Masks and Influenza Transmission


respirators_370pxThose bulky N95 respirator masks may not be any better than regular surgical masks in preventing influenza transmission.

According to this JAMA study, 446 nurses were assigned to use either regular surgical masks or the N95 respirator masks when caring for febrile patients with respiratory illnesses during the 2008-2009 influenza season. Of those nurses, 50 using plain surgical masks became infected with influenza and 48 using the N95 respirator masks became infected with influenza – a result that did not reach statistical significance.

Another interesting statistic in the study – roughly 70% of nurses who had influenza diagnosed by serology had no symptoms.

Not only does this study show that plain surgical masks and N95 masks are comparable in flu prevention, but it also shows the significant risk health care workers encounter when caring for patients with influenza like illnesses. Out of 446 nurses, 98 got influenza even when using masks. That’s a 22% transmission rate with mask use. According to this article in Science, transmissibility of influenza is estimated to be 27.3% without mask use. Based on these studies (and I haven’t done exhaustive research for others), the effectiveness of masks seems to make only a modest difference in reduction of influenza transmission.

The Science study showed that the largest factor in reducing transmission of influenza throughout the population was … immunization. According to the article, reaching 70% immunization, plus immunization of the high risk groups such as health care workers could avert a pandemic.

I have heard of several hospitals that require non-immunized staff to use N95 masks at all times during work hours.

The JAMA study made me wonder whether such a policy uses the masks like “nag screens” with computer programs: The masks may not affect influenza transmission that much, but they encourage workers to get immunized … just so they don’t have to keep wearing the masks.


  1. I have to tell you WC… I am extremely confused regarding the pros and cons doe getting the swine flu vaccine!

    I’ve heard things that scare me about getting it and things that scare me about not getting it.. which has resulted in none of us getting any shots.

    And this isn’t because I have never gotten a flu shot. I believe in childhood vaccinations, etc.

    But now.. and I forget which way this goes… one of the vaccines.. set you up to get a worse case of the other flu.

    Thimerosal is a concern for some. Possibility of neuro damage? Not sure why else.

    Some say it looks like the swine flu isn’t as virulent as it was.

    I recall your post about healthy younger people being affected more so.

    I am just confused… and admittedly afraid about it.. no matter which way i choose for myself and family.

  2. I never wear masks while examining flu patients. It just confirms all the wrong things. They see doctors and nurses in masks and they think, “Oh shit, I’m gonna die!”

    Good opportunity to educate. “No, Swine Flu is not Ebola… Yes, handwashing is how we prevent flu… See I wash my hands before and after touching you…”

    Nobody listens when you have that thing over your face.

  3. What I don’t understand in this study is how this can have any sort of meaning considering you cannot control for non-hospital exposure. Say a hcw wearing the n95 mask makes it through her day properly wearing her mask, adhering to hand hygiene, and other protective measures but then goes to pick up her daughter at a 20-kid daycare and is exposed. How would this not be a huge confounding situation. Considering the majority of time is spent outside the clinic (and most likely without mask protection) doesn’t this significantly drop the confidence to have in this study?

  4. Seaspray:

    I think you are talking about an unpublished Canadian study which suggested that people who got a (regular, seasonal) flu shot last year were more likely to get H1N1.

    This study has *not* been corroborated and, in fact, a study published in the BMJ this month suggests that the opposite is true (patients in Mexico who got seasonal flu shots last year were less likely to get H1N1).

    The CDC recommends that the following groups get vaccinated:
    pregnant women (because they are at higher risk for complications and to provide protection for infants who will not be vaccinated)
    household contacts of infants less than six months of age (because children must be older than six months to be vaccinated)
    healthcare workers (to reduce the spread of illness)
    All people from 6 months to 24 years of age (because many of our cases have been in young people)
    People younger than 64 who have health conditions that could cause them to be vulnerable to flu complications

      • Yikes! So much for this non-medical mind to sift through…but what I glean from the counter point article to, is that the study was skewed because of how the controls were conducted and is not an accurate representation to be relied upon. ??

        Also.. they mention 2008-2009 vaccine. Is that the current standard flu vaccine for now … or are we on the 2009-2010 vaccine?

        Because what I am wondering is if the study were accurate… couldn’t we just get the regular vaccine because isn’t one of the concerns that this H1N1 vaccine was made to quickly and hasn’t been tested long enough?

  5. “when caring for febrile patients with respiratory illnesses”

    this strikes me as the most problematic aspect of the study. what about other types of exposure? say from walking through a busy ER triage area? or before you realize that the patient has a fever and respiratory symptoms?

  6. At least limitations to the study are mentioned, albeit haphazardly IMHO. What skews the meaning & confidence for me is that they didn’t monitor the adherence to handwashing or other types of exposure. In fact, I got the feeling that they felt that, as droplet infection control measures included PPE (gowns, gloves & masks) handwashing probably wasn’t an important variable.
    I submit that to conclude that it was the failure of the mask to prevent infection in the percentage of cases mentioned, correlation has been mistaken as causation as the mentioned limitations have not been monitored, evaluated and used as exclusion data.
    But the devil’s advocate in me thinks this is a great study for the bean-counters to use as an excuse for not providing the more expensive masks.

  7. Apologies for hogging the blog, but I’d also like to add that we don’t use the fancy schmancy N95 masks in my corner of the world (rural NSW, Australia). We do use masks, but have the habit of putting them on patients with the aforementioned respiratory symptoms, and ask any visitors/relatives or any other person coming into the health care facility with said symptoms for whatever reason to don them as well. We also have a fairly robust handwashing program which has been extended to all people, not just staff, within the hospitals with the gel washes provided just about everywhere you can look.
    Will be interesting when the compliance audits come in! (My unit is responsible for the number crunching, among other things of a clinical governance nature)

  8. I have used these N95 masks and the cheap masks in my woodshop. Both are absolutely useless. Air freely flows around the edges of the mask because it does not make a good seal with the face. $30 later I have myself a good mask with great rubber seals. I’ve had the regular flu before, I don’t want it again.

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