Table 4.

Summary of the 19 papers which met the inclusion criteria for this review.

reference, year,
country, setting, NPI	measurements and data	conclusions
Li et al. [10]	infection rate (percentage):	increased ventilation, but with lower adherence to masks, caused increased transmission
	Dorm 1: 74%
location: China	Dorm 2, Zone N: 16–18%
setting: residential	Dorm 2, Zone M: 8%
NPI of interest:	ventilation:
EC: ventilation	Dorm 1: $12.9\text{--}32.4\hspace{0.17em}{\mathrm{m}}^3\hspace{0.17em}{\mathrm{h}}^{-1}$
oth: masks	Dorm 2, Zone N: $28\hspace{0.17em}{\mathrm{m}}^3\hspace{0.17em}{\mathrm{h}}^{-1}$
	Dorm 2, Zone M: $7.7\hspace{0.17em}{\mathrm{m}}^3\hspace{0.17em}{\mathrm{h}}^{-1}$
	masks (qualitative measurement):
	Dorm 1: personnel wore no masks
	Dorm 2, Zone N: personnel wore cloth masks, members wore no masks at night
	Dorm 2, Zone M: personnel wore cloth masks, members wore masks
	plot of ventilation rates VS infection rates: no clear trend observable.
Oginawati et al. [11]	thirty-eight houses of COVID-19 survivors divided into three clusters based on transmission (measured as no. of COVID-19 positive family members VS total no. of family members):	(i) increased ventilation shows reduced transmission
location: Indonesia	low transmission: 0–50%	(ii) Increased personal space shows increased transmission. The authors attribute this trend to presence of larger total household members, and therefore larger susceptible population, in larger houses.
setting: residential	intermediate transmission: 50–90%
NPI of interest: ventilation and occupancy	high transmission: 100%
	ventilation and personal space (occupancy) measured using measuring tape; Pearson correlation coefficient calculated:
	for ventilation $r=-0.522$
	for personal space $r=0.459$
Walshe et al. [12]	Outbreak in a mean processing plant with 111 confirmed positive asymptomatic cases (estimated attach rate of 38%) in five weeks. Four weeks after first case, 32 workers test positive, among which 16 (50%) were working in the boning hall. First three symptomatic workers were from the boning hall.	(i) increased transmission observed in the Boning hall with decreased ventilation and increased occupancy levels, in comparison to the abattoir
location: Ireland
setting: meat-processing plant (MPP)
NPI of interest:Ventilation and occupancy	air quality measurements (${\mathrm{CO}}_2$ concentrations, temperature, relative humidity, aerosol particle numbers) were compared between two regions:
	Boning hall with a relatively high proportion: ${\mathrm{CO}}_2$ concentration and average aerosol particle concentration increased throughout the day (except during breaks). Average temperature was ${100}^{\circ }\mathrm{C}$ and relative humidity 66%. Air re-circulation mechanism in place. ${\mathrm{CO}}_2$ decay suggests approximately $0.4\text{--}0.5\hspace{0.17em}\mathrm{ACH}$
	Abattoir with a relatively low proportion: ${\mathrm{CO}}_2$ concentration decreased throughout the day. Number of aerosol particles showed no significant change through the day. Average temperature was ${180}^{\circ }\mathrm{C}$ and relative humidity 71%. Air extraction mechanisms that provides approximately $8\hspace{0.17em}\mathrm{ACH}$ (based on volume of air extracted) in place.
	occupancy and screens differed between the two areas:
	boning hall: occupancy $-5.1\hspace{0.17em}{\mathrm{m}}^3$ floor area per person; screens in place
	abattoir: occupancy $-15.2\hspace{0.17em}{\mathrm{m}}^3$ floor area per person. Not clear if screens in place
	performance of an air filter also measured based on the changes to air quality; however transmission difference before and after installation of the filter not considered
	many other NPIs were also in place before the outbreak, but differences in transmission due to them not measured
Atnafie et al. [22]	Four hundred and eighteen people were studied, with 78 (18.7%) people with reported confirmed COVID-19 exposure and 78 (18.7%) people with confirmed COVID-19 infection. Data on perceived exposure of the worker, training need, as well as adherence to different NPIs including PPE usage, hand-washing habits, decontamination of high touch surfaces, changing of masks were collected through questionnaires and statistically correlated to infection rates.	surface disinfection was found to reduce transmission in healthcare settings
location: Ethiopia
setting: governmenthospitals and health centres
EC NPI—surface disinfection
	from statistical analysis they conclude that hospital workers in an institution that does not decontaminate high touch surfaces had 2.5 ($\mathrm{AOR}=2.52$, $95\mathrm{\%}\hspace{0.17em}\mathrm{CI}=1.12$–5.65) times the risk of getting infected when compared to workers from institutes that decontaminate high touch surfaces
Kerai et al. [23]	eighty-one HCWs with known COVID-19 exposure were considered as cases and 266 HCWs who were asymptomatic and tested negative for COVID-19 controls (no data on whether the control HCWs came in contact with COVID-19 positive patients); data collected by telephonic interviews	the practice of decontamination of high touch surfaces was found to be statistically correlated to reduced transmission
location: India
setting: tertiarycare-dedicated COVID-19 hospital
NPI: disinfection	the risk of infection was found to increase by a factor of 0.41 if high touch surfaces were not decontaminated
Telford et al. [24]	twenty-four LTCFs totalling 2580 residents with 1004 (39%) residents infected with COVID-19 were included in the study	(i) maximum occupancy limits in enclosed places were observed in facilities with a lower prevelance of COVID-19 infections
location: USA	— higher-prevelance group (greater than 39% infection proportion) — 11 LTCFs	(ii) no significant differences in the adherance to disinfection strategies were observed in the groups with higher and lower prevalence of COVID-19 infections
	— lower-prevelance group (lower than 39% infection proportion) — 13 LTCFs
setting: long-term care facilities (LTCFs)	adherence to NPIs of Hand Hygiene, Disinfection, Social Distancing, PPE and Symptom Screening were considered based on site visits (virtual or in-person)
NPI: room occupancy, disinfection
	maximum occupancy in small enclosed places (elevators, donning/doffing rooms etc.) was enforced in 10% of LTCFs in Higher prevalence group versus 64% in Lower-prevalence group, $p=0.01$
	differences in adherence to disinfection between the higher and lower prevalence groups was not statistically significant
Ou et al. [13]	an infected person travelled on two buses B1 and B2. Ventilation rates on the buses measured using tracer-decay and CFD	there was increased COVID-19 transmission on a bus with lower ventilation rates, compared to a bus with higher ventilation rates
location: China	—B1: infected people – 7/46 (and 1 more on a return trip without the index case)
setting: buses
NPI: ventilation	ventilation rate — 1.72 L/s per person.
	area occupied per passenger: $0.6\hspace{0.17em}{\mathrm{m}}^2$
	— B2: infected people — 2/17.
	ventilation rate — 3.22 $\mathrm{L}\hspace{0.17em}{\mathrm{s}}^{-1}$ per person
	area occupied per passenger: $0.72\hspace{0.17em}{\mathrm{m}}^2$
Pokora et al. [14]	twenty-two meat and poultry plants with 19 072 employees studied (among which 880 were infected) 6552 employees from plants with many (greater than $10$) infected were included for the statistical analysis. ventilation information was collected through questionnaires sent to the MPPs	the presence of a ventilation system was found to reduce the infection rates in meat processing plants
location: Germany
setting: meat processing plants (MPPs)
NPI: ventilation	they show that having a ventilation system reduced the chances of testing positive for COVID-19 (OR 0.388; 95% CI 0.299–0.503)
Gettings et al. [17]	one hundred sixty-nine schools with 91 893 studies (556 infected) studied; data collected through surveys	(i) ventilation by air dilution reduced COVID-19 transmission
location: USA
setting: classrooms	COVID-19 incidence (adjusted for Coutty level incidence) was impacted by different ventilation strategies. Incidence rates were:	(ii) ventilation by air dilution and purification/filteration reduced transmission more than when using dilution alone
NPI: ventilation and screens	— 39% lower in schools that reportedly improved ventilation
	— 35% lower using air dilution (opening doors, opening windows or using fans) alone
	— 48% lower using air dilution along with filtration/purification (HEPA filter and/or UVGI)	(iii) the use of screens does not impact transmission
	the COVID-19 incidence did not change significantly between schools that implemented barriers only in some/no classrooms compared to schools that implemented barriers in all classrooms
Feathers et al. [18] (2022)	reported 10–20 in-patients during the time of the outbreak and 10–20 ward sta.. during any time	improved ventilation through opening windows and using extractor fans may have reduced the incidence of COVID-19
location: UK
setting: hospices for palliative care patients	During the outbreak 26 patients and 30 sta.. were infected (measured by laboratory based RT-PCR testing). After the outbreak, implementation of following NPIs were advised:
NPI: ventilation	— EC NPI: improved ventilation through opening windows and leaving extractor fans in treatment rooms on
	— other NPI: universal staff masking and asymptomatic staff screening
	After the measures implemented (adherence to the NPIs not measured), the number of COVID-19 cases decreased to zero in three weeks. However, the decrease was noticed when the national COVID-19 incidence was decreasing.
Wang et al. [21]	Secondary transmission studied in 335 people from 124 families with at least 1 laboratory confirmed COVID-19 case. From 41 primary cases, the secondary attack rate was 23% (77/335). The effectiveness of many NPIs in reducing secondary transmission was studied. Data collected through questionnaires and interviews.	disinfection using chlorine or ethanol-based products was found to reduce secondary transmission in households
location: China
setting: residential
NPI: disinfection, ventilation, occupancy
	for reducing secondary transmission:
	— daily disinfection using chlorine or ethanol-based products was 77% effective in reducing secondary transmission (OR$=0.23$, 95% CI 0.07–0.84)
	— crowding of the household was not found to be significant
	— duration of ventilation was found to be significant in univariable analysis, but not in their multivariable logistic regression analysis
Guedes et al. [20]	One hundred twenty-one Dialysis facilities studied. The facilities managed 20 984 patients among which 1093 were confirmed of having COVID-19. Four hundred fifty-nine HCWs tested positive as well. Data collected through questionnaires.	improvements in ventilation and disinfection not found to be associated with decreased detection of COVID-19 clusters in haemodyalysis units
location: Brazil
setting: hemodialysis facilities
NPI: ventilation, disinfection	presence of COVID-19 clusters (defined as occurrence of more than 1 COVID-19 case within 7 days during the same dialysis shift) was found not to be associated with a range of NPIs; this included cleaning and disinfection and ventilation
Szablewski et al. [25]	six hundred twenty-seven people attended the camp and there were 351 COVID-19 positive cases (56% attack rate)	cabin occupancy rates may have been one of the factors that affected COVID-19 attack rates among campers
location: USA
setting: sleep-away youth camp	— During orientation: occupancy—cabins with median occupancy of 11 (range 1–23 occupants). Attack rate—median cabin attack rate of 50% ($\text{interquartile range (IQR) }=35\text{--}59\mathrm{\%}$).
NPI: occupancy	— During camp session: occupancy—cabins with median occupancy of 24 (range 1–26 occupants). Attack rate—median cabin attack rate of 67% ($\mathrm{IQR}=54\text{--}72\mathrm{\%}$).
Zhang et al. [28]	Tested a negative ionizer for reducing transmission in hamsters through direct contact and aerosol transmission. After housing three infected animals in the same cage with three naive animals for direct transmission study and with three naive animals separated by wire frames for aerosol transmission studies:	they found that the use of negative ionizer disinfection reduced aerosol transmission in hamsters
location, setting: Animal study	— without the ionizer 3/3 hamsters were infected
NPI: air purification (negative ionizer)	— with the ionizer 3/3 hamsters were protected from aerosol transmission
	— ionizer did not block direct contact transmission
Nabirova et al. [15]	The study considered 296 PCR-positive cases and controls were 536 PCR-negative cases who lived in the same camp. Data collected through telephonic interviews. The statistical analysis showed that among environmental factors, only working in air-conditioned spaces showed a correlation with increased transmission, with other factors considered showing no significant association.	they found no significant correlation between COVID-19 transmission and working in ventilated workstations
location: Kazakhstan	— working in ventilated spaces was found to be not correlated with transmission (AOR 0.68 95% CI 0.36–1.24)
setting: oilfield
NPI: ventilation
	occupancy was considered but not included in the analysis because of identified confounding factors
Baumgarte et al. [16]	Three hundred sixty-eight students and 117 staff were tested, out of which 33 students (two students acquired infection from outside) and three staff tested positive. Data collected from the health department and school management and through telephonic interviews. Spatial conditions of classrooms obtained from building plans and local data. From one index case (staff A), the affected classroom from the most affected to the least affected were:	a school classroom with poorer ventilation and slightly higher occupancy rates saw higher transmission of COVID-19 from an index patient
location: Germany	— C.1 with 3h exposure time (ET)—16 cases, attack rate (AR) 57.14%
setting: schools	— C.2 with 1:3hr ET—8 cases, AR 33.33%
NPI: ventilation and room occupancy	— C.3 with 1:3hr ET—3 cases, AR 12.5% and
	— C.4 with 0.45hr ET—1 case, AR 3.7%
	all of the classrooms had lessons taught by the staff A with C.1 and C.2 having the class on day 3 and C.3 and C.4 on day 4 of the outbreak (staff A had improved personal protection measures on day 4)
	in comparison to C.2, C.3 and C.4, the most affected class C.1 had
	— poorer ventilation configuration (not measured but analysed through the structure of the available windows)
	— and slightly higher occupancy ($5.3\hspace{0.17em}{\mathrm{m}}^3$ room volume per person in C.1 compared to $6.3\hspace{0.17em}{\mathrm{m}}^3$ is C.2 and C.3 and $5.6\hspace{0.17em}{\mathrm{m}}^3$ in C.4)
Cheng et al. [26]	compared two restaurant outbreaks R1 (in February 2021) and R2 (in December 2021)	(1) lower transmission observed in a restaurant with air purifiers installed and all diners vaccinated, when compared to a restraunt where no air purifiers were installed and all diners were unvaccinated
location: Hong Kong		(2) the restraunt with higher transmission also had lower occupancy rates
setting: restaurant	outbreak R2: 2.6% secondary attach rate from 1 index case:
NPI: air purification	— ACH: 2.0
	— air purifier: 14 UV-C air purifiers
	— occupancy: $1.19\hspace{0.17em}{\mathrm{m}}^2$ area per customer
	— vaccination status: all present had two doses of COVID-19 vaccines
	outbreak R1: 33.7% secondary attach rate (no. of index cases unknown):
	— ACH: 1.2
	— air purifier: none installed
	— occupancy: $1.82\hspace{0.17em}{\mathrm{m}}^2\hspace{0.17em}\text{per customer}$
	— vaccination status: all present not vaccinated
Fischer et al. [27]	Two donor hamsters inoculated with SARS-Cov-2 (two variants nCoV-WA1-2020 or hCoV-19/USA/KY-CDC-2-4242084/2021 (Delta)) were placed in a box separated by a tube from two naive hamsters. Two groups of set-ups were considered	the study found that UV-C light treatment reduced transmission in hamsters
location/setting: animal study	(1) with UV-C light treatment fitted inside the tube connecting the boxes that housed the donor and naive hamsters (representative of air treatment in a ducted system) and
NPI: air purification	(2) no UV-C light treatment. Transmission was measured using qRT-PCR and enzyme-linked immunosorbent assay. After 4 h of exposure, all animals in the the no UV-C treatment group had detectable viable virus. And no virus was detected in the group with UV-C light treatment.
Dancer et al. [19]	A window opening policy was enforced in three hospitals on 25th January 2021, along with multiple other NPIs that included daily surveillance, ward closures, universal masking, screening, restricting staff and patient movement and enhanced cleaning. Forty COVID-19 clusters were observed in three hospitals between 1st October and 25th January 2021, and only three clusters occurred between 25th January 2021, when window opening policy was implemented, and 31st March	window opening may have caused a decrease in outbreaks in hospital wards
location: Scotland
setting: hospital wards
NPI: ventilation	however, the decrease in outbreaks was observed when (1) the national COVID-19 incidence was decreasing, (2) when multiple NPIs were implemented together and (3) when the vaccination status of the population had changed