Plan for a 30% “clinical attack rate” for the first wave: ECDC

21 07 2009

The European Centre for Disease Control and Prevention (“ECDC”) have just put out their “Interim Risk Assessment 20th July 2009” for Swine Flu (I was notified about this via CIDRAP‘s email alert service, which is a great daily update on all things Swine Fluish). You can read the ECDC’s full risk assessment here.

You can find the entire Executive Summary to the report reproduced below. Just a few terms to understand first:

  • “Clinical attack rate” means the total proportion of the population that catch swine flu over the entire wave of the pandemic. So, it’s a measure of the total number of people who get sick (most of them mildly, of course). It’s not a measure of the hospitalisation rate or the death rate.
  • “Hospitalisation rate” means the proportion of those who get sick who then need to go to hospital. Can one of my readers confirm this please? I understand that it’s the proportion of people who get infected who then have to go to hospital, NOT the proportion of the total population who will end up having to go to hospital?
  • “Case fatality rate” is the proportion of people, who have caught swine flu, who will then die.
  • “Oseltamivir” is Tamiflu.
  • “Zanamivir” is Relenza.

Here’s the executive summary:

 There are no reports as yet of unusual presentations or transmission routes for this influenza compared to normal seasonal influenza viruses. There is no indication of risk of infection through food or potable drinks.
 If the pandemic behaves like previous ones, cumulative clinical attack rates over the first major wave of infection in 2009–10 might be expected to be in the range of 20% to 30%, with a reasonable planning assumption of 30%.
 Based on experience in North America, clinical attack rates will be highest in children and younger adults.
 Adults over 60 years seem, at present, to be the least affected age group, though there are indications from the USA that those few that are affected experience the highest risk of severe disease of any age group.
 The groups experiencing most of the severe disease and death are those in the risk groups of people with chronic underlying medical conditions (this includes morbid obesity), pregnant women and young children (especially under two years of age).
 Most of those infected experience a mild self-limiting illness, even in people in risk groups. However, as for seasonal influenza there are some people who experience more severe disease and some of these die despite medical care. These include a few people without any known underlying condition and outside other risk groups.
 A reasonable planning estimate for hospitalisation rates in Europe using the overall clinical attack rate as a base is in the in the range 1% to 2%. However, in the winter this may rise because of the presence of other respiratory infections.
 Local experience from the USA (New York City) indicates that, without preparation, this pandemic can severely stress healthcare systems.
 The observed case fatality rate based on the largest population reported to date, from the USA, is 0.4%. While in Europe the observed rate in the earliest affected country (the United Kingdom) is 0.3%. However, this is likely to be higher than the true figure, which may at present be more than the range of 0.1% to 0.2% of all clinical cases.
 As in seasonal influenza, case fatality rates are high in the very young, low in children and young adults and then increase with age.
 At the individual level the highest risk of hospitalisation for an affected person is: a) in the risk groups; and b) for young children and those over 60.
 As yet almost all the viruses have been sensitive to the antivirals known as neuraminidase inhibitors (oseltamivir and zanamivir) but they are resistant to adamantenes (amantidine and rimantidine). There have been a few pandemic virus isolates that have showed resistance to oseltamivir (though sensitive to zanamivir).
 The current seasonal influenza vaccine that contains a component effective against another A(H1N1) virus is not effective against the new pandemic A(H1N1) 2009 virus.
 It is impossible to predict when European countries will be affected, but a proper first wave seems inevitable for the autumn. The experience in one country (the United Kingdom) suggests that countries could be affected considerably earlier in the autumn than happens with seasonal influenza.
 It is too early to predict what the mix of pandemic and seasonal influenza viruses will be this autumn, although there will also be B influenza viruses, as they do not compete with A viruses.
 Pandemic viruses are unpredictable, and can change their characteristics as they evolve. Even pandemics usually slow down in summer, only to pick up in autumn, and the virus may even then come back, perhaps in a more aggressive form, like it happened in 1918–19.
 ECDC will work with Member States, other European Agencies, the European Commission, WHO and its other international partners to gather more information to update this Risk Assessment at intervals. Special attention will be paid to how the pandemic is developing in the first affected European countries and the temperate Southern Hemisphere countries.

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