Since the emergence of SARS-CoV-2 in late 2019, researchers have been trying to unpick seasonal characteristics of the novel coronavirus. Expectations of seasonal behaviour of COVID-19 are based on evidence from seasonal respiratory viruses, such as influenza and other human coronaviruses, which tend to peak in Northern Hemisphere winter (Figure 1)¹. After almost two years, the current state of knowledge still reveals several unknowns and uncertainties highlighting the ongoing need for further robust modelling analyses. 

Underlying mechanisms of COVID-19 seasonality include direct effects on virus stability and indirect effects via the human immune system and human behaviour (Figure 2).  

Simulated sunlight was found to rapidly inactivate SARS-CoV-2 on surfaces suggesting that persistence and exposure risk might vary substantially between indoor and outdoor spaces². Low temperatures were shown to favour the aerosol survival of the virus under laboratory conditions³. However, laboratory findings and implications should be interpreted with caution given that associations between meteorology and COVID-19 in real life settings are far more complex than in controlled laboratory environments. 

In cold and dry environments, the mucociliary clearance, i.e., the body’s ability to clear virus-containing particles once inhaled may be impaired¹. UV-B exposure can increase Vitamin D levels which might stimulate immune functions and antiviral defense mechanisms⁴. Acute and chronic exposure to air pollutants might lead to immune system dysregulation and inflammatory responses increasing the likelihood of cardiovascular and respiratory diseases. A significant fraction of COVID-19 infections lead to adverse long-term conditions affecting the heart, lungs and other organs which could be worsened by exposure to air pollution⁵.  

People tend to spend more time indoors when it is particularly hot, cold, or raining outside shifting transmission dynamics to indoor spaces. In addition, school closures can contribute to a reduction in SARS-CoV-2 transmission but are insufficient to prevent community transmission in the absence of other non-pharmaceutical interventions and increased vaccination coverage. Higher proportions of reported SARS-CoV-2 cases may be among children in the coming months considering the increasing percentage of adults being fully vaccinated against COVID-19. However, the exact influence that children may play in transmission scenarios and the short- and long-term burden of pediatric infections still need to be determined⁶. 

A combination of these mechanisms may modulate transmission dynamics highlighting the complex challenge of isolating one mechanistic pathway in epidemiological studies to understand its importance relative to the other drivers of transmission. At the start of the pandemic, the high proportion of susceptible individuals, limited spatial and temporal COVID-19 data and the impact of different types of non-pharmaceutical interventions adopted at different times after the onset of local outbreaks makes the detection of COVID-19 weather sensitivites a challenge⁷.  

In our study, we used a two-stage ecological modelling approach to estimate weather-dependent signatures in COVID-19 transmission in the early phase of the pandemic, using a dataset of 3 million COVID-19 cases reported until 31 May 2020, spanning 409 locations in 26 countries. We calculated the effective reproduction number (R_e) over a city-specific early-phase time-window of 10-20 days, for which local transmission had been established but before non-pharmaceutical interventions had intensified, as measured by the OxCGRT Government Response Index. We calculated mean levels of meteorological factors, including temperature and humidity observed in the same time window used to calculate R_e. Using a multilevel meta-regression approach, we modelled nonlinear effects of meteorological factors, while accounting for government interventions and socio-demographic factors.  

A weak non-monotonic association between temperature and R_e was identified, with a decrease of 0.087 (95% CI: 0.025; 0.148) for an increase in temperature between 10-20°C. Non-pharmaceutical interventions had a greater effect on R_e with a decrease of 0.285 (95% CI: 0.223; 0.347) for a 5th - 95th percentile increase in the government response index (Figure 3). The variation in the effective reproduction number explained by early government interventions was 6 times greater than for mean temperature. We find little evidence of meteorological conditions having influenced the early stages of local COVID-19 infection patterns and conclude that population behaviour and governmental intervention are more important drivers of transmission. 

Our findings are in line with statements of the First World Meteorological Organization’s COVID-19 Task Team Report published in March 2021 providing an assessment of the state of knowledge of associations between COVID-19 dynamics and meteorological and air quality factors. The report concludes that COVID-19 transmission patterns in 2020 appear to have been controlled primarily by government interventions rather than meteorological factors. Other relevant drivers include changes in human behaviour and demographics of affected populations and virus mutations⁷. 

Interdisciplinary research approaches will be needed to better understand COVID-19 seasonality and the potential interactions and implications of climate, weather, air quality and COVID-19 dynamics. From a health system perspective, the effects of COVID-19 restrictions on other respiratory viruses need to be considered to predict future scenarios, e.g., relaxed COVID-19 measures and increased human-to-human contacts might increase influenza incidence and overall hospital admissions in Northern Hemisphere winter. From a meteorological service perspective, lessons learned and analogies from co-developing infectious disease products such as dengue early warning systems could be used to co-produce useful COVID-19 related products and services with the health community⁸. From a disaster response perspective, the compound risk of climate extremes such as wildfires, floods and heat in combination with the COVID-19 pandemic needs to be addressed, with a focus on protecting vulnerable populations. Holistic approaches need to integrate the expertise and experiences of these different communities to understand the full picture of COVID-19 seasonality and derive necessary cross-sectoral action plans. 

References

1. Moriyama, M., Hugentobler, W. J. & Iwasaki, A. Seasonality of Respiratory Viral Infections. Annu. Rev. Virol. 7, 83–101 (2020).
2. Ratnesar-Shumate, S. et al. Simulated Sunlight Rapidly Inactivates SARS-CoV-2 on Surfaces. The Journal of Infectious Diseases 222, 214–222 (2020).
3. Dabisch, P. et al. The influence of temperature, humidity, and simulated sunlight on the infectivity of SARS-CoV-2 in aerosols. Aerosol Science and Technology 55, 142–153 (2021).
4. Cannell, J. J. et al. Epidemic influenza and vitamin D. Epidemiol. Infect. 134, 1129–1140 (2006).
5. Brunekreef, B. et al. Air pollution and COVID-19. Including elements of air pollution in rural areas, indoor air pollution and vulnerability and resilience aspects of our society against respiratory disease, social inequality stemming from air pollution, study for the committee on Environment, Public Health and Food Safety, Policy Department for Economic, Scientific and Quality of Life Policies, European Parliament, Luxembourg. (2021).
6. European Centre for Disease Prevention and Control. COVID-19 in children and the role of school settings in COVID-19 transmission - second update. Stockholm: ECDC (2021).
7. World Meteorological Organization. First Report of the WMO COVID-19 Task Team: Review on Meteorological and Air Quality Factors Affecting the COVID-19 Pandemic. (2021).
8. Lowe, R. et al. Building resilience to mosquito-borne diseases in the Caribbean. PLoS Biol 18, e3000791 (2020).