As we said in yesterday's column, viral pandemics don't operate according to news cycles, and this one has shown very little respect for the international dateline, anyway. It has been confirmed that individuals carrying the virus have made it as far as Israel and New Zealand now, so the World Health Organization has raised its pandemic alert level based on these confirmed cases. So far, however, it appears the vast majority of cases outside Mexico are instances where the individual traveled to Mexico and picked it up there. Within Mexico, the death total has now risen to 152, but many of those are suspected cases and there are no indications that anyone outside of that country has died as a result of the virus.
So, that quick rundown out of the way, it's time for the obvious caveat: all of those numbers may be out of date by the time you read this. So, we'll focus more on the choice of threat level made by the WHO and provide a bit more background on the flu virus itself.
The WHO provides a nice chart that details the categorization of its different pandemic flu phases. Prior to yesterday, it had rated the current flu outbreak at phase 3, which indicates that most of the activity was occurring in animals, with few cases of human infections. That was clearly out of date, as there appeared to be a significant amount of human-to-human transmission happening, so the question was really whether to raise it to phase four or five.
Phase four is characterized by sustained human-to-human transmission that allows community-level outbreaks, and indicates that a pandemic is possible. Phase five would involve the spread to multiple countries and is used to indicate that a progression to pandemic may occur at any point. The current situation might be viewed as somewhere around a 4.3—there are community-level outbreaks in multiple countries, but no indication of significant spread outside of people who haven't visited Mexico. These visitors have made wide contact with others during their travels, so the virus is essentially impossible to contain at this point, but, so far, there isn't much indication that the travelers are infecting people while in transit.
So, raising the alert level to phase four seems like a reasonable compromise, and should help public health authorities obtain the resources they need to track and contain the virus. So far, however, the WHO sees no need for travel restrictions.
Further musings on virus biology
Meanwhile, our description of flu virus biology attracted the attention of Joshua Ballanco, who is a PhD student in the biological science and longtime Ars reader. Ballanco pointed out that the NIH has already posted the sequences of several viral isolates online, and these can be used by interested parties to figure out what portions of the virus came from where. He also noted that part of the biochemistry that controls virus host range depends on the sugars that are linked to proteins at the cell surface. (The viral neuraminidase gene specifically interacts with these.) Pigs are a common host for generating mixed-lineage viruses such as the current one, as they have a sugar combination that allows them to play host to both avian and mammalian viral strains.
From here on out, we're going to engage in a bit of speculation about virus biology; treat it with appropriate caution.
As we suggested in the earlier column, this mix-and-match assembly of parts from different viruses may ensure that the virus is greeted as new by the immune system, but Ballanco suggests that it may also mean that the virus hasn't had a lot of time to adapt to human hosts. This may explain some of the virus' behavior. The relatively slow spread outside of Mexico may be a product of this lack of adaptation.
Meanwhile, the fact that its victims come from populations we tend to view as healthy may indicate that an overexcited immune system bears part of the blame, which may be a byproduct of the fact that our immune systems haven't encountered any close relatives of this specific virus before. The excessive reaction of the immune system may cause more damage than the virus.