Swine Flu: When an Outbreak is a Pandemic

In April, 2009, an outbreak of a new strain of the influenza A virus H1N1, or swine flu, started in Mexico City. Alarming health officials around the world was the fact that H1N1 seemed to be rapidly spreading, plus there were several deaths reported as well in young, previously health people. As of May 6, 2009, Mexico has confirmed over 800 cases of H1N1 and 22 other countries have reported outbreaks as well for a worldwide total reported cases of just over 1500. However, more recent reports have been more restrained than the initial ones and there is no apparent evidence of a pandemic, plus the virus, in most cases, seems to cause milder-than-expected flu symptoms. Also, its rate of spread seems to be no more than that of our typical seasonal flu.

What is a pandemic?

So what IS a "pandemic" anyway and why is the distinction between an outbreak, an epidemic and a pandemic important to health officials? For purposes of these definitions, we'll just be referring to these terms as they relate to the flu. An outbreak is simply the start of the flu infection in a localized geographic area. An epidemic occurs when a large geographic area is involved, and the CDC defines an epidemic based on a large outbreak which has a higher than expected mortality rate than expected.

A pandemic is a global outbreak of flu that exceeds the "normal" levels of mortality and infection levels for typical disease. The key word here is "global". Obviously, the definition of a pandemic is somewhat subjective, but in most cases, the difference between the normal background state and a pandemic state is strikingly obvious to health officials and the public alike.

There are two major factors which will effect whether or not a flu outbreak will lead to a pendemic: Pathogenesis and virulence. Pathogenesis refers to how easier a virus will cause disease and how easily it is spread. Obviously a virus that easily reproduces itself in the host and spread easily from one person to the next is more like to cause a pandemic. The virulence refers to HOW sick a certain virus will make the host and how easily it can cause death. Health officials at the CDC and WHO are obviously going to be more concerned with a virus with a 5% death rate than one which only causes a mild viral syndrome.

The WHO maps out a pandemic based on "phases," of which there are six. Phase 1 is when there is only non-human infections spreading and no animal-to-human transmission. In Phase 2 an animal influenza virus circulating among non-human animals which is known however to have caused infection in humans is identified, and is therefore considered a potential pandemic threat. Phase 3 sees animal-to-human transmission but no person-to-person transmission under normal living circumstances; when this is seem the risk of pandemic rises even higher. Phase 4 occurs when there is known human-to-human transmission of the flu virus. This allows outbreaks then epidemics to occurs and even further increases risk of a pandemic. In Phase 5, human-to-human spread is documented in at least two countries in one WHO region and may well indicated a pandemic is imminent.Phase 6, the pandemic phase, is characterized by outbreaks in at least one other country in a different WHO region in addition to the two or more countries defined in Phase 5. Designation of this phase will indicate that a global pandemic is under way, as per the WHO website. Lastly there is a post-peak period or phase in which the infection seems to die out in the infected areas; nevertheless, there is always the risk of new outbreaks elsewhere, starting the cycle all over again.

It is important to note that these WHO phases refer to levels of preparedness that the WHO recommends are different points in a flu outbreak. So actually we should refer to these as phases of the WHO preparedness plan, not phases of the epidemic. However, these phases are enacted at the different levels based on the perceived risk of a pandemic in a WHO region.

As of May 11, 2009, the WHO was recommending a Phase 5 level of preparedness. One can check on current level at the WHO website. However, recently a WHO official was thus quoted: "We believe there is sustained community transmission, person-to-person in 2 countries in 1 region, which is North America," said Keiji Fukuda, MD, MPH, assistant director-general ad. interim for health security and environment at the WHO. "We cannot predict the future, so it is possible that we will go up to phase 6...but it is also possible for the current situation to stabilize where it is now and then it is possible that we would go back down to phase 4 in the future,"

What else does epidemiology provide in understanding of the flu?

In addition to helping us follow outbreaks and identify characteristics of an outbreak versus a pandemic, epidemiologists study a variety of other features related to the flu. For example, risk factors for getting seriously ill or dying from the flu have been studied for sometime. And, for sometime we've known that patients with chronic diseases such as heart disease or diabetes are at more risk than others. Other high-risk groups include those with emphysema or COPD, end-stage kidney disease, or diseases that effect the immune system such as HIV or recent chemotherapy. However, a new recent finding with H1N1 flu is that obesity alone (that is even without diabetes or heart disease, etc), MAY be a risk factor for getting severely ill with this flu (as reported in Washingon Post on May 20, 2009). Another interesting finding by epidemiologists studying H1N1 is that they believe they have identified "Patient Zero." With any outbreak of a new infectious disease, "Patient Zero" is a label given to the believed first patient to have gotten ill, or with H1N1 be first patient to have caught the virus from a pig.

Patient Zero is thought to have been a four-year-old boy in the village of La Gloria, Veracruz, near a huge Smithfield hog operation. Some Mexican officials dispute this claim however and still insist that the human-to-human virus originated in China first.

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