To the Editor,
The seasonal cycle of respiratory viral diseases is widely recognized.1 A typical seasonal pattern of annual influenza epidemics is observed during the winter season in the northern hemisphere and during the summer season in the southern hemisphere.1,2 This knowledge has suggested that the transmission of SARS-CoV-2 could follow a similar seasonal pattern. Hence, many studies have been conducted to understand the seasonal pattern of COVID-19 since the beginning of the pandemic.3,4 However, a common limitation is the short timeframe since COVID-19 had only been prevalent for less than a year. Consequently, previously reported seasonal patterns could be incomplete and spurious due to the short study period.4
The first case of SARS-CoV-2 virus infection in humans was reported in December 2019 in Wuhan, China, and expanded worldwide. It has now been a year since the World Health Organisation declared the COVID-19 pandemic on March 9, 2020, and data corresponding to a complete seasonal cycle is already available. We aim to provide a descriptive view of the SARS-CoV-2 seasonality globally, comparing patterns between geographical regions (eFigure 1; https://links.lww.com/EE/A132). We collected data on the weekly incident cases of COVID-19 between March 2020 and February 2021 from the European Centre for Disease Control including data from 214 countries (eTable 1; https://links.lww.com/EE/A132).5 We studied the seasonality of COVID-19 in each geographical region using Poisson regression models by fitting periodic functions of time6 and calculating the predicted weekly incidence for each area.
The first months of the pandemic were very haphazard in most countries, while some established immediate lockdowns, others took action too late or indecisively.7,8 This fact could make data less comparable at the early stages.9 Nevertheless, Figure 1 shows a consistent pattern in the European regions, reporting winter peaks between November and January. Similarly, some regions in the Northern hemisphere (Northern America, Northern Africa, Eastern, South-eastern, and Western Asia) also show a similar pattern. The seasonality in most regions in the Southern hemisphere and the Equator’s intertropical convergence zone show a bimodal distribution with winter peaks between June and August and a second peak in January, which could be related to the appearance of new and more transmissible variants.10 Elsewhere, winter peaks appear heterogeneous with a gradual shift from Southern Asia to Oceania.
In summary, although the seasonal pattern of COVID-19 appears to resemble other respiratory viruses, more time, and research is needed to establish its entirety. At this stage, the global changes in seasonality can be partially explained by the impact of public health interventions aiming to reduce the transmission of SARS-CoV-211 and potentially further complicated by the appearance of new variants.10 However, different seasonal patterns could also be found between countries in the same geographical region. We expect the seasonal cycle of SARS-CoV-2 might become more evident in subsequent years. However, it will be necessary to consider the impact that vaccination may have on the population protective immunity12 and any economic measures that could increase contact in the near future.
1. Moriyama M, Hugentobler WJ, Iwasaki A. Seasonality of respiratory viral infections. Annu Rev Virol. 2020; 7:83–101
2. Hemmes JH, Winkler KC, Kool SM. Virus survival as a seasonal factor in influenza and poliomylitis. Antonie Van Leeuwenhoek. 1962; 28:221–233
3. World Meteorological OrganisationVirtual Symposium on Climatological, Meteorological and Environmental (CME) Factors in the COVID-19 Pandemic. August 4–6, 2020. Available at: https://public.wmo.int/en/events/meetings/covid-19-symposium/outcomes
. Accessed 1 February 2021
4. Smit AJ, Fitchett JM, Engelbrecht FA, et al. Winter is coming: a southern hemisphere perspective of the environmental drivers of SARS-CoV-2 and the potential seasonality of COVID-19. Int J Environ Res Public Health. 2020; 17:5634
5. European Centre for Disease Prevention and ControlSituation updates on COVID-19. Download COVID-19 datasets. https://www.ecdc.europa.eu/en/covid-19/data
. Accessed 28 February 2021
6. Stolwijk AM, Straatman H, Zielhuis GA. Studying seasonality by using sine and cosine functions in regression analysis. J Epidemiol Community Health. 1999; 53:235–238
7. Han E, Tan MMJ, Turk E, et al. Lessons learnt from easing COVID-19 restrictions: an analysis of countries and regions in Asia Pacific and Europe. Lancet. 2020; 396:1525–1534
8. Gelfand MJ, Jackson JC, Pan X, et al. The relationship between cultural tightness-looseness and COVID-19 cases and deaths: a global analysis. Lancet Planet Health. 2021; 5:e135–e144
9. Pearce N, Lawlor DA, Brickley EB. Comparisons between countries are essential for the control of COVID-19. Int J Epidemiol. 2020; 49:1059–1062
10. Walker SA, Vihta KD, Gethings O, et al. Increased infections, but not viral burden, with a new SARS-CoV-2 variant. medRxiv. doi: 10.1101/2021.01.13.21249721
11. Baker RE, Park SW, Yang W, Vecchi GA, Metcalf CJE, Grenfell BT. The impact of COVID-19 nonpharmaceutical interventions on the future dynamics of endemic infections. Proc Natl Acad Sci U S A. 2020; 117:30547–30553
12. Dan JM, Mateus J, Kato Y, et al. Immunological memory to SARS-CoV-2 assessed for up to 8 months after infection. Science. 2021; 371:eabf4063