virus, environmental virology, and developing an Ebola virus vaccine during an epidemic.
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Show notes at microbe.tv/twiv
The 1918 influenza pandemic was particularly lethal, not only for the very young and the very old (as observed for typical influenza), but unexpectedly also for young adults, 20Â to 40 years of age (pictured). It has been suggested that the increased lethality in young adults occurred because they lacked protective immunity that would be conferred by previous infection with a related virus. Reconstruction of the origins of the 1918 influenza virus provides support for this hypothesis.
Analysis of influenza virus genome sequences using a host-specific molecular clock together with seroarchaeology (analysis of stored sera for the presence of antibodies to influenza virus) indicates that the 1918 H1N1 virus arose ~1915 by reassortment of an avian influenza virus with an H1 virus that had previously emerged around 1907. The 1918 virus acquired the HA gene from the 1907 virus, and the NA gene and internal protein genes from an avian virus. This 1918 virus also infected pigs, in which descendants continue to circulate; however the human 1918 virus was displaced in 1922 by a reassortant with a distinct HA gene.
Seroarchaeology and mortality data indicate that an influenza pandemic in 1889-1893 was caused by an influenza H3N8 virus. This virus appears to have circulated until 1900, when it was replaced by a H1N8 virus (the N8 gene originating from the previously circulating H3N8 virus).
How do these events explain the unusual mortality pattern of the 1918 influenza A virus? High mortality among 20-40 year old adults might have been a consequence of their exposure to the H3N8 virus that circulated from 1889-1900. This infection provided no protection against the 1918 H1N1 virus. Protection of other age groups from lethal infection was likely a consequence of childhood exposure to N1 or H1 containing viruses (this may also have resulted in the lower than usual mortality in the elderly population). Influenza is typically highly lethal in very young children due to lack of immunologic memory.
These observations suggest that childhood exposure to influenza virus is a key predictor of virulence of a pandemic strain. Antibodies against the stalk of the HA protein protect against severe disease, but only within groups of HA subtypes (HA groups are determined by phylogenetic analysis). In 1918, antibodies against a group 2 HA subtype virus (H3) did not protect against severe disease caused by a group 1 HA subtype virus (H1). Childhood exposure might also determine mortality of seasonal influenza. For example, the high virulence of currently circulating H3N2 influenza viruses in those older than 65 years might be a consequence of infection with an H1N1 virus at a young age.
This logic can also explain mortality caused by influenza H5N1 and H7N9 viruses. Most fatalities caused by H5N1 viruses (the H5 is a group 1 HA) have been in individuals who were infected as children with an H3 virus (group 2 HA). Most fatalities caused by H7N9 viruses (group 2Â HA) have occurred in individuals who were infected as children with H1N1 or H2N2 viruses (group 1 HA).
The practical consequence of this work are clearly stated by the authors:
Immunization strategies that mimic the apparently powerful lifetime protection afforded by initial childhood exposure might dramatically reduce mortality due to both seasonal and novel IAV strains.
One notable characteristic of the four previous influenza pandemics is that they occurred in multiple waves. The 1918 pandemic began with outbreaks of low mortality in the spring and summer, followed by a more lethal wave in the winter. This pattern has fueled speculation that the current H1N1 pandemic strain will undergo mutation that leads to the emergence of a more lethal virus. What is the evidence that pandemic waves of increasing virulence are a consequence of viral mutation?
The only virus available from the 1918 pandemic was rescued from an Alaskan influenza victim who was buried in permafrost in November of that year, when higher mortality was already evident. This makes it impossible to correlate any genetic changes in the virus with increased virulence. Furthermore, as discussed on ProMedMail,
…there are many different ways of interpreting these differences other than more virulent virus. Some of these are differences in populations affected, more circulation of pneumococci and staphylococci during cold weather, more circulation of other viral pathogens, more virulence and larger inocula with the crowding and cold air inhaled.
The November 1918 influenza virus certainly has genetic and phenotypic properties expected of a virulent virus. These include the ability to multiply in the absence of trypsin*, lethality in mice and embryonated chicken eggs, and efficient replication in human bronchial epithelial cells. But we don’t know if these properties were absent from the virus that circulated in the spring of 1918.
Do the pandemics of 1957 and 1968, which also occurred in waves of increasing lethality, provide any information? Viruses are available from different stages of these pandemics, but to my knowledge the virulence studies have not been done.
This uncertainty makes it impossible to conclude that the 2009 H1N1 pandemic strain will become more virulent. Nevertheless, speculation is rampant, and accompanied the recent release of the Brazilian isolate. Another example is an amino acid change in the viral PB2 protein observed in some 2009 H1N1 isolates. According to Recombinomics,
Acquisition of E627K is a concern because it allows for optimal replication at 33 C, the temperature of a human nose in the winter, in contrast to E627, which is in the avian version of PB2 and allows for optimal replication at 41 C, the body temperature of birds. The appearance of E627K raises concerns that the level of swine flu with E627K will markedly increase in colder months. In 1918, the flu in the spring was mild, but the fall version of the virus, which had E627K, was much more virulent and targeted young, previously healthy adults…
If the amino acid at 627 is an important determinant of virulence, we would expect to find E627 in viruses isolated early in the 1918 pandemic – but such viruses are not available. Therefore the role of this amino acid change in virulence in humans cannot be tested. Further complicating the situation is that other amino acids in the viral PB2 protein can influence viral replication at low temperatures.
Fortunately, new H1N1 isolates are obtained every week, which provide a very accurate sampling of the entire pandemic. Should the new H1N1 strain become more virulent, it will be a relatively straightforward task to determine the genetic changes that accompany this property. Finally we will be able to determine if  pandemic waves of increasing virulence are a consequence of specific changes in the viral RNA.
*We’ll discuss the requirement of proteases for influenza virus replication next week.
Miller, M., Viboud, C., Balinska, M., & Simonsen, L. (2009). The Signature Features of Influenza Pandemics — Implications for Policy New England Journal of Medicine, 360 (25), 2595-2598 DOI: 10.1056/NEJMp0903906
Tumpey, T. (2005). Characterization of the Reconstructed 1918 Spanish Influenza Pandemic Virus Science, 310 (5745), 77-80 DOI: 10.1126/science.1119392.