Influenza
Influenza, commonly referred to as the flu, is an acute viral respiratory illness caused by influenza viruses, which are members of the Orthomyxoviridae family. The disease is characterized by sudden onset of fever, cough, sore throat, myalgia, headache, and malaise, and it can range from mild, self-limiting illness to severe, life-threatening complications, particularly in vulnerable populations such as the elderly, young children, pregnant women, and those with underlying chronic conditions. The pathogenesis of influenza involves the inhalation of aerosolized viral particles, which infect epithelial cells of the upper and lower respiratory tract. Viral replication triggers a robust host immune response, including the release of pro-inflammatory cytokines, which are responsible for many of the clinical symptoms. In severe cases, direct viral cytopathic effects and immune-mediated damage contribute to complications such as viral pneumonia, secondary bacterial infections, acute respiratory distress syndrome (ARDS), and multi-organ failure. The health impacts of influenza are considerable, with annual epidemics resulting in significant morbidity, mortality, and economic burden worldwide. Seasonal influenza epidemics are responsible for millions of severe cases and hundreds of thousands of deaths globally each year, while pandemic influenza outbreaks, caused by the emergence of novel viral strains, can lead to widespread and catastrophic health consequences.
Influenza A viruses are the most virulent human pathogens among the influenza types and are responsible for both seasonal epidemics and global pandemics. These viruses are classified based on the antigenic properties of their surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), resulting in subtypes such as H1N1 and H3N2. Influenza A viruses infect a wide range of hosts, including humans, birds, and other mammals, and their genetic diversity and capacity for antigenic shift and drift facilitate the emergence of novel strains with pandemic potential.
Influenza B viruses primarily infect humans and are associated with seasonal epidemics but not pandemics. They are divided into two main lineages: B/Yamagata and B/Victoria. While generally less genetically diverse than influenza A, influenza B can still cause significant morbidity, especially in children and young adults. The clinical presentation is similar to that of influenza A, but outbreaks tend to be less severe.
Influenza C viruses cause mild respiratory illness, predominantly in children, and are not associated with epidemics or pandemics. These viruses are less common and generally result in sporadic cases. The clinical symptoms are typically milder than those caused by influenza A or B, often resembling those of a common cold.
Influenza D viruses have been identified primarily in cattle and are not known to cause illness in humans. While they are genetically distinct from other influenza viruses, there is currently no evidence of significant human infection or disease caused by influenza D.
Influenza is a globally prevalent disease, with seasonal epidemics occurring annually in both temperate and tropical regions. In temperate climates, influenza activity typically peaks during the winter months, while in tropical areas, transmission can occur year-round with less pronounced seasonality. According to the World Health Organization, annual influenza epidemics are estimated to result in approximately 1 billion cases worldwide, including 3–5 million cases of severe illness and 290,000–650,000 respiratory deaths. The burden of disease is highest among young children, the elderly, pregnant women, and individuals with chronic medical conditions. Influenza pandemics, which arise from the emergence of antigenically novel influenza A viruses to which the population has little or no preexisting immunity, can lead to widespread morbidity and mortality, as exemplified by the 1918, 1957, 1968, and 2009 pandemics. Surveillance systems globally monitor circulating strains to inform vaccine composition and guide public health interventions. Vaccination remains the primary strategy for prevention and control, but antiviral drugs and non-pharmaceutical interventions also play important roles, particularly during outbreaks and pandemics.
The diagnosis of influenza is based on a combination of clinical assessment and laboratory testing. Clinically, influenza should be suspected in individuals presenting with acute onset of fever, cough, sore throat, myalgia, headache, and malaise, particularly during periods of known community transmission. However, the clinical features are not specific and can overlap with other respiratory infections. Laboratory confirmation is achieved through detection of influenza virus or its components in respiratory specimens. The gold standard for diagnosis is reverse transcription polymerase chain reaction (RT-PCR), which offers high sensitivity and specificity for identifying and subtyping influenza viruses. Other diagnostic methods include rapid molecular assays, viral culture, and rapid antigen detection tests, though the latter have lower sensitivity, especially in adults. Additional laboratory findings may include leukopenia or lymphopenia, but these are non-specific. Diagnostic criteria may be influenced by epidemiological context, underlying risk factors, and the need for timely initiation of antiviral therapy. In hospitalized patients or those with severe disease, comprehensive testing for both influenza and other respiratory pathogens is recommended, and imaging studies such as chest radiography may be indicated to assess for complications such as pneumonia.
Suraxavir marboxil is an antiviral agent indicated for the treatment of influenza, acting through inhibition of the cap-dependent endonuclease essential for viral replication. Baloxavir marboxil is another antiviral medication used to treat acute uncomplicated influenza, functioning as a selective inhibitor of the influenza virus cap-dependent endonuclease, thereby interfering with viral RNA transcription. Favipiravir is an antiviral drug indicated for the treatment of influenza and exerts its effect by inhibiting the RNA-dependent RNA polymerase of the virus, impeding viral genome replication. The influenza A (H1N1) monovalent vaccine is formulated to provide immunization against the H1N1 subtype of influenza A, contributing to the prevention of infection and associated complications. The intradermal seasonal flu vaccine is designed for annual immunization against circulating influenza strains, administered via the intradermal route to stimulate protective immunity. Laninamivir octanoate is a long-acting neuraminidase inhibitor utilized for the treatment and prophylaxis of influenza, reducing viral release from infected cells. Peramivir is an intravenous neuraminidase inhibitor indicated for the treatment of acute uncomplicated influenza, acting by blocking viral neuraminidase activity and limiting viral spread. The pandemic influenza vaccine (H1N1)v (split virion, inactivated, adjuvanted) is developed to confer immunity against pandemic H1N1 influenza strains, employing an adjuvanted, split virion formulation to enhance immunogenicity. The influenza A (H1N1) 2009 monovalent vaccine is specifically indicated for immunization against the 2009 pandemic H1N1 strain, aiming to reduce disease incidence and severity in target populations.
| Structure | Generic Name | CAS Registry Number | Molecular Formula | Molecular Weight |
|---|---|---|---|---|
 | suraxavir marboxil | 2364589-86-4 | C29 H25 F2 N3 O7 S | 597.587 |
 | baloxavir marboxil (Rec INN; USAN) | 1985606-14-1 | C27 H23 F2 N3 O7 S | 571.549 |
 | favipiravir (Rec INN; USAN) | 259793-96-9 | C5 H4 F N3 O2 | 157.103 |
| influenza A (H1N1) monovalent vaccine | ||||
| intradermal seasonal flu vaccine | ||||
 | laninamivir octanoate (Prop INNM) | 203120-46-1 | C21 H36 N4 O8 | 472.533 |
 | peramivir (Rec INN; USAN) | C15 H28 N4 O4 | 328.407 | |
| pandemic influenza vaccine (H1N1)v (split virion, inactivated, adjuvanted) | ||||
| influenza A (H1N1) 2009 Monovalent Vaccine |
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