Influenza virus transmission

sneezeInfluenza virus may be transmitted among humans in three ways: (1) by direct contact with infected individuals; (2) by contact with contaminated objects (called fomites, such as toys, doorknobs); and (3) by inhalation of virus-laden aerosols. The contribution of each mode to overall transmission of influenza is not known. However, CDC recommendations to control influenza virus transmission in health care settings include measures that minimize spread by aerosol and fomite mechanisms.

Respiratory transmission depends upon the production of aerosols that contain virus particles. Speaking, singing, and normal breathing all produce aerosols, while coughing and sneezing lead to more forceful expulsion. While coughing may produce several hundred droplets, a good sneeze can generate up to 20,ooo. Aerosolized particles produced by these activities are of different sizes. The largest droplets fall to the ground within a few meters and will transmit an infection only to those in the immediate vicinity. Other droplets travel a distance determined by their size. Those droplets 1-4 microns in diameter are called ‘droplet nuclei’; these remain suspended in the air for very long periods and may not only travel long distances, but can reach the lower respiratory tract. Inhalation of droplets and droplet nuclei places virus in the upper respiratory tract, where it may initiate infection.

The importance of aerosol transmission is illustrated by an outbreak of influenza aboard a commercial airplane in the late 1970s. The plane, carrying 54 persons, was delayed on the ground for three hours, during which time the ventilation system was not functional. Most of the travelers remained on board. Within 72 hours, nearly 75% of the passengers developed influenza. The source of the infection was a single person on the airplane with influenza.

Nasal secretions, which contain virus particles, are responsible for transmission by direct contact or by contaminated objects. An infected person will frequently touch their nose or conjunctiva, placing virus on the hand. Intimate or non-intimate contact (e.g. shaking hands) will transfer the virus to another person, who will then infect themselves by touching their nose or eyes. When contaminated hands touch other objects, the virus is transferred to them. In one study, 23-59% of objects from homes and day care facilities were shown to harbor influenza viral RNA. Others have shown that infectious influenza virus may be persist on paper currency for several weeks.

Influenza transmission can be reduced by covering your nose and mouth when coughing or sneezing, and by washing hands often with soap and water or alcohol-based hand cleaners. Note that CDC does not recommend the use of face masks for reducing viral spread. It is important to recognize that, in human infections, maximum levels of virus shedding may occur about a day before the peak of symptoms.

Ethical considerations preclude controlled influenza virus transmission studies in humans, and therefore animal models must be used. Ferrets are susceptible to many strains of influenza virus, and develop symptoms similar to those in humans. However these animals are costly and difficult to house, precluding their use in most large scale transmission studies. The guinea pig has recently been described as an alternative animal model for studying influenza virus transmission. These animals are susceptible to different viral strains, and the virus replicates to high titers in the respiratory tract. The guinea pig was used to show that transmission of influenza virus occurs by aerosols and through contaminated environmental surfaces. The efficiency of aerosol transmission in the guinea pig model is regulated by temperature and humidity.

Fabian, P., McDevitt, J., DeHaan, W., Fung, R., Cowling, B., Chan, K., Leung, G., & Milton, D. (2008). Influenza Virus in Human Exhaled Breath: An Observational Study PLoS ONE, 3 (7) DOI: 10.1371/journal.pone.0002691

Carrat, F., Vergu, E., Ferguson, N., Lemaitre, M., Cauchemez, S., Leach, S., & Valleron, A. (2008). Time Lines of Infection and Disease in Human Influenza: A Review of Volunteer Challenge Studies American Journal of Epidemiology, 167 (7), 775-785 DOI: 10.1093/aje/kwm375

Mubareka, S., Lowen, A., Steel, J., Coates, A., García‐Sastre, A., & Palese, P. (2009). Transmission of Influenza Virus via Aerosols and Fomites in the Guinea Pig Model The Journal of Infectious Diseases, 199 (6), 858-865 DOI: 10.1086/597073

Comments on this entry are closed.

  • anne 29 April 2009, 2:42 pm

    Thank you.. it's very clear.

  • Vicky 29 April 2009, 6:20 pm

    I was reading that some countries do not have the techniques for testing if a virus is of the present H1N1 type.
    Which exactly is the procedure for this test
    and why it is so complex/expensive?

  • profvrr 29 April 2009, 7:26 pm

    It is true that some of the assays are complex, and several are used.
    The older method is to isolate the virus in cell culture by
    inoculating a nasal swab specimen into cultured cells. Then a panel of
    antibodies against the 16 known HA subtypes are used in a variety of
    assays, such as neutralization assays, in which the capacity of the
    antibody to block infection is determined. For example, if the virus
    is an H1 virus, only antibodies against the H1 HA would block
    infectivity. Other assays are also used. Other diagnostic methods
    include direct fluorescent antibody testing, immunoassays, real-time
    reverse transcription-PCR, sequencing, and multiplex RT-PCR. Viral
    culture is the “gold standard” for typing and subtyping of influenza
    viruses, but takes 3 to 7 days to culture the virus. In experienced
    hands they are not complex, but they must be done properly to have
    confidence in them. Lab personnel need to be trained in the methods
    because clearly a great deal depends upon a reliable test.

  • Francis 30 April 2009, 12:59 pm

    Why does the CDC recommend against face masks when the science clearly shows they are exceedingly beneficial at stopping the transmission of flu-like disease?

    Here's a study I found on the CDC's own site clearly demonstrating the efficacy of properly fitted N95 face masks during the SARS outbreak.

  • Fugue 2 May 2009, 10:11 pm

    If face masks are useless, then why do surgeons wear them when they perform surgery? Sure, nobody with any common sense believes they offer 100% protection, but even a little extra protection is better than none. Another thing that I find outstanding is that nobody seems to be mentioning the use of protective gloves. Handwashing may be fine, but doesn't prevent germs from getting on your hands and spread around. Sure, gloves don't prevent germs from being spread along surfaces once on the glove, but – without gloves, most people touch their faces, scratch an itch, or rub their eyes without thinking just as part of normal reaction. Gloves create a more conscious awareness of the contact we make with our faces. And of course, gloves prevent germs from getting on your hands in the first place, which helps with the problem of poor handwashing techniques, (and lack of soap and water or lack of disinfecting lotion), or times when constant handwashing would be needed when in contact with a sick person. As a mother, I have found that I dramatically decreased my own chances of getting sick every flu season by using gloves when my child is sick. And parents know how challenging it can be to manage the constant bodily fluids that fly in every direction from a sick kid. They are little disease factories. Ideally, a glove that contains disinfectant or such on it's outer surface would be very ideal. Hmmm… wonder if anyone makes anything like that?

  • tarawillis 4 August 2009, 7:51 pm

    Hi, does anyone know what wavelengths H1N1 will absorb?

  • profvrr 5 August 2009, 3:01 pm

    280 and 260 nanometers.

  • Anonymous 15 November 2009, 9:43 am

    thanks. that will be really helpful for the project im doing on the flu

  • Anonymous 15 November 2009, 5:43 pm

    thanks. that will be really helpful for the project im doing on the flu

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  • Malissia 30 September 2010, 11:02 pm

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  • Ckirk769 1 February 2011, 11:10 pm

    What cell does the influenza virus infect?

  • profvrr 2 February 2011, 11:47 am

    Influenza virus infects the epithelial cells lining the respiratory tract.

  • Schlossmac 3 May 2011, 2:18 am

    So, can you help answer a question. I recently visited the island of Hawaii. I traveled to the more dap volcanic side of the island in the Valcano National park. It was rainy and misty and sub-tropical in some of the areas. While hiking along a trail I innadvertantly inhaled a single droplet of moisture. I almost instantly reacted negatively to it. Two days later I was laid out cold!. I have fought cancer before and I can tell you this little inhaled droplet was as bad as one of my bad treatment days with chemo. I do not believe that a virus is contracted necessarily from human droplets as there was no one on the trail anywhere around us. I will tell you I think something like this happened to me before with moist air. It is really scary! Any ideas>

  • profvrr 4 May 2011, 8:01 pm

    Hard to speculate on what happened. Unless there are animals around,
    the droplets won’t contain animal viruses, which are most likely to
    infect you. Plant viruses aren’t likely to make you sick – at least we
    don’t think so. An environmental toxin in the droplet? Allergic
    reaction perhaps?

  • Petapandabear 21 June 2011, 5:08 pm

    this was really helpful its eggxacly what I was looking for …….Thanks A Lot

  • guest 14 October 2011, 12:27 am

    Does the seasonal flu shot prevent or diminish the ability for the person who got the shot from carrying and or transmitting the virus?

  • reverse phone lookup 17 November 2011, 9:23 pm

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  • Jmzirger 1 December 2011, 5:16 pm

    Does anyone on this site have any information (preferrably with a literature reference) on the use of  the influenza virus as a model to study other viruses?

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  • Demoss00 28 February 2012, 9:53 am

    if a sibling has the flu can other health siblings in the housold be carriers to others outside the living areas?

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  • newsnose 7 August 2014, 5:18 pm

    I wonder if a type of fungi was in the droplet. Just a thought

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