During seasonal influenza, mortality is concentrated in the very young and the elderly, whereas during flu pandemics, young adults are often affected at a high rate. In temperate regions, the number of influenza cases varies from season to season. Lower vitamin D levels, presumably due to less sunlight, lower humidity, lower temperature, and minor changes in virus proteins caused by antigenic drift contribute to annual epidemics that peak during the winter season. In the northern hemisphere, this is from October to May (more narrowly December to April), and in the southern hemisphere, this is from May to October (more narrowly June to September). There are therefore two distinct influenza seasons every year in temperate regions, one in the northern hemisphere and one in the southern hemisphere. In tropical and subtropical regions, seasonality is more complex and appears to be affected by various climatic factors such as minimum temperature, hours of sunshine, maximum rainfall, and high humidity. Influenza may therefore occur year-round in these regions. Influenza epidemics in modern times have the tendency to start in the eastern or southern hemisphere, with Asia being a key reservoir of influenza viruses.IAV and IBV co-circulate, so the two have the same patterns of transmission. The seasonality of ICV, however, is poorly understood. ICV infection is most common in children under the age of 2, and by adulthood most people have been exposed to it.
Rapid influenza diagnostic tests (RIDTs) are a simple way of obtaining assay results, are low cost, and produce results quickly, at less than 30 minutes, so they are commonly used, but they cant distinguish between IAV and IBV or between IAV subtypes and are not as sensitive as nucleic-acid based tests.Nucleic acid-based tests (NATs) amplify and detect viral nucleic acid. Most of these tests take a few hours, but rapid molecular assays are as fast as RIDTs. Among NATs, reverse transcription polymerase chain reaction (RT-PCR) is the most traditional and considered the gold standard for diagnosing influenza because it is fast and can subtype IAV, but it is relatively expensive and more prone to false-positives than cultures. Other NATs that have been used include loop-mediated isothermal amplification-based assays, simple amplification-based assays, and nucleic acid sequence-based amplification. Nucleic acid sequencing methods can identify infection by obtaining the nucleic acid sequence of viral samples to identify the virus and antiviral drug resistance. The traditional method is Sanger sequencing, but it has been largely replaced by next-generation methods that have greater sequencing speed and throughput. Treatment
Treatment of influenza in cases of mild or moderate illness is supportive and includes anti-fever medications such as acetaminophen and ibuprofen, adequate fluid intake to avoid dehydration, and resting at home. Cough drops and throat sprays may be beneficial for sore throat. It is recommended to avoid alcohol and tobacco use while sick with the flu. Aspirin is not recommended to treat influenza in children due to an elevated risk of developing Reye syndrome.