Influenza in the US, Week 23

by Vincent Racaniello

Tuesday, July 07, 2009 at 02:04 PM EDT

The Centers for Disease Control and Prevention has published their weekly analysis of influenza activity for the week of 7-13 June 2009. They conclude that

…influenza activity decreased in the United States, however, there were still higher levels of influenza-like illness than is normal for this time of year.

During week 23, 38.7% of specimens (2,765 out of 7,149) tested positive for influenza virus. Of these, 82% (2,263) were identified as the pandemic H1N1 strain. The H3N2 strain, and last season’s H1N1 strain, accounted for only 21 and 22 of the positive specimens, respectively. These results are shown in the following graph:

The numbers raise at least two questions. First, the seasonal H3N2 and H1N1 strains are clearly disappearing. Is this because it is the end of the influenza season in the northern hemisphere, a time when influenza typically drops to very low levels? Or is it because the older viruses are being ‘forced out’ by the new pandemic strain? The fact that the pandemic H1N1 strain is also supplanting last year’s strains in Australia suggests that these viruses will not return in the fall.

The other question is why the pandemic H1N1 strain continues to cause a significant number of infections in the northern hemisphere in June. We’ve previously discussed how temperature and humidity are believed to be important factors in determining the seasonal patterns of influenza in temperate climates. Clearly the 2009 pandemic strain isn’t following these ‘rules’. Neither did the 1918 pandemic strain. Could it be that seasonality of influenza is regulated not only by temperature and humidity, but also by levels of population immunity? Perhaps, after a season of influenza, the high levels of immunity coupled with high temperatures and humidity together lead to reduced disease in the summer. Because population immunity to a pandemic strain is extremely low or nonexistent, the virus can circulate even in conditions of high temperature and humidity. This hypothesis is undoubtedly an oversimplification, and other factors, such as transmissibility, are also likely to play a role. For example, the pandemics of 1889, 1957, and 1968 did display seasonality in the northern hemisphere – see Figure 1 in the paper cited below.

Miller, M., Viboud, C., Balinska, M., & Simonsen, L. (2009). The Signature Features of Influenza Pandemics — Implications for Policy New England Journal of Medicine, 360 (25), 2595-2598 DOI: 10.1056/NEJMp0903906