Influenza, more commonly known as the flu, is a very contagious respiratory disease that circulates annually across the globe. Flu season in the northern hemisphere begins each year in October, with the number of cases historically peaking between December and February (3). However, a high number of cases may subsist into late spring (3). With flu season beginning, here is what to know about the seasonal flu.
As of October 28, 2022, the CDC reports that the number of flu cases in the past month have been higher than usual (2). Indeed, the FluServ-NET hospitalization network reported 443 flu-related hospitalizations in 13 states—the highest number to be recorded by this point in ten years (2). In particular, adults over 65 years of age and children 0-4 years old have been heavily affected. Indeed, as we enter the thick of flu season, it is imperative that those over 6 months old are vaccinated to prevent serious illness and complications from influenza.
There are four known types of influenza viruses: influenza A, B, C, and D (6). Only the first two, influenza A and B, are responsible for causing illness in humans. These viruses spread when an infected person coughs, sneezes, or talks, spraying tiny droplets carrying viral particles into other peoples’ mouths and noses (6). Symptoms of seasonal flu begin to appear 1 to 2 days after the virus has entered an infected person’s body (1). People may experience fever, chills, fatigue, and muscle aches, typically accompanied by a sore throat, dry cough, and runny nose (5). Most patients recover fully within one to two weeks (1). However, influenza can cause serious complications such as pneumonia or heart problems, especially in children and those with compromised immunity (1).
Seasonal flu vaccinations are critical to protecting the health of the community because influenza viruses, much like any other virus, mutate over time. There are two studied mechanisms by which these changes occur (6). The first, antigenic drift, happens when point mutations occur in viral RNA because errors in base pairings go uncaught by RNA-dependent RNA polymerase, the enzyme that catalyzes and corrects viral RNA replication (6). As a result, mutated viruses can possess immunologically distinct surface antigens, reducing the effectiveness of existing antibodies in humans (6). Antigenic shift results in even more dramatic changes. Genetic material is exchanged between subtypes of influenza viruses when multiple subtypes infect a single organism, forming a new viral subtype against which there is likely low preexisting immunity (6). Antigenic shift causes infrequent flu pandemics, while antigenic drift is responsible for the occurrence of seasonal flu yearly.
Due to these mechanisms of viral change, a single influenza vaccine does not provide long-term or lifetime immunity. Consequently, since 1952, the WHO has organized annual meetings to assess the types of viruses circulating across the globe and predict the viral strains that will play a key role in next year’s flu season (6). Selected viruses become “candidate vaccine viruses” and are used by vaccine manufacturers to create the next year’s influenza vaccine. In addition to the immunity provided by vaccines, antiviral drugs are used to lower the risk of complications, hospitalizations, and death in high-risk patients (4).
References
1. Demirden, S. Furkan et al. “Influenza Vaccine: An Engineering Vision from Virological Importance to Production.” Biotechnology and Bioprocess Engineering, 1–25. 24 Oct. 2022, doi:10.1007/s12257-022-0115-8
2. “Early Wave of Flu Brings Early Flu Hospitalizations.” Center for Disease Control and Prevention, 28 Oct 2022, https://www.cdc.gov/flu/spotlights/2022-2023/early-wave-hospitalizations.htm
3. “Flu Season.” Center for Disease Control and Prevention, 20 Sept 2022, https://www.cdc.gov/flu/about/season/flu-season.htm
4. “Flu Treatment.” Center for Disease Control and Prevention, 11 Oct 2022, https://www.cdc.gov/flu/treatment/index.html
5. “Influenza (Seasonal).” World Health Organization, 6 Nov 2018, https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal)
6. “Key Facts about Influenza (Flu).” Center for Disease Control and Prevention, 24 Oct 2022, https://www.cdc.gov/flu/about/keyfacts.htm