H1N1 Status: Updated with Latest CDC Data

Extreme Thinkover Guest Article

Dr. John Bogen, MD

Updated with the Latest CDC Data, November 12, 2009

What is the current status of the pandemic in the U.S?

The CDC reported that for Week 43 (ended October 31, 2009), both hospitalizations and deaths from influenza dipped slightly. A total of 18 pediatric deaths were reported for the week. Virologic surveillance of 14,151 specimens sent to U.S. labs for testing revealed that 37.2% tested positive for influenza, a slight decrease. Of those that tested positive, 0.3% were influenza B, and 99.7% were influenza A. Of the influenza A strains subtyped, 99.9% were the pandemic strain, and only 0.1% were strains associated with strains seen in prior seasons. http://www.cdc.gov/flu/weekly/

What do these data mean for the average patient? The seasonal influenza vaccine has so far had little value, since almost all the influenza currently circulating is the new pandemic strain. The pandemic vaccine has just now begun to be distributed and given to patients. We are still in the heart of the flu season. It is too early to tell from the data if the country itself has “peaked” in terms of the number of cases. There are also regional and local differences – some areas have already been hit hard, as evidenced by school closures. Other areas have not yet peaked. It is also too early to tell if the pandemic vaccine has made any difference in the overall numbers.

In usual “non-pandemic” influenza seasons, an estimated 36,000 deaths occur directly or indirectly from influenza in the U.S., with 90% of these in the elderly or in those with weakened immune systems. One piece of good news this season is that the elderly population seems to have some immunity to the pandemic H1N1 strain, probably due to different H1N1 strains that were in circulation until the 1957 H2N2 pandemic, at which time H2N2 replaced H1N1 as the seasonal strain. The bad news is that we are seeing more than the usual number of deaths in younger people (under age 65). People with weakened immune systems are still dying disproportionately, and we are seeing more than the usual number of deaths in previously healthy individuals including children and pregnant women.

According to a CDC report on November 12, 2009, here have been about 3,900 total deaths since the pandemic started. The estimated mortality rate has been about 0.022% for elderly, 0.024% ages 18-49, and 0.007% ages 0-17.

Data on vaccine safety are difficult to obtain at this time, since distribution of the vaccine has just begun. Several highly publicized anecdotes of serious reactions have appeared in the lay press. Most of these were with the seasonal vaccine. Ongoing clinical testing of the pandemic vaccine has continued to show good short-term safety. One must keep in mind that a temporal association between vaccine and symptom does not imply causality, but patients with serious reactions (e.g. anything more than local muscle soreness from the shot and the common few days of general malaise following the shot) can and should be reported to VAERS. It is quite obvious that, worst case scenario, that deaths from pandemic influenza greatly exceed the number of serious reactions from the vaccine.

The next few weeks will be critical. Between vaccine being distributed / administered and the pandemic running its course through communities, one would hope to see a downward trend in hospitalizations and deaths. The vast majority of unvaccinated patients who get pandemic influenza will be fine after a few days of misery. The vast majority of patients who get the vaccine prior to getting sick from pandemic influenza will not have a serious reaction to the vaccine and also will not get ill or die from the virus. If we see fewer deaths from influenza this season than the usual 36,000, that would be a good thing, but hardly a consolation to the families and friends of people who died from the pandemic.

If I may offer my educated opinion, I predict the pandemic strain will continue to be the dominant strain of influenza in the community. Next year’s seasonal influenza vaccine will include the current pandemic H1N1 strain (or a mutated version thereof that exists next spring), and influenza B. With luck, subsequent seasons will be mild because so many would have already gotten ill and developed immunity to the novel H1N1. The elderly will continue to have some natural immunity, and the vaccine and herd immunity will protect most of the rest of the population.

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