Swine flu: what did you expect?

by revere, cross-posted from Effect Measure

Usually "What did you expect?" is a rhetorical question, but we have a more serious point to make. Let's start with the familiar and move on to the less familiar. Many of you are coming here to find the latest news about swine flu. It's an imprecise term that covers two different things: what has happened that is new, in the sense of surprising and we didn't already know it would happen; and what is the current situation. Overnight (in the US) Europe (Spain) registered its first confirmed case. That's additional data but not surprising. We know this virus is seeded out there and we shouldn't be too surprised when we find it if we look hard enough in the days and weeks ahead. The other sense of news is about the current situation. The growing list of US states, and now countries, is relevant to understanding where we are at the moment. Similarly, the growing list of suspected cases will grow, probably very quickly. You should expect it. Those of you follow the H5N1 ("bird flu") story will recognize what is happening here. Flu of any kind has very non specific symptoms (cough, sore throat, fever, etc.) that are caused by numerous respiratory viruses (adenovirus, metapneumovirus, respiratory syncytial virus, parainfluenza, influenza B, etc). They are all circulating at this time of year. Some of them can cause severe disease, although mostly they just produce "spring" (or "summer") colds or flu (most of which isn't flu in the strict sense). In the context of this outbreak each of these respiratory virus cases has the potential to become a "suspect" case. Mexico City has 1614 "cases" and 103 deaths. Not all, or even most, of the cases have been confirmed by laboratory. They are really suspect cases. Some have been confirmed, most not. Confused? Expect that, too. In the opening days and weeks of an outbreak, everyone is confused. That's why we pool our information and try to sort it out.

Now to the less familiar parts of "what did you expect?" If there is normally so much respiratory disease around, why is this an outbreak or even an epidemic? It's a more difficult question than it appears, and it relates to "what did you expect?" An epidemic is an increase in the number of new cases beyond what you would expect. Four or five cases of human rabies in an area in the US would be an outbreak or even an epidemic. Hundreds of colds or even serious pneumonias in an urban area is normal. It's not an epidemic. What makes the swine flu an outbreak is that it is an infection with a virus we haven't seen before and which we believe may be new. Hence these cases are not what we expect and it is an outbreak. If it turned out that there had been the same number of cases in Mexico but from many different known viruses we might look for another explanation, for example, a change in insurance that changed care seeking behavior so cases were counted that weren't counted before.

Another thing that most people and probably most clinicians expect is that we know a lot about influenza. Perhaps because of the increased scientific interest since bird flu (an increased interest which will pay off handsomely in this outbreak, by the way) we do know quite a bit, but we also now know many of the things we thought we knew about flu, like the main ways it is transmitted from person to person, we don't really know. For example, how likely is it that you can get flu by touching a door knob or arm rest that someone with the flu just touched? Or that you can get the flu by sitting in the same emergency department waiting room (but not next to) other flu cases? These are open questions (see some of our many posts on this here, here, here, here). Why is flu seasonal? We don't know. We'll try to get to some more of these questions in the days ahead, since we have many new readers, but one big thing to know was emphasized by Acting CDC Director Richard Besser at the White House briefing yesterday: the influenza virus is highly unpredictable and our certain knowledge of it very scant. If you've seen one flu pandemic, you've seen one flu pandemic.

If this outbreak becomes a sustained worldwide one -- the definition of a pandemic -- you should not expect it to be the same as any other pandemic. It might be like 1918, 1957, 1968 or just a bad flu season. Or not.

But what did you expect?