Spread and incidence of pandemic flu

News stories typically state with great assurance that the new H1N1 virus originated in Mexico. But what is the data supporting this assertion? Well, the earliest samples identified with the virus came from patients in Mexico. But does that mean that the virus originated in Mexico? Not necessarily.

Statements about the origins and spread of any virus depend on accurate numbers from affected countries. Thus, any statement that we *know* that the new H1N1 virus originated in Mexico assumes that affected nations are both efficient at detecting infection and honest about reporting the presence of the virus in their country. Does the pattern of the reported incidence of the new H1N1 virus support this assertion?

Let us examine the dataset I described here and see if it supports this optimistic view.

150 countries have reported at least one case of the new H1N1 virus. Leaving another 49 which have not. Is this credible?   There are many examples of countries that have not reported any cases but which are surrounded by countries with infected patients. It seems unlikely that all countries with cases have reported them.

If one looks at the incidence of infection, taking into account population size, one sees an interesting pattern. Here are the top 10 countries with the number infected per 100,000:

  1. Brunei – 196
  2. Australia – 99
  3. Cayman Islands – 77
  4. Chile – 70
  5. New Zealand – 65
  6. Malta – 38
  7. Cyprus – 37
  8. Canada – 31
  9. Singapore – 25
  10. Samoa – 21

There are several points about the countries that are on the list, and the ones that are not, that I’d like to make. First, small countries (Brunei, Cayman Islands, Malta, Cyprus and Samoa) appear to be over-represented. This is not surprising since it is likely easier to identify all infections in a small country, assuming that the government of such a country wishes too.

The presence of Australia, New Zealand and Chile on the list is not surprising given that these three countries are currently experiencing the Southern Hemisphere winter. The presence of Canada in the top 10 is a bit surprising given that it is currently in its low flu season.

Singapore is located in the tropics where flu does not exhibit the same seasonality observed in temperate regions. It has a relatively strong science base and honest government. Thus, the numbers from this country probably give the best estimate for the incidence of pandemic flu in the tropics.

Perhaps more interesting than the countries on the list are the ones not on the list. The most notable is Mexico, the presumed epicenter of the pandemic. Since the virus is thought to have originated in Mexico in February 2009, one might expect that the incidence would be highest in this country. Instead, it comes in at number 18 on the list with 15 cases per 100,000. The United States is at number 20 on the list with 14 cases per 100,000.

Although the new H1N1 virus spread rapidly throughout the world via air travel, there is one country with extensive trade with the US that has a very low incidence of the virus – China. It is number 106 on the list with 0.5 cases per 100,000. How can this be? China implemented thermal scans on all incoming international flights and placed suspected infected foreigners under strict quarantine. If the figures provided by the Chinese government are accurate, one would have to conclude that movement restrictions are an incredibly effective intervention. Of course, there is another possibility – that they are lying about the number infected (as they did with SARS). Since flu virus is spread before people show symptoms, one wonders how thermal scanners can keep infected people out of China. Further, the draconian quarantines were only applied to foreigners. Infected Chinese citizens were told to go home and stay there for a few days. No guards, no fever checks. Finally, Hong Kong is reporting many more cases than Mainland China. Yet, large numbers of people pass freely between these two parts of China every day.

If one considers the possibility that not all countries are being honest about the incidence of pandemic flu within their borders, then we must also consider another hypothesis: the new H1N1 virus may not have originated in Mexico. If not, where?

My guess would be one of the countries not providing believable numbers.

China, I’m looking at you.

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2 thoughts on “Spread and incidence of pandemic flu

  1. early outbreaks in Mexico
    first confirmed sample from a baby with
    symptoms 24.Feb.
    sequences show that A/Mexico/4115 and A/Mexico/InDRE4487 are closest to the origin

    no flu surveillance in swine in Mexico.
    If there were any evidence that it originated elsewhere, then why doesn’t Mexico disclose it ?
    They have a team in LaGloria, but no info(yet).
    InDRE won’t say what persons/locations 4115,4487 are from – why ?

  2. gsgs, all our interpretations are based on a limited dataset. If China and other countries are hiding cases now, they may have hid cases earlier. If they hid cases, then they necessarily hid sequences.

    Mexican authorities cannot compel the Chinese government to release all the sequences they have from people or pigs. We certainly do know that the Chinese government has hid information in the past about flu sequences in humans and pigs. More than that, we cannot prove without their cooperation.

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