Influenza: A Century of Research

The first animal influenza A virus was isolated in 1931 by Richard Shope. The virus caused a highly contagious, influenza-like disease in pigs. Two years later, in 1933, the first human influenza A virus was isolated by Wilson Smith and colleagues. Soon after, in 1940, a representative of influenza virus type B was discovered by Thomas Francis, Jr. Being obligate intracellular parasites, viruses can be cultivated only within sensitive substrates. Three main substrates for the cultivation of influenza viruses are known: sensitive animals, embryonated chicken eggs, and tissue cultures. Today, in the twenties of the 21st century, sensitive animals are not often used for the isolation of the infectious virus. However, they are widely used to study and model a number of infectious diseases, including influenza. A list of these animals used for influenza research is very long, starting from ferrets and mice and ended with exotic zebrafish.

This chapter provides information on the structural organization and replication of influenza viruses in infected cells. The role of genes in the manifestation of pathogenicity and the attenuation of influenza viruses is discussed.

Influenza viruses undergo evolution by the shift (influenza A viruses) and the drift of envelope glycoproteins. Since predicting the direction of the antigenic evolution of influenza viruses is impossible, it is necessary to monitor wild viruses in nature. An important objective is to control the circulation of influenza viruses in natural reservoirs, interspecies transmission, especially the acquisition by avian influenza viruses and their ability to infect mammals, and detection of cases of stable transmissibility of such strains. In addition to antigenic evolution, it is necessary to identify and track the patterns of evolution of other biological properties of influenza viruses that might promote the evasion of viruses from the factors of nonspecific defense of the body and aggravate the epidemic process.

Each year, influenza causes a significant acute respiratory disease burden. Also, influenza pandemics periodically occur. Annual vaccination is the best tool for influenza prevention, but its effectiveness can vary from year to year. Antiviral treatment shortens the clinical course if administered within the first hours of the onset of the disease. Neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and polymerase inhibitors (baloxavir) are currently used as antivirals. Limited specificity of conventional vaccines and drug resistance of circulating viruses reduce the effectiveness of prophylaxis and treatment, requiring the development of new broadspectrum drugs.

Unfortunately, influenza can not be controlled effectively. A number of factors are responsible for insufficient control of this disease. Viral ubiquity and transmissibility, a high spreading rate of the influenza virus, antigenic selection, antiviral treatment etc., are responsible for the control of influenza. The main properties of the influenza virus described in the previous chapters that may influence the control of influenza are listed in this chapter.

The emergence of novel respiratory viruses has often been accompanied by the disappearance of existing circulating strains. In this chapter, the impact of pandemics on the circulation of seasonal respiratory viruses is discussed. Some issues arising from the spread of pandemic viruses and underlying the choices of a strategy of fighting COVID-19 are under debate.

This short final chapter has been written in the form of a conclusion; it is devoted to summarizing the 100-year study of influenza. The main achievements in the study and fight against this infection are listed. The possibility of defeating influenza in the foreseeable future is discussed.