A new strain of bird influenza (H7N9) has been causing infections in people in China. There have been about 100 documented human cases and about one-third of these patients have died. It is feared that this could be the emergence of a more deadly strain of the flu and the start of a new, severe pandemic of influenza.
Typically, the flu virus that circulates each year is only slightly different from the strains of influenza that circulated in previous years. However, every 10 to 20 years, a new flu virus appears when a strain of influenza is transmitted to humans from another species and begins to cause widespread infection in the human population.
A new influenza virus can occur when an animal is simultaneously infected with a human flu virus and an influenza virus that normally only infects its own species. The two viruses combine in such a way that the animal virus is now able to infect humans and spread easily from person to person. Because the human population has no pre-existing immunity, the virus spreads more quickly, infects a larger portion of the population and results in more serious illness, more hospitalizations and more deaths than the typical seasonal flu.
In a typical flu season, 15 to 60 million Americans are infected, 226,000 are hospitalized and 30,000 to 50,000 die (about 0.1 percent of those infected). In contrast, in pandemic influenza, up to 50 percent of the population can be infected and the rate of mortality can be as high as 5 percent of infected individuals. Consequently, a severe outbreak of pandemic influenza would have the potential to cause millions of deaths in the United States.
Could the H7N9 bird flu virus have the potential to cause a new pandemic in the United States? It is clearly a very deadly virus. Of the 131 documented cases, 123 were hospitalized and 39 died. The fatality rate among hospitalized patients was as high as 33 percent. It is thought that many mild, unreported cases occur and the true fatality rate is much lower but could be as high as 3 percent. Consequently, the virus has the potential to cause a severe outbreak of the flu in which the mortality rate would be as high as some of history’s most severe pandemics.
Fortunately, H7N9 does not appear to spread easily among people. All of the current cases have occurred in people who have had direct contact with birds. There is limited or no transmission from person to person. Consequently, the virus has been unable to spread beyond the initial small clusters of cases. However, in the past, other bird flu viruses have developed efficient human-to-human transmission and have caused widespread pandemics. For example, one of the worst pandemics, the Spanish Flu of 1918 to 1920, which resulted in about 50 million deaths worldwide, is thought to be the result of bird flu.
Fortunately, no cases of this virus have been detected in the United States, and public health officials are able to prepare for the possibility of the pandemic. The Centers for Disease Control has sequenced the virus, is preparing a test kit to be able to detect this strain of the flu and is working on an H7N9 vaccine. In short, H7N9 could happen here, but, hopefully, we will be prepared.
Dr. John Goldman is Program Director of Internal Medicine at PinnacleHealth.
