WHO alerted the world last week of a flu outbreak in Mexico that is caused by
a new swine virus. The 'swine flu' has caused >80 deaths since March this
year. The same strain is confirmed to have caused cases in California and now
some ill people have travelled by air internationally, so cases are being
reported worldwide. Is this a pandemic and how dangerous is it?

Pandemic flu has been on the cards for some time, and we've all been worrying
that avian flu from South East Asia was the candidate. Instead a swine strain
appears to have taken off in Mexico. I think this outbreak illustrates why
studying global disease is so important – epidemics can begin in out of the way
places, or places with less well developed surveillance systems, then unleash
themselves on the rest of the world in days. Now we may discover whether all the
pandemic influenza planning that's been happening works – stockpiling drugs,
emergency vaccines, border checks and the new international health regulations.

CABI has (timing!) just published a book on communicable disease
epidemiology: Communicable diseases
epidemiology and control
by Roger Webber. I
dipped into the first few chapters to see how epidemics spread and what factors
are important in getting them going:

For an epidemic to get going each case needs to cause more than one new case.
Latent period (time from infection to start of infectiousness) incubation period
(time from infection to clinical symptoms), period of communicability (how long the
disease can be passed on for) all affect how likely a disease is to be
transmitted to a susceptible person, according to Webber. The infectiousness of
the virus and the number of susceptible people in the population are also
crucial factors.

The most recent well known pandemic scare was with SARS which in the end infected >8000 people and killed >900 – how does that compare with influenza on the above factors?

First – with flu the infectious period
predates symptoms (latent period < incubation period) by 2 days. That means healthy
looking people without any symptoms can spread the virus. This contrasts with
the SARS epidemic where symptoms and infectivity coincided and so cases could be
picked up early. Border screening of travellers for influenza would have limited
usefulness. Dave Simpson has looked into this- see his blog today.

Second -we could all be susceptible to a new flu virus. Because influenza
virus mutates so quickly there is little immunity in the general population to
flu viruses that have significant genetic changes like this one appears to. So
the case is similar to SARS where there was no previous immunity. Disease
control by vaccination could reduce the number of susceptible people, slowing
the epidemic. But how long to a new vaccine, especially when recent research (according
to Global Health database records) appears to have been concentrated on pandemic avian
influenza vaccines?

Third, the route of infection is airborne – huge numbers of people can be
infected by one person. With SARS closer contact was needed to spread disease
and this limited the epidemic. Even so sharing an elevator was enough for some
SARS cases, according to the section on SARS in Webber's book.

How deadly is the new flu virus?

This new swine influenza virus seems to be less fatal than
the avian flu we have been watching for the last few years and less fatal than SARS. The US cases of
swine influenza A/H1N1 all appear to be mild. The Mexican cases are more severe. Going on figures reported by the
BBC about 6-8% of Mexican cases have died. The avian flu in South East Asia
kills a much greater percentage of patients (about 2/3 of patients).

Unfortunately this estimate is a bit of a difficult one because working out
how deadly a virus is relies on accurate reporting of illness and deaths. In an
epidemic that may not be the case. For Mexico its certainly unclear how many of
the influenza cases are swine flu since only 18 samples have been tested and

The difference in severity between Mexican and US cases suggests there could some other
factor at work in Mexico as well as the swine flu. A point to consider here
is that looking in Global Health I found that deaths in previous pandemics often
result from bacterial infections of people weakened by the flu, not by the virus
itself (see the paper Predominant
role of bacterial pneumonia as a cause of death in pandemic influenza:
implications for pandemic influenza preparedness
This could also cause confusion on the reporting of deaths due to the virus.

More information:

US Government site,

Search Global Health ( if you subscribe)

Map of the epidemic:

Get the book:
Communicable diseases
epidemiology and control

Leave a Reply