The latest "influenza" to visit itself upon the citizens of the world reminds us that epidemics are part of the natural interaction of people and microbes throughout civilization. Yet each time we react as though we have discovered something new. The only really new things are our surprise and consistent inability to recall all of the lessons from prior visitations.
In September 1976, when my colleagues and I reported two patients in Virginia with Swine flu, it may have added some concern that the original cluster at Fort Dix had spread beyond New Jersey. Yet the 230 cases among trainees at the army base with 13 severe illnesses and one death in February 1976 had already alarmed the country. In fact, by March of that year, President Gerald Ford announced federal funding for a national vaccination program. Subsequently, there were two unexpected outcomes: The Swine flu epidemic never materialized, and the vaccine unexpectedly caused a 7-fold increase in a neurological side effect leading to weakness and paralysis, the Guillian Barre Syndrome. A shakeup at CDC followed, and for years the public's willingness to accept vaccines was diminished. Those who boldly predict the outcome of influenza do so at their own peril.
The 2009 Swine flu outbreak that began in Mexico is different. Cases have developed in countries on at least five continents over a brief time period. The key mystery is why 160 deaths have been reported in Mexico. Sadly, the U.S. reported its first death in a 23-month-old Mexican toddler visiting in Texas.
First let's look at the meaning of the deaths in context of the expected mortality rates. In the 1918-19 Avian flu pandemic, the mortality was 2.5% -- 25 times the rate we see with the seasonal arrival of flu each year of one in a thousand or 0.1%. If the 160 deaths in Mexico are truly related to swine flu and the disease is very virulent -- for example with a very high 1% mortality -- the real number of cases must be 16,000 -- not the 2,500 currently reported. On the other hand, if the new Swine flu is acting more like our seasonal flu, the real number of cases in Mexico is 160,000, 0.1% of which accounts for the 160 deaths.
Whatever the true virulence of this Swine flu virus and its case mortality rate, the reported number of cases is grossly understated in Mexico. This also implies that the epidemic there is probably several months in duration.
The first death in the U.S. among 64 cases, which is also the first death globally outside of Mexico among 100 or more cases, is difficult to put into perspective with any confidence -- especially since the baby came from Mexico. The death of a baby in the U.S. is tragic, but each year we see 36,000 deaths from influenza in our country. Influenza is always serious -- even without an exotic strain from another country. We cannot at this time predict the mortality of the Swine flu outbreak, but only put in a range of 0.1% of cases to something higher such as 0.5% (seen in the 1957 "Asian" flu and 1968 "Hong Kong" flu pandemics). A 1% mortality may be worst case scenario and seems unlikely.
If more deaths are occurring in Mexico than the rest of the world, why would this happen? In both the U.S. and Mexico the same virus is attacking the same age cohort -- young adults. Then any big differences would be ascribed to differences in patients. Is it possible that patients in Mexico have co-infection with a second virus? Do they have high rates of secondary bacterial pneumonia with Staphylococcus aureus or Pneumococcus? Is the particulate air pollution in Mexico City damaging airways and lungs causing more disease? Do patients have an unusually robust immune response to the virus -- the so-called cytokine storm? We do not know the answers to any of these questions, but the key point is this: CDC in concert with health authorities in Mexico should perform sophisticated viral and bacterial studies, and pathology analyses on patients with severe disease and on patients who have died. Our understanding of how to manage this viral infection will be enhanced with such information.
As we prepare ourselves in the U.S. for more illnesses and more deaths from the new Swine flu, it is the uncertainty that is especially challenging. Knowing the cause of severe illnesses and deaths and the true mortality will be of critical importance.
Dr. Wenzel is the former President of the International Society for Infectious Diseases and Chairman of the Department of Internal Medicine at Virginia Commonwealth University in Richmond, VA.
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