Blowing in the wind

Last month, the Italian region of Marche announced they had installed ventilation in schools and it had reduced Covid infections by a 82% (Reuters, Stuff, La Repubblica).  A report was supposed to follow, but this is all I’ve been able to find.  It’s not really surprising that Covid rates went down with improved ventilation, but what’s currently available is very low on detail. Ventilation was installed in 3% of classrooms (or for 3% of classes, I’m not certain), and this 3% was compared to those that didn’t get new ventilation.  The reported benefits were:

That’s great! But. Things you’d really like to know when you think about how much this should change policy in other countries:

• How were the schools with new ventilation chosen, and how were the different ventilation levels chosen? How did their Covid rates compare before the change?
• How was Covid measured? Was there any systematic testing or was it just a matter of who got sick and then got tested? Is this symptomatic infections or all infections? Do you know anything about their testing rates?
• Was there any attempt to decide if Covid cases were connected with school or were household infections or something else?
• Did the ventilation involve any measurement of air mixing and effective air changes, or does this study show you don’t need that?
• Were students wearing masks? What were the isolation rules for infections?
• What are the uncertainty intervals on those efficacy estimates? How many students and Covid cases in each group are the estimates based on?

In particular, the relationship between air changes and transmission risk looks very close to what you might expect from just diluting the air — but it really shouldn’t! The ventilation should only have changed Covid risk while students were at school; it shouldn’t have reduced the risk of transmission at home or in other places.  To get an 82.5% reduction in total infections, they must have been doing much better than 82.5% reduction in infections at school.  For example, if 82.5% of infections in the schools without new ventilation happened at school, you’d need to abolish those at-school infections completely to get 82.5% overall effectiveness.  If 90% of infections happened at school, you’d need 92% effectiveness in reducing at-school infections to get 82.5% overall effectiveness.

If the point of the Italian study is just that ventilation is beneficial, it really isn’t major news and it’s not all that helpful to other countries. If the detailed estimates are to be useful, we need to know what they are detailed estimates of.