The swine flu - for the most part - seems to have fallen off the radar of the mainstream news. There still several things about the swine flu that are not known. In particular, it is not known how severe this particular flu outbreak will be with respect to illness and death relative to other influenza viruses.

According to the WHO, disease surveillance authorities in Mexico began noticing cases of a disease initially described as a mild influenza like illness starting on March 18. The number of cases has risen steadily since then, and again, according the the WHO, there are now 10243 cases and 80 deaths in 41 countries worldwide. The large majority of these, 5469 and 6 deaths, have occurred within the United States; the second largest number of cases, 3648 and 72 deaths, have occurred within the epicenter of the disease - Mexico. That there are more cases of flu in the USA relative to Mexico, the epicenter of the disease, most probably speaks to the relative reaction of Mexico as compared to the USA.

I thought it might be noteworthy to plot the progress of the disease to illustrate this idea. Below is a semi-log plot that follows the number of flu cases, beginning on April 29, 2009, the day that WHO first began posting any meaningful data. [caption id="attachment_63" align="aligncenter" width="497" caption="Semi-log plot: Number of Flu Cases vs. Date "][/caption]

Perhaps the most noteworthy item concerning this data is the fact that the number of cases in the USA surpassed the number of cases in Mexico relatively early on in the outbreak. The disease was handled completely differently in the two countries. In the USA, very little action was taken - though some action was taken in some isolated places. For example, 858 schools were closed throughout Texas, affecting more than half a million students; a number of schools were closed throughout California, and NYC has closed at least 16 schools.

The response in Mexico has been somewhat different; Mexico reacted to the virus by closing businesses, restaurants and schools, canceled concerts and sporting events, and advised citizenry to stay indoors as much as possible for five days. This strategy - logically - is thought to be largely responsible for keeping swine flu under control and limiting spread through Mexico. In other words the Mexican response was much more logically consistent with actually controlling an outbreak than that of the USA; and the data clearly demonstrate this.

By any standard the US response to this unknown, novel, out-of-season, potential pandemic has been lackluster and nonchalant.Whether or not a less passive approach was necessary remains to be seen. The virulence of any given disease is defined by it's case fatality ratio (CFR); a higher CFR is indicative of a more virulent virus. The tricky thing is how CFR is defined. Ideally it should be defined as number of deaths, per affected individual, however how those numbers is defined is also often tricky. In any case, the CFR is the best estimate or description of a disease's virulence. Estimates for the CFR of the seasonal flu are approximately 0.1%. The 1918 Spanish Flu Pandemic has been estimated to have a CFR ranging from 2% to 20%, depending on which sources you check, but is most commonly given as 2.5%. This particular virus, depending on which statistics you look at. Considering only the number of deaths per infected individual, the USA, Mexico, and worldwide have CFR's of 0.110%, 1.97%, and 0.78% respectively.

These CFR's are likely overestimated as well; it's reasonable to assume that the actual number of cases is higher - probably significantly higher - than the reported number of cases. A certain number of persons with mild symptoms, and those - such as myself - who simply like to avoid MD's (Unfortunately, I can't avoid other Ph.D.'s, I"m surrounded by them) will not seek medical attention, and will simply go unreported. These unreported cases will lower the CFR.

These results are probably not statistically significant as of yet. Flu seasons often occur in waves; in the US, there are often two waves of flu activity, with one wave occurring in late November and lasting into January, while another wave often occurs in January, peaking in and becoming most severe in mid-February, and finally tapering off in late March.You can see this for yourself here.

A realistic CFR cannot be calculated until there is more data, and the infection has started to subside. It could be too early to say, but thus far, it doesn't appear that this strain is particularly virulent, and the number of new cases (according to my graph) seems to be tapering off. There is speculation that this virus could mutate and become more virulent, and this certainly could occur. In my opinion, this is not very likely.

Mutations can and do occur in viruses - generally more so than in complex organisms - however, the in the evolution of viruses, in general, is for them to evolve to become less virulent, not more virulent over time. Secondly, this particular strain of flu virus is no more likely to mutate than is any other flu virus, including the seasonal flu. Thus far, the data seem to indicate that the dreaded H1N1 swine flu is no more virulent than, and no more likely to cause death than the typical seasonal flu that comes and goes each year.