Is the world due for an influenza pandemic?
Research has shown that an influenza pandemic is more likely to strike in the spring or summer than at the height of seasonal influenza season. But is there any way to predict exactly when the next one will occur? Infectious Disease News asked Peter Palese, PhD, professor and chair of the department of microbiology in the Icahn School of Medicine at Mount Sinai in New York, if the world is due.
Influenza pandemics are caused by novel influenza viruses, which are responsible for worldwide epidemics with high morbidity and mortality. The most dramatic influenza pandemic in recorded history occurred in 1918-1919. It is estimated that up to 100 million people died worldwide, and the United States likely had 750,000 deaths in a population of 100 million people. This led to an 11-year dip in the average life expectancy at that time. We have had three additional pandemics over the last 100 years: in 1957, caused by the H2N2 virus; in 1968, caused by the H3N2 virus; and in 2009, caused by a novel H1N1 virus (pH1N1). Viruses derived from the 1918 virus circulated for 39 years, viruses belonging to the H2N2 kind were prevalent for 11 years and H3N2 viruses have been with us for 51 years. For the last 10 years we have had pH1N1 viruses co-circulating with H3N2 viruses. It is difficult to discern a pattern for influenza pandemics when we have pandemic viruses arising after 11 or 39 years and H3N2 viruses circulating in the human population for more than 51 years. It is thought that these novel pandemic viruses are the result of a reassortment event involving the exchange of genes/RNAs from a human influenza virus with the minichromosomes/RNAs of an animal influenza virus.
Because there is no discernable pattern for the emergence of pandemic influenza virus strains, the old saying applies: It is difficult to make predictions, especially about the future. On the other hand, because novel pandemic strains arise from a mixing of genetic information between human and animal strains and we now have many more people and many more commercial chickens and pigs, which are breeding grounds for influenza viruses, such an event requiring close contact of people and animals is more likely than just 100 years ago.
From all we know about influenza viruses, we still do not understand what is important for person-to-person transmission and what makes an influenza virus highly virulent for humans. Thus, the best hope for successfully fighting a new pandemic virus is a better understanding of the biology of the virus and the development of improved vaccines and antivirals. It is very likely that new pandemic influenza viruses will emerge, but we have no good model for predicting such an event and for gauging the severity of the next pandemic.
Disclosure: Palese reports coauthoring patents covering the development of universal influenza virus vaccines that have been filed by the Icahn School of Medicine.
Research has shown that an influenza pandemic is more likely to strike in the spring or summer than at the height of seasonal influenza season. But is there any way to predict exactly when the next one will occur? Infectious Disease News asked Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security, if the world is due.
The world is overdue for an influenza pandemic. Influenza is an infectious disease that has been recognized since the time of Hippocrates and has caused perpetual seasonal outbreaks, which occurred with such regular periodicity that they were believed to be under the influence of the stars. Influenza pandemics were heralded by the appearance of a novel strain of influenza A, the result of reassortment with other influenza A viruses, usually from avian species. In recent recorded history, influenza pandemics occurred in 1918, 1957, 1968 and 2009 (though many pandemics occurred before but are not well documented). Of these, the 1918 pandemic was a civilization-shattering global biological catastrophe, perhaps killing 100 million people and infecting one-third of the world’s population with just a 1% case fatality rate.
The conditions of 2019 seem very different from those in 1918. The development of antibiotics, antivirals, vaccines and sophisticated critical care interventions are important factors that might mitigate the impact of a pandemic virus. These advances are, however, likely outweighed by the impact of shortened travel times — the virus now travels at the speed of a jet, not a boat — as well as the growth of megacities and the proliferation of large avian and swine farming and market facilities
Of the various combinations of hemagglutinin and neuraminidase proteins that characterize influenza A viruses, all circulate naturally in avian species. These churnings of viral proteins provide a constant stream of novel combinations that could emerge in humans at any time.
Of candidate pandemic viruses, H7N9 is the most notable. This avian origin virus, which has caused over 1,500 human infections in China since 2013, is now in its seventh wave of human infections. The fatality rate of this virus is approximately 40% in hospitalized patients but has thus far been incapable of sustained human-to-human transmission. However, it has now developed worrying mutations that are the harbingers of human tropism, antiviral resistance and vaccine evasion.
Influenza remains the epitome of a pandemic pathogen, and its hydralike nature will continually generate new variants that constitute a global catastrophic biological risk — and challenge — to the human species. Until an effective universal influenza vaccine that is broadly protective against all strains of the virus is developed, influenza will remain a clear and present looming pandemic threat that merits enhanced vigilance.
Disclosure: Adalja reports no relevant financial disclosures.
Research has shown that an influenza pandemic is more likely to strike in the spring or summer than at the height of seasonal influenza season. But is there any way to predict exactly when the next one will occur? Infectious Disease News asked Arnold S. Monto, MD, professor of epidemiology and global public health at the University of Michigan School of Public Health, if the world is due.
Each summer, over the last several years, human cases of swine-variant influenza have been recognized, especially in the Midwestern states. Given the history of the 2009 pandemic, which had its origin as a swine influenza reassortant, when these viruses were first detected, there was concern that this might represent a new variant of pandemic potential. However, it was soon realized that infection with these viruses was acquired as a result of the kind of human-swine interaction that takes place at state and county fairs. It became clear that, although these viruses did cause typically milder illness in those directly infected from pigs at the fairs, there was limited evidence of further human-to-human spread; in other words, there was no sustained transmission.
With the summer season approaching, it is likely that these transmission events will again occur and be recognized. At the same time, with awareness, there is an opportunity to reduce the kind of swine-human interaction that makes transmission more likely. Surveillance for these events continues to be necessary, although it is highly unlikely that these viruses will acquire the ability to spread easily from human to human, with the influenza virus, surprises do sometimes occur. Reducing transmission from pigs to humans will make this possibility even less likely.
Disclosure: Monto reports no relevant financial disclosures.