The media is all a-twitter with news of a University of Michigan study, published this week in the New England Journal of Medicine, showing that in adults the injected flu vaccine is far more effective than the nasal spray version (also known as FluMist).

As noted here by NPR yesterday during their interview with the lead author of the study, the nasal spray provides

only half as much protection as conventional flu shots.

I’ve read quite a few news stories about this study, but none of them discuss reasons for this result.  Another teachable moment lost, unless you read on, of course.

This result is not at all surprising.  Leaving out for the present the possibility of manufacturing and related issues, the explanation is simple.  The inhaled material (attenuated live virus) has to get into a person’s system, and this journey is fraught with variables and impediments to travel.

Perhaps the most important consideration is mucus.  The attenuated virus particles must pass through mucus (just as the actual flu does) to have an effect.  As we all know from blowing our noses, mucus varies in consistency, and it will vary in thickness at places the virus must pass through.   (By thickness, I mean both the distance measured from one side of a mucus layer to the other, as well as how thick–think viscosity–the material is.)

Naturally, some people will have more and thicker mucus than others, for various reasons, including colds, food habits, and local air pollution (which can spark creation of more mucus).  So,  these people with more mucus will receive a lower dose  of spray vaccine than those with less mucus, and thus won’t be as likely to be fully immunized.  The vaccine company has  no doubt tried to compensate for this factor by using a high  dose, getting people to blow their noses, and other means, but they can’t control this factor very well in my view.  Further, they likely face a top-end maximum that they can put in each dose, because this is a live virus, albeit an attenuated one.

Other factors include the fact that different people will be able to inhale more strongly than others, again for various reasons.

But an injected dose is an exact amount into the bloodstream, with no mucus or inhalation factors coming into play.