Pandemic A/H1N1 influenza update: How far have we progressed? — Part 2

Aug. 1, 2010
Each week the CDC collects and analyzes information about the incidence and severity of influenza in the United States.

John A. Molinari, PhD

For more on this topic, go to and search using the following key words: infection control, A/H1N1 influenza, pandemic, outbreak, Dr. John Molinari.

Each week the CDC collects and analyzes information about the incidence and severity of influenza in the United States. While this data has served as a national barometer of seasonal flu activity for many years, people have paid more attention to it since April 2009. This was when the first reports of human cases of pandemic A/H1N1 influenza (swine flu) were announced by Mexican and U.S. public health agencies. My column last month presented a brief summary of some major aspects of the resultant global outbreak. Fortunately, as of late May 2010, the most recent evaluations of pandemic flu disease continue to indicate no widespread or regional influenza activity in the U.S. for a fifth consecutive week. Most states are reporting no activity or only sporadic activity.

Throughout the pandemic, many people, including health professionals, have asked questions that need to be addressed in order to help better understand the scope and implications of this respiratory disease. The following will attempt to address a few of the questions. For additional information on H1N1 flu, please log on to the CDC Web site at

From where did the A/H1N1 influenza virus come? The virus was originally referred to as swine flu because laboratory testing showed that its gene segments were similar to influenza viruses that had most recently been identified in and known to circulate among pigs. Initial genetic analysis suggests that this virus resulted from reassortment, a process through which multiple influenza viruses can exchange genetic information during co-infection of a single human or animal host. Reassortment can result in abrupt, major changes in influenza viruses, termed "antigenic shift."

When resultant progeny viruses emerge, they will contain gene segments from each of the infecting parent viruses, and therefore may have different characteristics than either of the parental viruses. If these new type influenza A viruses are introduced into the human population and are passed via human-to-human respiratory transmission, infected persons have little or no protective immunity. Extensive investigation of A/H1N1 genetics indicated that this particular virus is very different from other influenza viruses that normally circulate in North American pigs.

In addition to containing two genes from viruses that normally circulate in European and American swine, A/H1N1 contains avian and human genes. Thus, the A/H1N1 influenza virus is referred to as a "quadruple resassortment" virus.

Can a person get sick with A/H1N1 flu more than once? Since recovery from influenza stimulates a protective humoral (antibody) immune response against the etiologic virus, it is not likely that a person would be reinfected with the same virus. A possible exception may occur in people with weakened immune systems. These immuno- compromised individuals may be unable to develop a sufficient immune response. Thus, it may be possible for them to become infected with the same virus more than once.

Who has been most impacted by the A/H1N1 pandemic, according to CDC estimates? CDC's latest estimates of A/H1N1-associated cases, hospitalizations, and deaths continue to highlight the fact that people younger than 65 years of age are more severely affected by this disease as compared to people 65 and older. About 90% of estimated hospitalizations and 87% of estimated deaths caused by the pandemic virus since April 2009 have occurred in people younger than age 65. This information contrasts with long-term data for seasonal flu in which about 60% of flu-related deaths occur in people 65 years and older.

Can I get the flu from receiving either the trivalent seasonal or monovalent pandemic flu vaccine? This question is the most common one. Despite reports of severe illness and the potential for rapid spread of infection in patient-care facilities, many health-care professionals still fail to adequately protect themselves. Dental-care providers are among the most at-risk health-care workers for contracting influenza. Yet I often hear comments such as, "I received the vaccine once years ago, and came down with the flu a few days later from the injection so I am never getting that shot again." This perception is conveyed to others and can make them fearful of vaccinations. Health-care providers must be armed with the appropriate information to counteract this.

With regard to the "flu shot" vaccine, you should be aware that neither the seasonal flu vaccines administered by intramuscular injection nor the A/H1N1 pandemic vaccine administered in the same manner contain live influenza viruses. After cultivation in chick embryo tissue cells, harvested viruses are treated and inactivated with formalin.

These inactivated microbes are then fragmented into viral components that are used as the immunizing agents. Thus, in addition to an absence of viable virions, the final preparation does not even contain any intact viruses.

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