A Brief Introduction to Virology
Rae-Ellen W. Kavey, Allison B. Kavey in Viral Pandemics, 2020
The process of recombination identified by the Cold Springs Harbor team – two virus particles simultaneously infecting the same cell exchanging parts of their genetic material to produce new hybrid forms of the original viruses – was found to occur primarily in DNA viruses and was recognized as resulting in new viral strains which could infect previously resistant hosts, an important part of the infectivity of viruses. A particular form of recombination called “reassortment” occurred only in RNA viruses with segmented RNA, like the influenza virus. In this setting, whole segments of genetic material are exchanged, resulting in progeny with immediate and major antigenic change, a process called genetic shift.30 Based on findings like these, it was recognized that reassortment could yield an entirely new and antigenically novel virus strain. In the 30 minutes needed for a virus replication cycle, an infinitesimal killer virus could emerge, highly infectious, easily transmittable and lethal. It was molecular genetic work that identified this most important characteristic of RNA viruses: endless evolution by both spontaneous mutation and by genetic reassortment leading to continuous emergence of new, antigenically novel strains. Faced with an entirely new virus strain, human hosts have little if any resistance, so these emerging strains have high infectivity. This is especially true when reassortment occurs between an animal virus and a human virus.
Order Articulavirales
Paul Pumpens, Peter Pushko, Philippe Le Mercier in Virus-Like Particles, 2022
It is worth mentioning in this connection that the reassortant influenza viruses have been generated long before by coinfection of embryonated chicken eggs with influenza type A and type B viruses (Gotlieb and Hirst 1954). More recently, the live attenuated influenza viruses containing HA from A/PR8(H1), A/HK68 (H3), or A/VN (H5) strains in the backbone of the B/Yamagata/88 virus were reported (Hai et al. 2011). The reassortment phenomenon is routinely used in licensed influenza vaccines. In the reassortant vaccine virus, the vaccine-relevant HA and NA glycoproteins are embedded into the envelope, whereas other viral proteins are derived from type-specific donor strains, such as the A/PR/8/34 (H1N1) or B/Ann Arbor/1/66 viruses (Chen Z et al. 2008).
Evolution of Herpes Simplex Viruses
Marie Studahl, Paola Cinque, Tomas Bergström in Herpes Simplex Viruses, 2017
The conceptual and practical effects of finding high levels of recombination in HSV-1 natural infection on our understanding of the genetic patterns in virus populations are substantial: (i) genetic exchange increases the probability that individual deleterious mutations are lost from the population by selection (82), though the importance of this effect in HSV is difficult to quantify; (ii) similarly, the reassortment of uniquely occurring mutations has dramatic implications for the selection of advantageous combinations of variants, so the rate of virus evolution in response to environmental stimuli is potentially increased (82); (iii) instead of a clonal evolutionary process in which all segments of DNA along the genome are assumed to share the same history, virus genealogies must now be thought of as complex, non-tree-like structures. Therefore, phylogenetic (tree-based) approaches become inapplicable because trees are an inadequate representation of the relationships between strains in a population and even networks capable of representing limited recombination are useful only for short contiguous sequences (92). Adding recombination to models of sequence evolution increases the complexity of computations, for example in estimating the age of a particular mutation in the context of population history. On the other hand, the presence of recombination, with the incorporation of many partially independent genealogies into a single data set, means that there is more information available to infer the underlying population history than if the whole genome were evolving as one locus (94,95). We note that in interspecies studies the working assumption of congruent phylogenies for each gene remains, to the best of our present understanding, unaffected by intraspecies recombination.
The risk of a swine influenza pandemic: still a concern?
Published in Expert Review of Respiratory Medicine, 2019
Paul Loubet, Vincent Enouf, Odile Launay
The ecology of IAV is complex and involves a broad range of avian and mammalian host species. Influenza viruses have high mutation rates and are constantly changing, which enables the virus to quickly adapt to changes in the host environment, as is the case during interspecies transmission. The rapid evolution results from two mechanisms: reassortment and point mutations. Reassortment occurs when two different strains infect the same cell of a given host, allowing for the exchange of intact gene segments. When reassortment involves either the HA or NA segments, it is termed antigenic shift. Point mutations occur due to an error-prone polymerase devoid of a proof-reading and correction mechanism. When point mutations are fixed in the HA or NA segments it is called antigenic drift. Both of these mechanisms play pivotal roles in the emergence of novel influenza viruses that could jump the host barrier. Once the virus jumps into a new host, it must adapt and change to be able to spread and become established in the new population [2].
Influenza vaccine programs for children in low- and middle-income countries: current status and way forward
Published in Expert Review of Vaccines, 2019
Justin R Ortiz, Kathleen M Neuzil
Influenza viruses circulate globally, affect people of all ages, and cause annual disease outbreaks. Influenza A viruses undergo frequent antigenic mutations (antigenic drift) that allow them to evade immune protection and to cause repeated infections in an individual over a lifetime [13]. Novel virus strains may emerge through genetic reassortment between different strains in a common host (antigenic shift) [14]. Further, non-human strains may directly infect and cause disease in humans [15]. Reassortment events or direct infection of humans by animal viruses have the potential to cause a pandemic if humans have little to no pre-existing immunity to the virus, if person-to-person transmission is sustained, and the infection causes clinical disease [16]. An influenza pandemic could be catastrophic for human health and have major effects on the functioning of societies and economies [13].
Recommended hospital preparations for future cases and outbreaks of novel influenza viruses
Published in Expert Review of Respiratory Medicine, 2020
Seasonal influenza, in addition to periodic pandemics, is a major health burden with a significant morbidity and mortality. Epidemics of seasonal influenza occur in all parts of the world every year and an individual may build up some degree of immunity toward certain strains of influenza virus following repeated exposure with increased protection through influenza vaccinations. The background immunity will be lost when a novel influenza virus emerges through an antigenic shift or genetic reassortment leading to a pandemic. There have been 4 influenza pandemics since the 20th century: the Spanish Influenza in 1918–19 (an estimated 20–50 million deaths globally), the Asian Influenza in 1957–58 (1–4 million deaths globally), the Hong Kong Influenza in 1968–69 (1–4 million deaths worldwide) and the 2009 pandemic due to A(H1N1) (100,000–400,000 deaths worldwide). The estimated case fatality rates were 2–3%, <0.2%, <0.2% and 0.02% for the 4 pandemics, respectively [1].
Related Knowledge Centers
- Antigenic Shift
- Chromosomal Crossover
- Orthomyxoviridae
- Virus
- Pandemic
- Genetics
- 1957–1958 Influenza Pandemic
- Hong Kong Flu
- Influenza A Virus Subtype H1N1
- 2009 Swine Flu Pandemic