Scientific papers based on global data and information obtained from specific countries (China, Brazil, Australia, Spain, and the US) have provided different conclusions on the effect of temperate and humidity (seasonality/climate) on COVID-19 transmission. Every research paper examined has stated that indoor environments posed a higher risk of COVID-19 transmission than outdoor environments. Below, we have provided a summary of ten studies each on the effect of seasonality on COVID-19 transmission and the effect of indoor vs. outdoor transmission.
EFFECT OF SEASONALITY ON COVID-19 TRANSMISSION
1. Impact of Meteorological Factors on the COVID-19 Transmission: A Multi-City Study in China (April 9, 2020)
- Based on an examination of 17 Chinese cities, the researchers have concluded that “weather with low temperature, mild diurnal temperature range and low humidity likely favor the transmission of COVID-19.”
- They first explored the associations between “COVID-19 confirmed case counts, meteorological factors, and MSI using non-linear regression.”
- Next, generalized linear models and meta-analysis were used to analyze 17 cities with over 50 confirmed cases.
2. Temperature Significantly Changes COVID-19 Transmission in (Sub)Tropical Cities of Brazil (April 25, 2020)
- Based on an examination of 27 Brazilian cities, the researchers have determined that “there is no evidence supporting that case counts of COVID-19 could decline when the weather becomes warmer (above 25.8°C).”
- They applied a generalized additive model to “explore the linear and non-linear relationship between annual average temperature compensation and confirmed cases.”
- A polynomial linear regression model was used to “represent the behavior of the growth curve of COVID-19” in the cities examined.
3. No Association of COVID-19 Transmission with Temperature or UV Radiation in Chinese Cities (May 7, 2020)
- Based on data obtained from 224 Chinese cities, the researchers have concluded that their study “does not support the hypothesis that high temperature and UV radiation can reduce the transmission of COVID-19.”
- This study has developed a model that provided the association between cumulative incidence rate and basic reproduction number and the temperature and ultraviolet in cities outside and inside of Hubei province.
4. The Role of Climate During the COVID-19 Epidemic in New South Wales, Australia (May 12, 2020)
- Based on data obtained from New South Wales, Australia, this study has determined that “during periods of low relative humidity, the public health system should anticipate an increased number of COVID-19 cases.”
- The researchers used “time series analysis to investigate the relationship between 749 cases of locally acquired COVID19 and daily rainfall, 9 a.m. and 3 p.m. temperature, and 9 a.m. and 3 p.m. relative humidity.”
5. A Spatio-Temporal Analysis of the Environmental Correlates of COVID-19 Incidence in Spain (June 8, 2020)
- Based on data collected from select Spanish provinces, this study has concluded that the incidence of COVID-19 is “lower at higher temperatures and higher levels of humidity,” and there is a positive association between sunshine and the incidence of COVID-19.
- The researchers used spatial Seemingly Unrelated Regressions to “model the incidence of reported cases of the disease per 100,000 population as an interregional contagion process, in addition to a function of temperature, humidity, and sunshine.”
6. Short-Term Effects of Specific Humidity and Temperature on COVID-19 Morbidity in Select US Cities (June 9, 2020)
- Based on an examination of eight US cities, the researchers have determined that the “influence of meteorological parameters on COVID-19 was modest and not uniform throughout select study locations.”
- They used a “case-crossover and distributed lag non-linear model” to study the association between daily temperature and humidity and COVID-19 morbidity.
7. The Effects of Regional Climatic Condition on the Spread of COVID-19 at Global Scale (June 9, 2020)
- This global study has concluded that “most of the countries located in the relatively lower temperature region show a rapid increase in the COVID-19 cases than the countries locating in the warmer climatic regions despite their better socio-economic conditions.”
- The researchers applied the coefficient of determination to analyze the “association among regional climatological parameters with average data of high temperature, low temperature, and daylight hours with total and death cases of COVD-19.”
8. Temperature, Humidity, and Latitude Analysis to Estimate Potential Spread and Seasonality of Coronavirus Disease 2019 (June 11, 2020)
- Based on an examination of 50 cities worldwide, the researchers have determined that the “distribution of substantial community outbreaks of COVID-19 along restricted latitude, temperature, and humidity measurements was consistent with the behavior of a seasonal respiratory virus.”
- The statistical analysis methods used for this study included ERA-5 reanalysis (climate data), Mann-Whitney test (temperature and transmission), and linear regression analysis (climate data and number of cases).
9. Transmissibility of COVID-19 in 11 Major Cities in China and Its Association with Temperature and Humidity in Beijing, Shanghai, Guangzhou, and Chengdu (July 10, 2020)
- Based on an examination of 11 Chinese cities, this study has concluded that the transmission of COVID-19 “will be suppressed as the weather warms.”
- The researchers used the Susceptible-Exposed-Infectious-Recovered model, correlation analysis, and regression analysis to examine the “association between the transmissibility of COVID-19 and the weather variables” such as temperature and humidity.
10. Susceptible Supply Limits the Role of Climate in the Early SARS-CoV-2 Pandemic (July 17, 2020)
- This global study has determined that “without effective control measures, strong outbreaks are likely in more humid climates and summer weather will not substantially limit pandemic growth.”
- The researchers used a “climate-dependent epidemic model to simulate the SARS-CoV-2 (COVID-19) pandemic, testing different scenarios of climate dependence based on known coronavirus biology.”
EFFECT OF INDOOR VS OUTDOOR TRANSMISSION
1. Seasonality of Respiratory Viral Infections (March 20, 2020)
- This study examined “how outdoor and indoor climates are linked to the seasonality of viral respiratory infections,” which include COVID-19 (SARS-CoV-2).
- The researchers have noted that the “corollary implication is that indoor climate and air change rates, modulated by outdoor seasonal conditions, are the key drivers of seasonal patterns in epidemiology.”
- Additionally, this study has determined that the “overwhelming majority of person-to-person transmission events happen indoors” and “exposure to outdoor contributes to alteration of respiratory defense on the existing virome.”
2. Airborne Transmission Route of COVID-19: Why 2 Meters/6 Feet of Inter-Personal Distance Could Not Be Enough (April 23, 2020)
- This study discussed the length of person-to-person distancing required in indoor and outdoor environments.
- The researchers have suggested that “more extensive distancing measures (over 10 meters) should be adopted inside indoor environments when face masks are not used,” and the distance could be reduced to 2 meters when face masks are used.
- This study also concluded that “low risk of contagion is guaranteed even for inter-personal distance shorter than 10 meters” in outdoor conditions as “droplets nuclei are subjected to higher dispersion in the atmosphere.”
3. Simulated Sunlight Rapidly Inactivates SARS-CoV-2 on Surfaces (May 20, 2020)
- This study has determined that “sunlight may rapidly inactivate SARS-CoV-2 (COVID-19) on surfaces, suggesting that surface persistence, and subsequently exposure risk, may vary significantly between indoor and outdoor environments.”
- The researchers have suggested that the “potential for fomite transmission may be significantly reduced in outdoor environments exposed to direct sunlight relative to indoor environments.”
4. COVID-19 and Other Coronavirus: Airborne Indoor and Outdoor Transmission? State of Evidence (May 30, 2020)
- Based on an examination of available literature and its own experimental hypothesis project, the researchers of this study have concluded that transmission of COVID-19 is more likely to occur indoor as viral aerosols will dilute in outdoor settings.
5. What Settings Have Been Linked To SARS-Cov-2 Transmission Clusters? (June 5, 2020)
- Based on an examination of 201 COVID-19 clusters worldwide, this study has concluded that the vast majority of clusters occurred in an indoor or mixed indoor/outdoor setting.
6. Reducing Transmission of SARS-CoV-2 (June 26, 2020)
- This study examined the factors that determined the probability of COVID-19 transmission in indoor and outdoor environments.
- The researchers have suggested that the probability of becoming infected indoors will primarily depend on the total amount of SARS-CoV-2 inhaled, and other factors such as “the amount of ventilation, number of people, how long one visits an indoor facility, and activities that affect airflow.”
- In outdoor environments, breeze and winds will affect the “concentrations and distance traveled, and whether respiratory viruses remain infectious in aerosols.”
7. Identifying Airborne Transmission as The Dominant Route for the Spread of COVID-19 (June 30, 2020)
- This study examined the difference in how COVID-19 transmission occurs in indoor and outdoor environments.
- In an indoor setting, “virus-bearing aerosols from human atomization are readily accumulated, and elevated levels of airborne viruses facilitate transmission from person to person.”
- In an outdoor setting, the transmission of airborne viruses is “subject to dilution, although virus accumulation still occurs due to stagnation under polluted urban conditions.”
8. Do the Benefits of Exercise in Indoor and Outdoor Environments During the COVID-19 Pandemic Outweigh the Risks of Infection? (July 17, 2020)
- This study has suggested that people should avoid exercising in “indoor environments with higher occupancy” and adopt social distancing (at least 2 meters/6 feet) or side-by-side arrangement when exercising outdoor.
- The researchers have concluded that the benefits of exercising during the pandemic may outweigh the risks of infection, but “caution is needed in both indoor…and outdoor environments.”
9. An Overview on the Role of Relative Humidity in Airborne Transmission of SARS-CoV-2 in Indoor Environments (July 21, 2020)
- This study has determined that in indoor places with less humidity, the “chances of airborne transmission of SARS-CoV-2 are higher than that of humid places.”
- The researchers have also noted that “indoor air contributes to population exposures more than those outdoors.”
10. Two Meters or One: What Is the Evidence for Physical Distancing in COVID-19? (August 25, 2020)
- This study has published a table (figure 3) that provided the “risk of SARS-CoV-2 (COVID-19) transmission from asymptomatic people in different settings and for different occupation times, venting, and crowding levels.”
- According to the table, indoor environments with poor ventilation posed higher risks than well-ventilated indoor and outdoor environments.