Seasonality of influenza revisited

19 February 2009

239997306_60c718f6c7_mThe cute guinea pig returns for another installment on why influenza is prevalent during winter months in temperate climates.

We previously discussed work by Palese and colleagues in which a guinea pig model for influenza virus transmission was used to conclude that spread of influenza virus in aerosols is dependent upon temperature and relative humidity. They found that transmission of infection was most efficient when the humidity was 20-35%; it was blocked at 80% humidity. The authors concluded that conditions found during winter, low temperature and humidity, favor spread of the infection. Lower humidity favors virion stability and smaller virus-laden droplets, which have a better chance to travel longer distances.

Another group re-analyzed Palese’s data and found that relative humidity explains only a small amount of the variability in influenza virus transmission and survival – 12% and 36%, respectively. When they converted the measurements of moisture to absolute humidity, the results were striking: 50% of the variability in transmission and 90% of the variability in survival could be explained by absolute humidity.

Changes in relative humidity, the authors argue, do not match the seasonal patterns of influenza transmission. Although relative humidity indoors is low in the winter, outdoor levels peak during this season. Absolute humidity, on the other hand, has a seasonal cycle with low values both indoors and outdoors during winter months, consistent with the increased transmission of influenza. 

The conclusion of both papers is the same: humidification of indoor air during the winter might be an effective means of decreasing influenza virus spread. 

On the weather report, the amount of moisture in the air is given as a percentage. This is relative humidity – the ratio of the partial pressure of water vapor in a gaseous mixture of air and water vapor to the saturated vapor pressure of water at a given temperature. Absolute humidity is the actual amount of water vapor in a liter of gas. Any easy way to remember what measurement of humidity is important for influenza transmission is that it’s not what you hear on the weather report.

J. Shaman, M. Kohn (2009). Absolute humidity modulates influenza survival, transmission, and seasonality Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0806852106

  • Kelly

    I'm finding your blog quite facinating! Can you clarify for me, what is the difference between relative humidity and absolute humidity? Which is being referred to in weather reports?
    Thanks!

  • http://www.virology.ws profvrr

    Great question! We are talking about that on TWiV today (my virology
    podcast, twiv.tv) but since it won't be out for a few days, here is
    the answer: relative humidity is the ratio of the partial pressure of
    water vapor in a gaseous mixture of air and water vapor to the
    saturated vapor pressure of water at a given temperature. It is
    expressed as a percentage; the weatherman (and woman) uses this term.
    Absolute humidity – is the actual amount of water vapor in a liter of
    gas. This is not used by the weatherperson.

  • Kelly

    Prof,
    Thank you so much for the clarification! :)

  • Scott

    Your articles are wonderful. If you don't mind, could you explain the relative humidity a bit further. I used to think I was intelligent, but I cannot seem to wrap my brain around your relative humidity explanation to Kelly!

  • Scott

    Also, my daughter just tested positive for Influenza B. Our doctor has offered us some Tamiflu to avoid getting the flu, but we are very concerned about the side effects and are likely going to trust our quarantine system versus the antiviral. What is your advice on this? Thanks.

  • http://www.virology.ws profvrr

    How was your daughter tested for influenza? Does she have symptoms? It
    is likely that it's too late for Tamiflu to be effective; it has to be
    given early in infection. I can't advise you what to do, but remember
    that influenza B is usually (but not always) milder than influenza A.

  • http://www.virology.ws profvrr

    Good question. We actually didn't explain it very well on TWiV.

    Humidity measurements are all about determining how much water is in
    the air. The measurements are made with instruments designed for that
    purpose.

    Absolute humidity is the amount of water in a volume of air. It is
    usually expressed as grams of water per liter of air. Another way to
    look at it: the mass of water vapor divided by the mass of dry air in
    a volume of air at a given temperature. The hotter the air is, the
    more water it can contain.

    Relative humidity is the ratio of the current absolute humidity to the
    highest possible absolute humidity. The latter depends on the current
    air temperature.

    So they are just different ways of expressing how much water is in the
    air. One is absolute, (mass of water per volume of air), the other is
    relative – how much water is in the air compared to the maximum water
    the air can hold.

  • http://www.virology.ws profvrr

    How was your daughter tested for influenza? Does she have symptoms? It
    is likely that it's too late for Tamiflu to be effective; it has to be
    given early in infection. I can't advise you what to do, but remember
    that influenza B is usually (but not always) milder than influenza A.

  • http://www.virology.ws profvrr

    Good question. We actually didn't explain it very well on TWiV.

    Humidity measurements are all about determining how much water is in
    the air. The measurements are made with instruments designed for that
    purpose.

    Absolute humidity is the amount of water in a volume of air. It is
    usually expressed as grams of water per liter of air. Another way to
    look at it: the mass of water vapor divided by the mass of dry air in
    a volume of air at a given temperature. The hotter the air is, the
    more water it can contain.

    Relative humidity is the ratio of the current absolute humidity to the
    highest possible absolute humidity. The latter depends on the current
    air temperature.

    So they are just different ways of expressing how much water is in the
    air. One is absolute, (mass of water per volume of air), the other is
    relative – how much water is in the air compared to the maximum water
    the air can hold.

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  • fbedoya

    How come if the influenza virus has a high mutagenic rate and a high infectious capacity, there is only one virus strain identified every year? Wouldn't there be more than one strains of influenza virus detected per year?
    Thank you.

  • http://www.virology.ws profvrr

    A high mutation rate means the viral RNA changes, but that does not
    always translate into a different protein. There are more evolutionary
    constraints on protein than on RNA. Many changes occur in influenza
    viral RNA, but few make it into viral protein. Among the viruses
    circulating each year there are certainly many variants, especially at
    the RNA level, but these do not all lead to antigenic changes.

  • Guest

    There are more than one identified each year.

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