A network of ~150 laboratories in the United States participate in virological surveillance for influenza. The results of these analyses are publicly available, and provide extremely interesting characterization of the pandemic spread of the new H1N1 strain. During week 22, 6,664 samples were submitted for testing, of which 2,681 were positive for influenza virus. Of these, 2,071 (89%) were identified as the new H1N1 strain. The previous seasonal influenza A (H1), A (H3), and B viruses continue to co-circulate. These observations are summarized on this graph:
I have noted previously the unusual upsurge in influenza activity during weeks 17 and 18, which coincided with the emergence of the new H1N1 virus. The number of virus-positive specimens dropped during week 19, but has since risen. More importantly, the percent positive specimens – the black line – continues to rise. This observation suggests that the new H1N1 virus will continue to circulate, at least for the next month. Increased sampling by clinicians in the face of a pandemic probably also contributes to the rising numbers of influenza positive specimens.
Also of interest is the declining circulation of the two influenza virus strains of the previous season – H3N2 and H1N1. Of the 2,681 influenza positive samples in week 22, 9 were identified as the previous seasonal H1N1 strain, and 22 as the H3N2 strain. The circulation of these seasonal influenza viruses into June is probably not abnormal. Because of the pandemic H1N1 strain, more samples are being tested than is usual. Clinicians stop testing for influenza virus at the end of May as the ‘flu season’ wanes. CDC stops its influenza surveillance in May, and resumes in the fall. This year CDC will probably keep up the influenza reporting through the summer. According to Dr. Anthony Fiore, a medical epidemiologist at CDC,
Now people who don’t normally look for flu are looking more than they ever have. We’re seeing that some of the respiratory illnesses that occur even late in the season may be due to seasonal flu viruses that we didn’t appreciate in the past. I guess we don’t know that anything different is occurring right now with seasonal flu viruses. This is an artifact of clinicians looking very hard for flu viruses at a time they don’t normally look.
The textbooks have always said that influenza viruses can be isolated from large northern hemisphere cities throughout the summer. The increased surveillance this year will provide a more detailed picture of the extent of influenza virus spread.
I’m also looking forward to reviewing influenza surveillance data from the southern hemisphere in the coming months. The Department of Health and Ageing of Australia provides similar data as the CDC, but their information has not been updated since 22 May and consequently the impact of circulation of the new H1N1 strain is not evident.