**1. Introduction**

The socioeconomic losses associated with viral infections of the respiratory tract are enormous at present. In Russia, acute respiratory infections (ARIs), including influenza, occupy a leading position for infectious diseases [1]. According to the Russian Federal Service for Surveillance on Consumer Rights Protection and Human Wellbeing (Rospotrebnadzor), 27.3–41.2 million cases of these diseases are registered in Russia each year. The total economic damage from ARIs in Russia ranges from 40 to 100 billion rubles annually [2]. In the United States, more than 400,000 people per year are hospitalized with respiratory viral infections [3].

Influenza is a serious medical and social problem for humanity. This acute infectious disease is caused by an enveloped ribonucleic acid (RNA) containing virus belonging to the family Orthomyxoviridae. Every year, more than 500 million people in the world get the flu; about 2 million of them die [4]. In Russia, every seventh person is involved in the influenza virus' annual epidemic [5]. When a new antigenic variant of the virus appears, a new pandemic covers all regions of the Earth. It is characterized by high morbidity with a large number of patients requiring hospitalization and mortality

among all age groups of the population [6]. The 2009 pandemic of influenza A (H1N1)pdm09 was accompanied by wide coverage of the population of many countries of the world, including the Russian Federation, with serious clinical complications and high mortality [7,8]. Adverse outcomes were observed not only among immunocompromised individuals in high risk groups but also in healthy young people with no significant previous pathology, including pregnant women. The influenza virus can cause disease not only in humans but also in various animals (e.g., birds, pigs, horses) [9]. In recent years, cases of human infection with avian influenza viruses of the H5N1, H7N7, and H7N9 subtypes have been reported [10].

Despite the presence of a significant number of anti-influenza drugs, this infection is still dangerous, as annual flu epidemics remain insufficiently controlled. Recently developed drugs are quite effective against currently circulating influenza virus strains, but their use can lead to the selection of resistant viral strains [11]. In this regard, there is a need for new therapeutic approaches and drugs with a broad spectrum of activity. Sulphated polysaccharides of algae (red and brown), such as carrageenans and fucoidans fulfill these requirements. This review presents the in vitro and in vivo results of experimental and clinical studies demonstrating the excellent effectiveness of these compounds in combating influenza infection and explain the need to use them as a potential basis for the creation of new drugs with a broad spectrum of activity. Despite their pronounced antiviral properties, they have not been entered the category of drugs yet, because of the difficulties of these compounds' standardization [12].

The purpose of this review is to draw the attention of researchers working on the problem of prevention and therapy of influenza to coordinate their efforts for the creation of standard samples of these highly active biopolymers.
