The epidemiologic data for the H1N1 outbreak in Mexico City was presented May 20, 2009, at The Vaccine Dinner Club at Emory School of Medicine. The CDC also presented U.S. data for H1N1 at this meeting.

Guillermo Ruiz-Palacios, M.D., of the National Institute in Mexico City presented a classic influenza epidemic curve with a case fatality rate of 2.3%. A triple re-assortment virus, H1N1 marked a distinct genetic shift. The deaths in the 20 to 50 age group comprised 75% of the mortality. The pneumonia curve occurred in parallel with the influenza curve as expected.   

Almost simultaneously the H1N1 has appeared world-wide. The WHO reports over 19,000 cases in over 60 countries with a case fatality rate of 0.71%. The U.S. reports 6764 cases and a death rate of 0.15% as of May 27, 2009.

CDC epidemiologist Dan Jernigan, M.D., provided several important insights at the same meeting. In the U.S., 62% of the cases are in the 5 to 24 age group. Approximately 35% occur with vomiting and/or diarrhea, thus an increased risk of dehydration when associated with high fever. Asthma/COPD are present in 40% of cases. The use of steroids for asthma may increase the likelihood of viral pneumonia. Diabetes was present in 15% of cases.

The ultimate effect of H1N1 in the U.S. will not be known until the fall. A case fatality rate of 0.15%-0.5% seems likely. Given the improvement in critical care in the interim, this seems substantially worse than 1957.

We continue to prepare for the full impact of H1N1 in the United States. Dr. Ruiz-Palacios placed triage teams at four different entrances to his hospital. This approach seems prudent.

Expanded hydration capability also seems warranted. It may be feasible to expand the ED capability for hydration therapy. If facilities are overloaded, some patients may be hydrated and then sent home.

Preparing for an influenza epidemic at local hospitals with an expected 5% hospitalization rate is a work in progress. A summer break from influenza will allow us to assess the needs. As with any mass casualty event, we need to prepare to get the resources to where the patients are.

Michael P Dailey MS MD FACP