How to spread the new H7N9 flu

People are naturally nervous about the new strain of influenza virus (H7N9) which, as of 5 April had killed 6 people. Eerily the new flu emerged in Asia almost exactly 1 decade after the SARS epidemic. The causative agent of SARS is a Coronavirus, a completely different virus from influenza. To add to the confusion, in the past 9 months a new strain of Coronavirus has emerged in the Middle East, with a high initial case fatality rate (11/16).

My wife and I were in Hong Kong for the past few weeks for a happy family event, and the horror of the SARS epidemic was widely reported on TV and newspaper (interviews with survivors etc. etc.). I’d estimate that, on any given street in Hong Kong, Beijing, Kyoto, Osaka between 1 – 5% of people are wearing masks covering their nose and mouth. This was not a reaction to the news of the new flu (which came out only 6 days ago), but an attempt by those ill with any febrile illness to avoid spreading it.

First, a bit of reassurance. The most severe cases of any epidemic are always the ones by which it announces its presence. So the high case fatality rate (6/16) reported so far almost certainly won’t hold up. Later on, after the causative organism has been identified, and the immune response to it worked out (involving proteins attacking the bug — e.g. the antibodies), we always find that there were hordes of people who had only a mild illness or who weren’t sick at all (except for AIDS, but even there the most severe cases came to light first). However you don’t need a high fatality rate for an epidemic to be lethal — the 1918 flu had a case fatality rate of 10 – 20% yet it killed between 50 and 100 million people.

You couldn’t come up with a better way to spread a respiratory virus that what happened at JFK airport at two midnights ago (it’s the morning of the 6th here in the USA). Two large flights arrived at about the same time — the Cathay Pacific flight we took from Hong Kong had around 200 people. The other flight seemed equally large. Everyone had to go through immigration, showing passports etc. So upwards of several hundred people came off the planes and into a corridor perhaps 15 – 20 feet wide 10 feet high and several hundred feet long. There was one immigration official for US citizens and perhaps 2 more for the non citizens. So there we were, for well over an hour packed cheek by jowl in this space as people were processed through.

If you can think of a better way to spread an infectious virus between a group of unrelated people who will disperse all over New York City, and probably all over America, I’d like to hear it.

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  • Chasity P. Whitfield  On May 4, 2013 at 6:53 pm

    Technically, CFRs are actually risks (or ” incidence proportions “) and take values between 0 and 1. They are not rates , incidence rates , or ratios (none of which are limited to the range 0-1). If one wants to be very precise, the term “case fatality rate” is incorrect, because the time from disease onset to death is not taken into account. Nevertheless, the term case fatality rate (and the abbreviation “CFR”) is often used in the scientific literature.

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