Is the second wave over?

People interested in influenza pandemics often discuss “waves” of infection. The idea is that infections start out slowly; then more and more people are rapidly infected until a peak is reached; finally, the number of people who are newly infected drops precipitously.

In the three pandemics of the 20th century, there were multiple waves of infection. The second wave of the 1918 “Spanish” Flu was especially lethal. It appeared to kill a much higher proportion of young people who were infected than normal. Since many consider what we are experiencing now the second wave of the 2009 H1N1 pandemic, there is a lot of discussion of how lethal the virus is and whether we are already at the peak. In my opinion, it is premature to conclude that this virus is not especially lethal or that the second wave is over.

One thing we tend to forget about waves of infection is that they are local. Most of the graphs that we are familiar with illustrate waves within a single city. If one looks at reports of infections across the US, one does not see an homogenous pattern. Instead, there are staggered start points for waves which appear to be primarily determined by when schools opened. Since schools opened early in many Southern states, cities in this region were hit first. There are some indications that the daily number of new infections is declining in these areas. On the other hand, in the Northeast, where schools opened last, the number of new infections has just begun rising. Given that different regions are at different points in the school-opening-wave, it makes little sense to average them together to come up with a national curve. It is more accurate to say that some States are past their initial school-opening wave while others are just beginning it. On a national level, there may be a temporary lull in the number of new infections until the heavily populated Northeast reaches its peak, which may not occur for several weeks.

One of the odd things about this pandemic is how few adults have been infected. As many have observed, all of the large outbreaks have been among children, either at school or summer camp. There is not a single documented large scale outbreak among adults, as far as I know. Is this due to some pre-existing immunity? Maybe. But the data on this issue is mixed. Another possibility is that the environmental conditions for efficient transmission among adults have not yet occurred.

The normal flu season has just begun. It is entirely possible that what we have seen thus far is just small echo of the original wave of infections from the Spring. New strains of pandemic H1N1 are likely on their way from Asia to Europe and North America. We have no idea how transmissible or lethal these new strains will be.

Any effort to downplay the final outcome of the current pandemic based on the assumption that “the worst is over” should be regarded with skepticism.


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