Pandemic A/H1N1 influenza update: How far have we progressed? — Part 1

July 1, 2010
Evidence describing the beginning of the most recent global flu pandemic was first reported in April 2009.

John A. Molinari, PhD

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Evidence describing the beginning of the most recent global flu pandemic was first reported in April 2009. The initial CDC press conference was held April 23 and described the early diagnosis of the first cases in the United States (five in California, two in Texas).

The investigation into the emergence of this new pandemic was initiated as a result of reports describing an unusual occurrence of pneumonia-like cases and deaths in rural Mexico. The resultant rapid, worldwide, human-to- human spread of pandemic viruses has led to the observation that, as of April 30, 2010, more than 214 countries have reported laboratory confirmed cases of A/H1N1 flu.

When the outbreak was first recognized, the CDC worked to develop a system for tracking and reporting the number of confirmed H1N1 cases, hospitalizations, and deaths. This early attempt resulted in a substantial underreporting of cases, and was replaced with a more accurate process for estimating A/H1N1disease prevalence based on the number of laboratory confirmed cases reported.

When initial results were analyzed for cases reported in April to July 2009, the CDC determined that there were an estimated 79 cases for every reported case, and every hospitalized swine flu case could represent an average of 2.7 hospitalized people. The CDC continues to use this method to provide an estimated range of total U.S. pandemic influenza cases, hospitalizations, and deaths.

The following statistics calculated from April 2009 to March 2010 reflect the impact this highly contagious respiratory infection has exerted on the country’s population:

Approximately 60 million cases of A/H1N1 influenza have occurred, with an estimated range of 43 to 88 million

Estimated hospitalizations range from 192,000 to 398,000 with a midlevel of approximately 270,000

A/H1N1-related deaths are thought to number approximately 12,200, with a range of 8,720 to 18,050

Clinical disease during the pandemic has ranged from mild to severe, with most people having recovered without medical treatment

The spread of A/H1N1 virus occurs in the same way that seasonal influenza is transmitted, mainly from an infected person to another person through coughing, sneezing, or talking. Other less frequent modes of transmission involve a person touching a surface or an object with viruses on it, and then touching his or her mouth or nose.

Remember, the incidence of these pandemic infections is in addition to the annual occurrence of approximately 200,000 flu-related hospitalizations and 36,000 deaths attributable to seasonal influenza viruses. Also, in contrast with long-term data that shows the majority of seasonal flu hospitalizations occurring in persons 65 years of age and older, age-related information collected for A/H1N1 cases continues to indicate that people younger than 65 are more severely affected by this disease.

The CDC reports that approximately 90% of the estimated hospitalizations and 87% of estimated deaths (approximately 9,400) have occurred in persons younger than 65.

While the pandemic continues, in recent months the increase in total number of cases has been small. This correlates with a period of generally low flu activity in the U.S. Influenza pandemics generally occur in waves. As noted, the first wave for A/H1N1 flu began in spring 2009. This was followed by a second wave in the fall, with the peak number of infections developing at the end of October 2009.

To date, we have not seen early signs that a third wave of disease activity is imminent; however, it remains possible. If the current disease trend continues, it is quite possible that the pandemic A/H1N1 virus will gradually evolve into a less virulent seasonal virus and circulate among the population for an extended period — perhaps years.

Preliminary evidence that the virus is undergoing further genetic changes to a seasonal virus is suggested by the fact that a 2009 A/H1N1 virus has been selected as a component for inclusion in the southern hemisphere’s upcoming seasonal flu vaccine.

Despite the large volume of published information and other media reports available, many people — including health professionals — still have questions that need to be answered. This is important as the discussion can increase their understanding of the A/H1N1 issues and enable them to better answer inquiries from inquisitive and confused patients. The second installment of this two-part series will address a few of the common questions asked about influenza.

Dr. John A. Molinari is director of infection control for THE DENTAL ADVISOR. Previously, he was professor and chairman of the Department of Biomedical Sciences at the University of Detroit Mercy School of Dentistry. Contact him at [email protected].

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