How does flu spread?

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Flu is an infectious disease caused by the influenza virus. Symptoms can range from mild to severe depending on the type of influenza virus, age and general health of the person infected. Common symptoms include fever, sore throat, runny nose, headache, muscle pain and general tiredness which generally appear two days after exposure to the virus and last between one to two weeks. It is difficult to differentiate between the common cold and flu in the early stage of an infection, however flu symptoms are generally more severe and with a quicker onset1.

Flu Transmission

The flu virus typically enters the body through the surfaces of the nose, throat and lung. As such there are three main ways in which flu is spread; directly, when an infected individual coughs or sneezes directly into the face of another person spreading the virus onto the receptive surfaces described above. Through airborne infection, whereby and infected individual coughs or sneezes releasing flu virus particles as an aerosol into the air which then land on the receptive surfaces and infect a new individual. Finally, the indirect route whereby an infected person contaminates surfaces such as their hand or a door-knob, an un-infected individual is then contaminated with these particles and then infects themselves2.

When an infected individual sneezes or coughs hundreds of thousands of virus particles are released. Although the majority of these particles will be contained within larger droplets which quickly settle out of the air smaller particles (one-twentieth to one-two-hundredth of a millimetre in size) can remain airborne and are capable of infection3. The survival time of the flu virus in the air varies depending on the levels of humidity and sunlight, preferring a dry and dark environment.

If these particles settle, or are directly passed onto other surfaces then they have the potential to infect further people. The length of survival time for the flu virus depends on the surface; surviving for up-to three days on hard surfaces such as wood, or just fifteen minutes on dry tissues. However, if the virus particles are enclosed in mucus they are able to survive for much longer periods4.

An individual infected with flu is infectious whilst they are producing and shedding virus particles. This begins approximately one day after infection, peaks at around two days and typically lasts up to seven days1. As symptoms typically arise around two days after infection it is easy for infected people to spread the virus unknowingly.

Flu Seasonality

Flu displays a strong seasonality with many outbreaks occurring in winter, with separate northern and southern hemisphere outbreaks spread throughout the year. The exact reason for this is unknown although there are several possible explanations; firstly, that in winter people are indoors more often resulting in more close contact. Secondly, colder temperatures lead to drier air which dehydrates the mucus of receptive surfaces making them easier for virus particles to infect. Finally, as previously mentioned virus particles can survive for longer periods of time in colder areas with little sunlight5.

Prevention and Treatment

The easiest and most effective way to reduce the spread of flu is to encourage good personal health and hygiene habits such as avoiding touching receptive surfaces such as the nose or mouth coupled with frequent handwashing. Infected individuals should cover their mouth and nose when coughing or sneezing, preferably with a disposable tissue and ideally remain at home when sick so as not to infect others. Public areas should be routinely cleaned especially surfaces that are frequently touched such as bathroom taps or door handles.

A vaccine against the flu is available and is recommended by the World Health Organization (WHO) for high-risk groups such as pregnant women, children, the elderly, those with chronic illnesses and front line health care workers 2. Due to the high mutation rate of the flu virus an individual vaccination usually only protects an individual for a few years. Every year the WHO tries to predict which variants of flu will be circulating and recommends the development of vaccines to combat them. As this development can take several months and it is not possible to vaccinate against all forms of flu at once certain variants do get overlooked, or a new strain can arise during this time.

There is currently no cure for the flu. Infected individuals are encouraged to drink plenty of liquids, rest at home and take medication such as paracetamol to help with symptoms. High risk individuals as listed above should be observed to ensure no complications arise. As flu is a virus antibiotics have no effect on the infection. Antivrial drugs are available although they have limited benefit, reducing time of symptoms by approximately one day6, and are also becoming less effective due to the development of resistance7.

Author: Aaron G

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References :

+ Carrat, F. et al. Time lines of infection and disease in human influenza: a review of volunteer challenge studies. Am J Epidemiol 167, 775-785 (2008).

+ Influenza virus infections in humans, (2014).

+ Cole, E. C. & Cook, C. E. Characterization of infectious aerosols in health care facilities: an aid to effective engineering controls and preventive strategies. Am J Infect Control 26, 453-464 (1998).

+ Bean, B. et al. Survival of influenza viruses on environmental surfaces. J Infect Dis 146, 47-51 (1982).

+ Lowen, A. C., Mubareka, S., Steel, J. & Palese, P. Influenza virus transmission is dependent on relative humidity and temperature. Plos Pathog 3, 1470-1476 (2007).

+ Michiels, B., Van Puyenbroeck, K., Verhoeven, V., Vermeire, E. & Coenen, S. The value of neuraminidase inhibitors for the prevention and treatment of seasonal influenza: a systematic review of systematic reviews. PLoS One 8, e60348, (2013).

+ Moscona, A. Global transmission of oseltamivir-resistant influenza. N Engl J Med 360, 953-956, (2009).

+ Moscona, A. Global transmission of oseltamivir-resistant influenza. N Engl J Med 360, 953-956, (2009).


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