Chris Iddon

New study looking at influenza transmission

1 infected person shared small low humidity and poorly ventilated room with 8 uninfected subjects (plus 2 observers). Chatting, playing uno, bit of exercise

5000+ppm CO2

no one got infected 🧵 1/n

This is the second similar study that the authors have conducted. The ethics are recognised

Evaluating modes of influenza transmission (EMIT-2): Insights from lack of transmission in a controlled transmission trial with naturally infected donors Author summary Human-to-human influenza virus transmission under controlled conditions could provide insights leading to better control of epidemics and pandemics. However, a previous study using labo...

Despite creating an environment that would be hypothesised to support flu transmission, other factors matter:
immunity of recipients
viral emission rates
close range transmission
number of coughs
tbh i think this shows that "it's complicated" /end journals.plos.org/plospathogen...

Although there was low ventilation rates in these rooms, there was high mixing rates and the authors postulate that this could have rapidly mixed exhaled plumes that ordinarily would hold more concentrated viral material and support close range transmission 5/n

air sample in the room only detected low levels of viral genomic material, and exhaled breath samples were also low in viral material. Items were passed around, tablet, microphone and marker pen. Some viral material found on one sample from the pen 4/n

This study included multiple interactions between donors and recipients, some the CO2 only got to 2400ppm, but the authors note that despite the donors having high viral load from swabs they emitted low levels of virus 3/n

There's plenty that would baulk at CO2 = 5000ppm, raw dogging the air etc
but as we've been pointing out for years, if the infector isn't emitting much virus then your inhaled dose will be low, even if ventilation is poor 2/n
a link to the other place x.com/moog77/statu...

New study looking at influenza transmission

1 infected person shared small low humidity and poorly ventilated room with 8 uninfected subjects (plus 2 observers). Chatting, playing uno, bit of exercise

5000+ppm CO2

no one got infected 🧵 1/n

thanks for the comprehensive explanations :)

16+ venue, my kids will be disappointed. Any other gigs in the pipeline?

An interesting study which provides some more evidence of the impact of HEPA filter treatments on school absenteeism. Overall, the study offers little reliable evidence that HEPA air cleaners meaningfully reduce student absenteeism. proquest.com/docview/3281...

The study does control for important confounders (school effects, classroom effects, seasonality).

But it can’t control for individual health, household exposure, immunity, or behaviour—so residual confounding remains likely.

This doesn’t mean the researchers did anything wrong—sensitivity analyses are useful.

But without a single pre-specified primary model, statistically significant results should be treated cautiously.

Why the uncertainty?
The study ran many overlapping models (different seasons, moving averages, interactions).
Results change depending on the model → classic risk of model-dependence (and over-interpretation).

And importantly:
⚠️ In Winter, some models show higher absenteeism in HEPA treated classrooms compared to controls—especially at higher equivalent ventilation rates.

Other models show no statistically significant effect at all once you adjust for school, classroom, season, and time trends.

In other words: the “benefit” disappears depending on how you model the data.

Some statistical models show small reductions in illness-related absenteeism in HEPA classrooms—mostly in Fall and Spring.

But the effect sizes are tiny (fractions of a day per student).

But the big question was absenteeism.
Do cleaner classrooms = fewer sick days?
Short answer: the evidence is mixed and weak.

They didn't see much difference in particle counts between control and treatment (T) classrooms when occupied, but they only took a couple of 24hr measurements. Other studies with continual measurement have shown reductions in PM with filters present. Unoccupied = PM reduction

The study installed portable air purifiers (HEPA-based, some with extra filtration layers) in many classrooms and compared illness-related absenteeism to control classrooms with no purifiers.

New thesis on HEPA air cleaners in school classrooms:
🧵 TL;DR: In this school study, HEPA air cleaners showed little reliable impact on absenteeism, and air-quality improvements were limited during actual classroom use.

yeah the deposition stuff is interesting (tho predictable). But their conclusions on ventilation and PAC are dependent on the size of the PAC and ventilation provision, of which there are myriad combinations (here i think they use 2), and obvious from their relative ach

my point is that it's comparing a larger equivalent removal with a smaller removal and going oh look the larger removal removes more. Completely pointless to me. You can see that 3.15ach is greater than 0.5ach without having to run a model.

what's the point of comparing ventilation at 0.5ach with a PAC with equivalent ventilation of 3.15ach and going, oh the PAC is better at removing particles? They model a 40m3 room which is tiny. If PAC_1 was in a 300m3 room with 0.5ach ventilation, the PAC_1 will now be running at 0.42ach 🤷‍♀️

They will provide air purifiers with HEPA filters running at 486m3/h (approx 3ach) or at 48m3/h.

Classroom air purifiers for reducing school absence: study protocol for a pragmatic, cluster-randomised, parallel, two-arm, group sequential superiority trial Respiratory infections like influenza and SARS-CoV-2 pose significant global health risks due to their high transmissibility and severity. SARS-CoV-2 has caused over 7 million deaths worldwide, and the Lancet Commission estimates a >20% chance of a ...

New study protocol for a pragmatic, cluster-randomised, parallel, two-arm, group sequential superiority trial of HEPA filters in school classrooms to commence later this year in Norway pmc.ncbi.nlm.nih.gov/articles/PMC...
-Using school absence as outcome
-be interesting to see the results

The stuff you find when you actually read the RCTs in a systematic review...

This paper is one of the foundational studies on vitamin D to prevent respiratory infections in kids. Cited 1,400 times as per Google Scholar.

Associations of air cleaning interventions in primary school classrooms with indoor environmental quality and perceived symptoms Air cleaners are used to remove various pollutants from air in different types of indoor environments. Studies on the air cleaners’ efficiency to remo…

The authors conclude that the air quality in these classrooms is already good, the ventilation is good even without the air cleaners, so they suppose that the impact of air cleaners is limited in such environments.
sciencedirect.com/science/arti...

Kids also reported their perceived perception of the indoor air quality and their perception of the air quality improved with the presence of air cleaners, even when the air cleaners were off

A placebo effect - demonstrating the power of suggestion

6 schools and 27 classrooms under baseline, air cleaner off, HEPA on and SHAM (air cleaner on but no HEPA filter). Air cleaners also included activated carbon to remove TVOC.
If kids had symptoms they scored these, air cleaner reduces symptom severity with and without HEPA