Some useful numbers on the pandemic flu

flu virus from CDC
Support for H1N1 Pandemic Alert:

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SCIENCE: Early Look at H1N1 Flu Supports Pandemic Alert

An early look at the H1N1 flu outbreak, released Monday by Science, suggests the World Health Organization was justified in its recent decision to raise the global pandemic alert level.


Some important numbers have now been calculated for this swine flu virus. The critical things to be aware of for any flu are its virulence and its transmissibility. At the moment, this flu does not seem to be as virulent as the earlier 1918 flu which killed, by some estimates 2.5-5% of those infected.

But that is comparing the current outbreak to the more deadly second and third waves of the Spanish flu. It is better to compare this to the initial outbreak of the Spanish flu in 1918.

Here are some of the preliminary numbers from the paper. This swine flu appeared in late April to have an estimated case fatality ratio (CFR) of 0.3% to 1.5%. (The CFR is a measure of the percentage of deaths in a population.) These numbers were based on the original outbreak. Including more recent data gives an upper bound for CFR of about 0.6%.

This puts it, at the end of April, in Category 3 of the CDC Pandemic Severity Index. This was already worse than the last two pandemic outbreaks in 1957 and in 1968. This fatality rate is higher than normal flu, which is about 0.1%.

The authors also estimated the transmissibility of the new virus – its reproductive number. This is a measure of how many people a single case infects. Flu is normally not very easily transmitted from human to human. Whereas measles has a reproductive number of 12-16, influenza has one of about 1.2-3. The number that has been estimated for the current flu outbreak is about 1.5. That does not seem so bad.

But these are actually worrisome facts and why it is a good thing that the WHO brought us all through this process as strongly as they did.

When the 1918 flu first hit, in the spring of 1918, it too had a slightly higher mortality rate than normal. About 0.5% Say a Category 3. That was in the late spring. But 4 months later, during normal flu season, it came back with a vengeance. Its death rate now was over 5 times higher. It had become a Category 5.

Similarly with the reproductive number. The initial outbreak of the Spanish flu has a number of about 1.5. It had more than doubled to 3.5 by the next wave.

The numbers that WHO was working with suggested a flu with similar virulence and transmissibility as the initial wave of Spanish flu. The precautions they took may have slowed the progression of the virus in this initial wave.

While more recent data indicate that the virulence of swine flu may not be as high as in the initial outbreak in Mexico, the estimated numbers suggest continuing caution and observation.

So the fact that this swine flu is not as deadly as the 1918 flu pandemic is not very relevant at this point. In fact, it is about as virulent as the Spanish flu at the same point in the outbreaks. Its CFR and reproductive number are also similar to that seen with the initial wave of Spanish Flu.

Finally, the authors mention that the presence of cross-immunity might be a possibility for the difference in mortality for the swine flu. That is, previous exposure to a different Type A influenza virus may confer some protection to a population. This was suggested by some of the data that indicated that children less that 15 years old were much more susceptible to the virus than those over 15.

Thus, some populations may have some initial protection not found in other populations. The difference in mortality rates between Mexico and the US may not just be a consequence of health care but could be because the US population was exposed in an earlier year to a Type A not seen in Mexico. I am sure this conjecture will be a focus of intense investigation.

The swine flu will spend a few months in the Southern Hemisphere before returning. WHO and others will be watching its progress there to see if real virulency and increased transmissibility are acquired.

I think we are far from done with this virus. I’m just hoping the numbers do not change much.

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