Risk Analysis Journal: A COVID-19 Cluster Analysis in An Office

#17,763

While the threat of COVID has diminished greatly since the pre-vaccine days of 2020, it is unlikely to be the last novel virus we will have to deal with in the years ahead.  

Since many of the early assumptions about the SARS-CoV-2 virus were overly optimistic - and that undoubtedly cost lives - it worthwhile to glean what we can from the experience. 

Some of the early informational `misfires' of the first year of the COVID pandemic included:

• The belief that COVID was not asymptomatically transmitted
• SARS-CoV-2 was not an airborne virus
• A distance of Six feet from an infected person was considered `safe'
• It g required a lengthy (15 minutes face to face) exposure to contract the virus
• Masks were not useful for the public, and could increase the risk of infection
• Herd immunity would end the pandemic within 6 to 18 months
• Once infected or vaccinated, reinfection was unlikely

Governments and political leaders instinctively seek to minimize threats and reassure the public, even when the available evidence is weak or inconclusive. Gradually, most of these assumptions were revised, although you still see a few hanging on today. 

Since the next pandemic may well have a higher case fatality rate, it would behoove us not to repeat these same mistakes. 

Today we've a statistical analysis/simulation based on an actual office outbreak of COVID in Japan in 2021, which not only strongly suggests long-range aerosol transmission of the virus, it also estimates the effectiveness of masks (both regular & fitted), and the value of better ventilation in buildings. 

Fair warning, this is a lengthy, detailed, and quite technical read which incorporates a good deal of statistical gymnastics, most of which are well above my pay grade. I've only reproduced the abstract and conclusions. 

Those with a better grasp of statistics may wish to follow the link to read the report in its entirety.  I'll have a brief postscript after the break. 

A COVID-19 cluster analysis in an office: Assessing the long-range aerosol and fomite transmissions with infection control measures

Atsushi Mizukoshi, Jiro Okumura, Kenichi Azuma
First published: 07 November 2023
https://doi.org/10.1111/risa.14249

Abstract

Simulated exposure to severe acute respiratory syndrome coronavirus 2 in the environment was demonstrated based on the actual coronavirus disease 2019 cluster occurrence in an office, with a projected risk considering the likely transmission pathways via aerosols and fomites.

A total of 35/85 occupants were infected, with the attack rate in the first stage as 0.30. It was inferred that the aerosol transmission at long-range produced the cluster at virus concentration in the saliva of the infected cases on the basis of the simulation, more than 108 PFU mL−1. 

Additionally, all wearing masks effectiveness was estimated to be 61%–81% and 88%–95% reduction in risk for long-range aerosol transmission in the normal and fit state of the masks, respectively, and a 99.8% or above decline in risk of fomite transmission. 

The ventilation effectiveness for long-range aerosol transmission was also calculated to be 12%–29% and 36%–66% reductions with increases from one air change per hour (ACH) to two ACH and six ACH, respectively.

Furthermore, the virus concentration reduction in the saliva to 1/3 corresponded to the risk reduction for long-range aerosol transmission by 60%–64% and 40%–51% with and without masks, respectively.

(SNIP)

The objectives of this study are to verify the quantitative risk from each transmission pathway based on clustered cases, to verify parameters regarding transmissibility such as concentration in the saliva and dose–response function parameter, and to quantify the control measure effects such as masks and ventilation. The key scientific questions were whether long-range aerosol transmission or fomite transmission was dominant quantitatively in COVID-19 cluster cases in an office and how much the COVID-19 risk from each pathway could be reduced by each infection control measure.

(SNIP)

5 CONCLUSION

The transmission pathways for this study were hinged on the actual COVID-19 cluster in the office environment, wherein many occupants were in wide-open office space. The key findings were as follows:

• Long-range transmission from inhalation of aerosols became dominant following the control measures for infection by putting on masks, whereas transmission from indirect contact via fomites was rare.

• The dominance of long-range aerosol transmission was consistent with the reviewed evidence suggesting the possibility of long distance airborne transmission of SARS-CoV-2 in indoor settings (Azimi et al., 2021; Duval et al., 2022).

• Additionally, if the infected case(s) with a high concentration in the saliva existed, this transmission may occur.

• The effectiveness of all wearing masks was a 61%–81% and 88%–95% in the normal and fit state of the masks, respectively, in long-range aerosol transmission and a 99.9% or above reduction in the risk of fomite transmission.

• Moreover, ventilation was also effective, of which risk reductions for long-range transmission were 12%–29% and 36%–66% with air change rate increases from one ACH to two ACH and six ACH, respectively. The virus concentration reduction in the saliva to one third corresponded to the risk reduction for long-range transmission by 60%–64% and 40%–51% with and without masks, respectively.

Thus, infection control measures against long-range aerosol transmission are crucial, especially for SARS-CoV-2 variants, such as increasing mask removal efficiency through proper wearing, ensuring a low virus concentration via ventilation, and possibly, decreasing the virus body load and emission of infected cases, such as early vaccination.

When the next pandemic comes, during the first 6 to 12 months our first (and likely only) defense will be NPIs (non-pharmaceutical interventions), like face masks, social distancing, hand hygiene, and better indoor ventilation.  

While the internet is filled with revisionist history and claims that masks don't work, every major study we've seen (see here, here, here, here, here, and here)  tells us the opposite is true. 

Face masks and other NPIs can significantly reduce the spread of any respiratory virus.  A lesson that we'd be far better off remembering, rather than having to re-learn it the hard way.

Again.