Paradigm Shift Intervention Monitoring
Commentary
Swine
H1N1 Exlosion In The United States
Recombinomics Commentary
15:18
June 9, 2009
Two thousand seventy-four (31.1%) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories and reported to CDC/Influenza Division were positive for influenza.
The above comments, from the week 21 CDC report cite a decrease of flu activity in the United States, but measurement of activity is compromised by the announcements of states throughout the country of plans to limit testing to severe cases and clusters. Moreover, even for clusters, the number of samples tested is very small, so even when dozens or hundreds of students are swine H1N1 infected, the official tally for the outbreak is usually just one or two.
However, the ratios within the report indicate activity is high and increasing. For week 21, the flu detected is almost exclusively swine H1N1. Seasonal flu H1N1 was identified in just 4 samples, and only 36 tested positive for H3N2. In contrast there were 1558 confirmed swine H1N1 samples, as well as 148 unsubtypable samples which are awaiting swine H1N1 confirmations. Thus, over 95% of influenza A is swine H1N1 and this extends to all influenza infections, because only 17 samples were positive for influenza B.
These dramatic differences are due in part to declines in seasonal flu. Normally, there is no week 21 report, because the flu season for the northern hemisphere runs from week 40 to week 20, which is justified by the low levels of seasonal flu at this time.
However, swine H1N1 is a novel flu virus which has just entered the human population, so natural immunity is limited. Moreover, the swine H1N1 has an avian PB2, which has optimal activity at 41C, in contrast to seasonal flu, which is most active at 33 C. Thus, the swine H1N1 is expanding in the warm off season.
This is most easily seen in the detection frequency, which, as noted above was 31.1% for week 21. This level is higher than the 25.1% in week 6, which was the peak week for flu in the 2008/2009 season. Thus, even though testing has been largely scaled back, the detection frequency is at record levels and above the peak week in February, reported at the height of the seasonal flu season.
The effect of the limited testing can be see in a recent Effect Measure blog from the Boston area, where flu activity is increasing (see updated map). All six members of the Revere''s extended family had flu symptoms, and one family member was tested and was influenza A positive. The high incidence of swine H1N1 throughout the country, and the dramatic acceleration in the Boston area, leaves little doubt that the 100% attack rate is due to an aggressive swine H1N1. Similarly, the range of symptoms in the family members also highlights reasons behind the lack of testing, since many members had relatively mild symptoms. Similarly, flu-like symptoms were widespread at the associated pre-school, but there was no indication that the school would close or notify parents of the widespread infections. Similarly, the lack of additional testing on the influenza A positive patients strongly suggests that none of these H1N1 infected cases will be reported. However, the high level of swine H1N1 at this time allows for a close approximation by simply measuring influenza A and tabulating people with influenza-like symptoms.
The lack of reporting has create a false impression of the extent of spread. Similarly, the mild nature of most infections has blunted the impact of the fatal infections of patients between the ages of 20-55. Normally, 90% of flu infections are in patients 65 or older. The swine flu however targets younger patients, and most deaths are in this younger age group.
Moreover, sever cases are on the rise. Manitoba just asked for an additional 15 ventilators to treat patients in Winnipeg. The vast majority of the 26 patients currently on ventilators are in the 20-55 age group, and more than half are from the aboriginal population.
The high frequency of the severe influenza in these patients raises concerns that the accelerating spread of swine H1N1 worldwide will lead to severe cases in selected populations in Asia and Africa, leading to additional concerns.
Thus far, the 2009 pandemic parallels the 1918 pandemic, which was a swine H1N1 that had moved into a human population in the spring of 1918, leading to mild cases. However, the virus turned lethal in the fall, after adapting to its human hosts, which devastating consequences, including the deaths of 20-50imillion people, which were largely in the 25-44 age group.
The increasing parallels between the pandemics of 2009 and 1918 continue to create increasing concerns.
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