- Download PDF Copy
By Dr. Tomislav Meštrović, MD, Ph.D.Reviewed by Sophia Coveney
Influenza is a highly infectious viral disease that can occur as a pandemic, epidemic, outbreak and in form of sporadic cases. A majority of human infections are caused by either type A or B influenza viruses. Type A has been associated with widespread epidemics and pandemics, while type B has been infrequently implicated in regional epidemics. Influenza type C infections cause only a mild respiratory illness.
Yearly influenza epidemics can affect all populations, but children younger than the age of two, adults older than 65, as well as people with chronic medical conditions or weakened immune systems, bear the highest risk of complications. The annual attack rate is estimated at 5–10% in adults and 20–30% in children.
In temperate climates seasonal epidemics of the virus occur mainly during the wintertime, compared to tropical regions where it may occur throughout the year, resulting in much more irregular outbreaks. The epidemics caused by the influenza virus are estimated to result in about 3 to 5 million cases of severe illness, and up to 500 thousand deaths worldwide.
Influenza virus. Image Credit: pinkeyes / Shutterstock.com
Modes of transmission
Influenza virus is transmitted among humans in three main ways: by direct contact with infected individuals, through contaminated objects (such as hairbrushes or towels – often called fomites) and by inhaling virus-laden aerosols. The contribution of each mode of transmission to the overall spread of influenza is not known.
The production of aerosols that contain virus particles is necessary for respiratory transmission. Speaking, singing and even normal breathing can produce an adequate amount of aerosols. Meanwhile, sneezing and coughing lead to more forceful expulsion.
Aerosolized particles produced by the aforementioned activities have different sizes. The largest droplets fall to the ground within a few meters and will infect only those in the immediate vicinity. Distance that other droplets cross is often determined by their size.
Droplets that are between 1 and 4 microns in diameter are known as “droplet nuclei”. These remain suspended in the air for long periods of time and not only they have the ability to travel long distances, but they can also reach the lower parts of the respiratory tract. Inhalation of droplets and droplet nuclei situates the influenza virus in the upper respiratory tract, where it has the opportunity to initiate infection.
Related Stories
- New influenza vaccine strategies aim to enhance protection with T-cell responses
- New study warns of potential pandemic risk from α-H1N2 swine flu virus
- New project launched to accelerate development and accessibility of H5N1 mRNA vaccine candidates
Nasal secretions that contain virus particles are responsible for transmission by direct contact or via contaminated objects. An infected individual most often touches the nose or conjunctiva, thus placing the virus on the hand. Any ensuing contact (for example, shaking hands) can transfer the virus to another person, who will then infect themselves by simply touching their eyes or nose.
In addition, the virus transmission also happens upon touching other objects with contaminated hands. Researchers have shown that up to 60% of objects from homes and daycare facilities were shown to harbor influenza viral RNA. Infectious influenza virus may also persist on paper bills for several weeks, which is another possible way of spread.
Changes of the influenza virus: drift and shift
Viral glycoproteins hemagglutinin (HA) and neuraminidase (NA) change periodically due to sequential evolution within immune or partially immune populations. Such periodic changes result in antigenic mutants of the virus, which are selected as the predominant virus because the antecedent virus is suppressed by a specific antibody arising in the previously infected population. Such cycle then repeats continuously.
One type of change is called antigenic drift, which occurs when small changes in the virus happen continuously over time. Surface antigens of the virus undergo minor change, and since the protection that remains from past exposures to similar viruses is incomplete, an epidemic can ensue. Antigenic drift can occur in all three types of influenza virus.
On the other hand, antigenic shift represents an abrupt, major change in one or both surface antigens (HA or NA) that occurs at varying intervals. It is a result of a genetic recombination (an exchange of gene segments) between influenza A viruses – most often those that affect humans and birds. If the virus spreads efficiently from person to person, an antigenic shift may result in a worldwide pandemic.
References
- http://www.cdc.gov/flu/professionals/index.htm
- https://www.who.int/
- https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/365793/Green_Book_Chapter_19_v6_0.pdf
- http://www.virology.ws/influenza-101/
- http://ideas.health.vic.gov.au/bluebook/influenza.asp
- Nicholson KG, Webster RG, Hay AJ. Textbook of Influenza. Blackwell Science, Oxford, 1998.
- Lamb RA, Krug RM. Orthomyxoviridae: The viruses and their Replication. In: Fields Virology fourth edition, Knipe DM, Howley PM eds, Lippincott, Philadelphia 2001, pp 1487-1531.
Further Reading
- All Influenza Content
- Influenza - What is Influenza?
- Types of Influenza
- Influenza Immunization
- Influenza Prognosis
More...
Last Updated: Dec 23, 2022
Written by
Dr. Tomislav Meštrović
Dr. Tomislav Meštrović is a medical doctor (MD) with a Ph.D. in biomedical and health sciences, specialist in the field of clinical microbiology, and an Assistant Professor at Croatia's youngest university - University North. In addition to his interest in clinical, research and lecturing activities, his immense passion for medical writing and scientific communication goes back to his student days. He enjoys contributing back to the community. In his spare time, Tomislav is a movie buff and an avid traveler.
- Download PDF Copy
Citations
Please use one of the following formats to cite this article in your essay, paper or report:
APA
Meštrović, Tomislav. (2022, December 23). Influenza Epidemiology. News-Medical. Retrieved on September 15, 2024 from https://www.news-medical.net/health/Influenza-Epidemiology.aspx.
MLA
Meštrović, Tomislav. "Influenza Epidemiology". News-Medical. 15 September 2024. <https://www.news-medical.net/health/Influenza-Epidemiology.aspx>.
Chicago
Meštrović, Tomislav. "Influenza Epidemiology". News-Medical. https://www.news-medical.net/health/Influenza-Epidemiology.aspx. (accessed September 15, 2024).
Harvard
Meštrović, Tomislav. 2022. Influenza Epidemiology. News-Medical, viewed 15 September 2024, https://www.news-medical.net/health/Influenza-Epidemiology.aspx.
Suggested Reading
Stem cell therapy advances: MSCs show potential in managing COVID-19 and influenza infections
A call for robust H5N1 influenza preparedness and response
Study reveals distinct genetic risk factors for influenza and COVID-19
Optimizing influenza vaccine strategies with nanoparticle immunization
Study reveals a promising approach to developing universal influenza vaccine
VUMC researchers discover potent antibody against influenza B
H5N1 influenza outbreak in Texas dairy cattle sparks multi-species transmission concerns
Flu viruses can use a second entry pathway to infect cells
Comments
The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
Post a new comment
Login
(Logout)
Post
Terms
While we only use edited and approved content for Azthena answers, it may on occasions provide incorrect responses. Please confirm any data provided with the related suppliers or authors. We do not provide medical advice, if you search for medical information you must always consult a medical professional before acting on any information provided.
Your questions, but not your email details will be shared with OpenAI and retained for 30 days in accordance with their privacy principles.
Please do not ask questions that use sensitive or confidential information.
Read the full .