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Influenza A: Epidemiology

by John Fisher, MD
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    00:01 Now, the attack rates are the highest in the young, but the mortality is highest in older individuals.

    00:14 So what then are other risk factors for the excess mortality and morbidity? Well, people with cardiovascular and pulmonary conditions, if they get influenza, you can imagine why they would have a problem.

    00:33 Patients with chronic metabolic disease like diabetics, patients with renal dysfunction who already have problems with, for example, pulmonary edema, patients who are immunosuppressed.

    00:50 They can be infected by common and unusual organisms.

    00:55 Pregnant women are particularly prone if they get influenza during pregnancy to having problems with influenza pneumonia, not just the respiratory tree, but actually the lung parenchyma in the second and third trimesters.

    01:13 And I’m not sure we clearly understand why, but it may be because the diaphragm is being elevated by the growing baby and that may compress the lungs and produce some normal atelectasis.

    01:29 And then obese persons are at risk for severe complications of influenza.

    01:37 Now, epidemic disease would be some kind of outbreak in one location, a city, town, or a country.

    01:46 And we usually find that it’s the single strain doing all this damage and it starts abruptly.

    01:54 In temperate climates we primarily see it in the winter.

    01:58 And these mini influenza epidemics, they peak in two to three weeks and they last about five to six weeks where doctors are diagnosing the flu right and left.

    02:10 Now you would think that someone who has not been vaccinated against the current strain would be very susceptible.

    02:17 Well, they are, but it only affects about 10% to 20% of unvaccinated persons.

    02:25 Now, the vaccine efficacy ranges between 50% and 90%.

    02:32 It’s not as effective for example in patients who have immunodeficiency or in elderly individuals.

    02:42 So let’s try to explain a little bit about these epidemics and pandemics that occur with influenza.

    02:50 There are several outbreaks every few years that we don’t seem to have much immunity to.

    02:57 And that’s usually the result of antigenic drift.

    03:02 And what we’re talking about are these spikes.

    03:05 Remember, the virus is helpless without these spikes, so it would be minor changes in the hemagglutinin spikes or minor changes in the neuraminidase spikes.

    03:19 And if we have good antibodies to both, we’re going to be pretty much immune to influenza.

    03:24 But if they change, either one, we can be susceptible.

    03:30 We might not have a severe flu, but we may be susceptible.

    03:36 So what’s happening with these spikes is they get new amino acids in them, new antigenic sites.

    03:48 Now, we can get a very serious change in influenza with something called antigenic shift.

    03:55 This would be some kind of radical change in the hemagglutinin spikes, neuraminidase spikes, or both.

    04:04 And when there’s a radical change that we haven’t seen in years, for example, then think of the world.

    04:11 Most of the world hasn’t seen a virus like this, there are very few people, for example, that are still alive from the 1918 epidemic.

    04:23 So if the population has little or no prior immunity, then that virus is going to wreak havoc on the population and there are going to be much higher attack rates and it’s going to begin abruptly, maybe at a time of the year when you don’t expect it.

    04:40 So, once again, antigenic shift is the emergence of another antigenically variant influenza virus which results in worldwide disease.

    04:51 And of interest, it may involve waterfowl because, believe it or not, waterfowl can get influenza.

    04:59 They don’t get human influenza, but there’s a distinct type of bird flu.

    05:05 It may go to other animals, and then to humans.

    05:09 So first you see, for example, human influenza strain and a bird influenza strain.

    05:17 Now, they are similar viruses but the bird flu wouldn’t necessarily infect a human.

    05:26 However, if there’s an intermediate host, such as swine, these viruses mix and the genes from these viruses mix and out of the swine comes a new human strain, and it may be one that no one has seen and that’s why it spreads to the general population.

    05:54 Alternatively, the human exposed to bird flu may mix with their own influenza strain and come up with a new virus strain and that can spread to the general population.

    06:14 So antigenic drift is a minor change in one of these spikes, an antigenic shift is some major change that no one has seen in years.


    About the Lecture

    The lecture Influenza A: Epidemiology by John Fisher, MD is from the course Upper Respiratory Infections. It contains the following chapters:

    • Influenza A – Epidemiology
    • Pandemic Disease-Pattern

    Included Quiz Questions

    1. A 1-year-old infant.
    2. A 54-year-old with diabetes.
    3. A healthy 28-year-old pregnant woman in her second trimester.
    4. A 52-year-old with congestive heart failure.
    5. A 21-year-old with mild persistent asthma.
    1. 50-90%
    2. Usually greater than 90%
    3. Usually less than 50%
    4. 10-20%
    5. 75-85%
    1. Antigenic drift is gradual accumulation of new amino acids on antigenic HA and/or NA antigenic sites, while antigenic shift is a radical change in NA and/or HA antigenic site.
    2. Antigenic drift is a sudden antigenic change while antigenic shift is gradual antigenic change.
    3. Antigenic drift often leads to pandemics, while antigenic shifts lead to epidemics.
    4. Antigenic drift often occurs in an unexpected season, while antigenic shift typically occurs in the winter in temperate climates.
    5. Antigenic drift is due to genetic combination of virus that affect various species, while antigenic shift is limited to human viruses only.
    1. Younger school aged children.
    2. Pregnant women.
    3. The geriatric population.
    4. Men and women in the third and fourth decades of life.
    5. College aged students.

    Author of lecture Influenza A: Epidemiology

     John Fisher, MD

    John Fisher, MD


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