Swine flu: what does "so far, so good" mean?

by revere, cross-posted from Effect Measure

Breathing easier, may be an apt phrase for an almost audible collective sigh of relief. So far, the incipient swine flu pandemic is not extremely nasty. Is this perhaps premature? The world's premier scientific journal, Nature, and many flu scientists, suggest it is:

Complacency, not overreaction, is the greatest danger posed by the flu pandemic. That's a message scientists would do well to help get across.[snip]

There is ample reason for concern: a new flu virus has emerged to which humans have no immunity, and it is spreading from person to person. That has happened only three times in the past century.

[snip]

Yet at this early stage, the consequences of the pandemic are so uncertain that communicating the risks is a delicate matter. Influenza viruses evolve rapidly, making it extremely difficult to predict what this strain might look like a few months from now.

[snip]

The risk is not hyping the pandemic threat, but underplaying it. (Editorial, Nature)

 

No one on the public health side has over reacted. When an outbreak or pandemic is unfolding, you get only one chance. The window is a narrow one. CDC (and WHO) have acquitted themselves well, so far. CDC's daily briefings have been straightforward and informative. The public, understandably, has bounced back and forth from fear to relief and back again. I don't think either pole is avoidable. If the scientists are baffled and uncertain, the public has a right to be, too. But CDC has been steady and consistent in their approach (more so than HHS or Homeland Security, in my view) and they clearly understand the most important thing: this outbreak is still evolving and it is premature to write it off.

The idea this is "mild" flu virus is so far true, but mild is a relative term, as we have pointed out here. And "so far" is another important qualifier. This virus is spreading relatively quickly, but it isn't everywhere and not much time has passed. With exceedingly virulent strains like the 1918 virus many people died relatively quickly but with flu death usually comes weeks after the onset of infection. Not that many people have been infected as yet. If the pace of new cases accelerates, we will likely seem many more hospitalizations (currently there are only 35 in the US) and more deaths (currently there are two):

"My own personal view is that it is too early to say what the severity is," says [Marc Lipsitch of the Harvard School of Public Health], who works with the CDC as part of a 'Team B' made up of academic researchers and other outside advisers. One reason the disease appears mild is that in the United States there has been only one reported death among 286 cases. But such case-fatality rates, reported one week after a case is confirmed, can underestimate actual mortality rates, because they can overlook patients who remain ill and subsequently die. This was demonstrated in 2003, when the reported case-fatality rate of severe acute respiratory syndrome (SARS) in Asia was initially put at a few per cent â only to be found to be closer to 20% in studies that followed up cases. (Declan Butler, Nature)

So the question remains. Are we like the man who jumps from the Empire State Building and says as he passes the 14th floor, "So far, so good"?

Let's hope not, but with influenza one cannot be sure of anything. In the coming months we will be watching the southern hemisphere closely to see what happens as they enter their flu season. That may give us a clue as to what to expect in the fall.

. . . or it may not.