Background While possibility of airborne transmission in the spread of common respiratory infections, there is no consensus on the relative importance of airborne infection route in real-life. This study aimed to investigate the significance of the airborne transmissions and the effectiveness of air cleaning in reducing infections among children in daycare. Methods A cross-over study was conducted in four daycare centers in Helsinki. All children attending the daycare were invited to participate (n = 262) and the sole inclusion criterion was that the children were expected to stay in the same day care center for the two-year duration of the study. 51 subjects were included in the final analysis. Clean air flow rate was increased by 2.1-2.9 times compared to baseline mechanical ventilation of the premises. The effect of intervention was assessed using negative binomial regression. Results The intervention reduced incident infections from 0.95 to 0.78 infections per child per month among the children (primary outcome) in daycare. The reduction attributed to intervention in the statistical model was 18.0 % (95% CI 2.1-31.3 %, p = 0.028). Conclusions We observed a significant decrease in incident infections without implementing any other infection mitigation strategies but air cleaning. Our results challenge the current paradigm which emphasizes fomite and contact transmission and infection control measures that target these pathways. As ventilation and air cleaning can only affect particles able to float in the air stream, our results support the significance of airborne transmission among common respiratory pathogens as well as air cleaning as an infection control measure.

The authors have declared no competing interest.

NCT05569330

The study was a part of the E3 Excellence in Pandemic Response -project funded by Business Finland [grant number 4793/31/2021], Helsinki University Hospital Coinnovation fund, Finnish Medical Foundation (VV), FLS (ES).

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Ethics committee of Helsinki and Uusimaa gave ethical approval for this work (HUS/14231/2022)

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