Annual Seasonal Flu Outbreaks Caused by Influenza A and B Virus Infections
Seasonal influenza is an acute respiratory infection caused by different types and subtypes of influenza viruses. Primarily infecting the nose, throat, and lungs, influenza viruses transmit through respiratory droplets, which may spread due to sneezing, coughing, blowing one’s nose, or contact with contaminated surfaces. Today, influenza viruses present a worldwide distribution and a year-round disease burden. Although most epidemics in temperate climates emerge during the winter months, tropical regions may experience influenza epidemics throughout the year. Read along to learn more about the seasonal flu outbreaks, the characteristics of different types of influenza viruses, and which symptoms to look out for throughout this flu season.
What causes seasonal influenza?
Influenza viruses are RNA viruses of the family Orthomyxoviridae. According to their unique antigenic and biological properties, they are classified into 4 main types, which are A, B, C, and D. Among these, Influenza A and Influenza B are the only types of influenza to cause human disease of any concern. With more than 110 different subtypes, including the avian flu and the swine flu, Influenza A is known to cause the most severe disease. In fact, Influenza A has been responsible for all influenza pandemics and the majority of the influenza epidemics in the history of humanity, which includes the Spanish Flu (1918), Asian Flu (1957), Hong Kong Flu (1968), Bird Flu (2004), and Swine Flu (2009).
In comparison to Influenza A, Influenza B viruses are characterized by a lower mutation rate, pathogenicity, and complication risk. In fact, Influenza B viruses are currently only classified into two antigenically distinct lineages, which are B/Yamagata and B/Victoria. Of the remaining two types of influenza, Influenza C viruses are known to cause mild disease and relatively few outbreaks, whereas Influenza D has not been observed to affect humans.
As commonly observed with RNA genome viruses, the influenza virus mutates rapidly, frequently, and continuously due to antigenic drifts and antigenic shifts within its genetic material. Given the virus’s rapid mutation rate, it is always possible to get infected with influenza. In fact, flu vaccines are updated before every flu season in accordance with the estimations of the influenza subtype in circulation.
How common are seasonal influenza outbreaks?
In the majority of the northern hemisphere, seasonal influenza outbreaks commonly occur as regular annual epidemics during the period between November and April. These outbreaks often affect most countries for one or two months. In the southern hemisphere, on the other hand, the majority of influenza infections occur between June and October. Unlike temperate areas, equatorial regions experience influenza outbreaks all year round. In addition to these annual outbreaks, sporadic infections of influenza also occur. However, most sporadic cases of flu emerge due to the import of cases from tropical areas, and their frequency is very low.
What are the symptoms of seasonal influenza?
Not all people infected with influenza experience symptoms. In fact, it has been estimated that around 75% of all seasonal influenza infections during a typical flu season are asymptomatic. In symptomatic cases, symptoms of uncomplicated influenza often appear with a rapid onset and include fever, runny or congested nose, sore throat, cough, headache, myalgia, and headache. Alongside these respiratory and systemic symptoms, children infected with influenza may also experience gastrointestinal symptoms such as stomachache, nausea, vomiting, and diarrhea. While the majority of the infected people shortly recover without medical attention, influenza can cause severe disease and complications among high-risk groups, which include young children, the elderly, pregnant women, people with compromised immunity, and those with pre-existing medical conditions.
What should you expect this flu season?
Although the first influenza season since the onset of the pandemic was nearly non-existent in many countries, epidemiologists and virologists around the world are now urging caution over the increasing simultaneous circulation of influenza A/B, COVID-19, and other common respiratory viruses such as RSV. Indeed, two years of pandemic confinement measures such as masking, social distancing, and travel restrictions have caused the majority of the world population to have much lower levels of exposure to influenza viruses to have built up sufficient immunity against them. With the removal of all pandemic measures, experts are warning that this flu season may be characterized by a higher-than-usual rise in both COVID-19 and influenza cases.
How to protect yourself against seasonal influenza?
Although the majority of healthy individuals typically recover without the need for medical attention, following a nutritious diet, resting, and staying hydrated can help shorten the process. As mentioned above, there is also a seasonal influenza vaccine composed of both Influenza A and Influenza B. It is upgraded each year depending on estimations concerning the primary circulating subtype for the upcoming flu season. In a flu season, when the vaccine viruses match the circulating viruses, influenza vaccines have been shown to significantly reduce the risk of severe disease and complications. Therefore, especially for people in high-risk groups, experts recommend vaccination each year before the onset of the flu season to ensure maximum immunity against the virus.
Since the seasonal pattern, disease onset, and clinical presentation of influenza can be very similar to many other respiratory infections, such as the common cold, RSV, and COVID-19, symptoms alone may not be sufficient for its differentiation. Mainly during periods such as the flu season, where multiple of these viruses may be circulating simultaneously, there may be a growing need for the use of diagnostic tests. To address these needs, our Influenza A/B Rapid Test Kit, RapidFor™ SARS-CoV-2 & FLU A/B Antigen Combo Test Kit, and SARS-CoV 2 + FLU A/B + RSV Combo Test Kit enable the detection and differentiation of influenza A/B infections in minutes.
REFERENCES
Ghebrehewet, S., MacPherson, P., & Ho, A. (2016). Influenza. BMJ, i6258. https://doi.org/10.1136/bmj.i6258
Eichberg, J., Maiworm, E., Oberpaul, M., Czudai-Matwich, V., Lüddecke, T., Vilcinskas, A., & Hardes, K. (2022). Antiviral Potential of Natural Resources against Influenza Virus Infections. Viruses, 14(11), 2452. https://doi.org/10.3390/v14112452
Skelton, R. M., & Huber, V. C. (2022). Comparing Influenza Virus Biology for Understanding Influenza D Virus. Viruses, 14(5), 1036. https://doi.org/10.3390/v14051036
