Seasonality of Infectious Diseases
Seasonality has been recognized as a major feature of numerous infectious diseases of public health importance. Many of us are already familiar with periodic outbreaks of influenza in winter, chickenpox in the spring, and the once much-feared polio in the summer. Indeed, researchers have shown that some infectious diseases exhibit seasonal patterns, which result in periodic increases in disease incidence that overlap with the seasons or other annual conditions. Although the exact mechanisms underlying the seasonality of many infectious diseases remain largely unclear, recent studies have found evidence suggesting that each acute infectious disease may have its own window of occurrence throughout the year. Alongside acute infectious diseases, findings have demonstrated that outbreaks of chronic infectious diseases may also exhibit a degree of seasonality.
According to World Health Organization (WHO), despite the advancements in vaccines, therapeutics, and infection management strategies, infectious diseases such as lower respiratory infections, diarrheal diseases, and tuberculosis were among the leading causes of mortality worldwide in 2016. Improving our understanding of the seasonality characterizing infectious diseases may create consequential benefits for public health soon. Read along to learn more about disease seasonality, its potential causes, its implications for the COVID-19 pandemic, and its significance for public health.
What is seasonality?
According to Fisman (2007), seasonality refers to the periodic surge in disease incidence corresponding to seasons or other calendar periods. Seasonality is a widespread and even potentially unifying feature of acute and epidemic-prone diseases. In fact, a study by Martinez (2018) explores the concept of a local epidemic calendar, which may be marked by epidemics in the absence of control measures. Based on data collected from the World Health Organization (WHO), the US Centers for Disease Control and Prevention (CDC), and the European Centre for Disease Prevention and Control (ECDC) covering 69 infectious diseases, Martinez (2018) has demonstrated that at least 68 of these infectious diseases possess a seasonal character. Moreover, the study has determined that like acute infectious diseases, some chronic infectious diseases such as hepatitis B and HIV/AIDS also occur at greater regularity during certain periods of the year.
However, the study also illustrates significant variation among the seasonality of disease incidence based on geographical location. For instance, Martinez notes that Hepatitis C peaks during winter in India, and during spring or summer in Egypt, China, and Mexico. Conversely, while dry seasons are associated with Guinea worm disease and Lassa fever in Nigeria, this period is linked to Hepatitis A in Brazil.
What are the potential drivers of seasonality?
The underlying dynamics shaping seasonality is poorly investigated and understood for the majority of infectious diseases affecting public health. Still, many studies have pointed out to the relationships between the pathogen, the environment, and human behavior in an effort to explain the seasonality of different diseases. According to Martinez (2018), the four seasonal drivers of seasonality is environmental factors, host behavior, host phenology, and exogenous biotic factors. Environmental factors include climate conditions such as temperature, rain, and humidity which can affect both hosts and pathogens. Host behavior refers to human activity, which may result in different levels of contact and transmission throughout the year. Host phenology, on the other hand, covers host life history, annual cycles, and endogenous circannual rhythms, which may alter the physiology of the host. Finally, exogenous biotic factors include the interactions occurring within hosts and among hosts, reservoirs, and vectors.
Accordingly, while the underlying dynamics of seasonality is further complicated by confounding variables, it is possible that influenza, for instance, might peak in winter due to a combination of various factors such as humidity, temperature, elevated levels of close contact among people, and reduction in vitamin D levels. For diseases such as chikungunya and dengue fever, the seasonality can be largely explained by the elevation in the numbers of vectors during the rainy seasons.
For viruses, on the other hand, studies underline the impact of environmental conditions and viability outside the human body. According to Ramalingam (2019), especially enveloped viruses have been found to exhibit a highly definite seasonality due to their vulnerability to adverse conditions. Accordingly, while enveloped viruses such as RSV, influenza, and human metapneumovirus peak during the winter months and circulate for around three months each year, non-enveloped viruses such as rhinoviruses and adenoviruses have been found to circulate for over half the year.
What are the implications of seasonality for the COVID-19 pandemic?
Although it is an enveloped virus, there is currently no decisive findings on whether SARS-CoV-2 might mimic influenza, RSV, and other enveloped viruses in terms of seasonality. Although some of its siblings such as SARS and MERS have not circulated long enough for a seasonal cycle to be observed, most of the human coronaviruses which cause respiratory disease were found to exhibit a winter seasonality. Still, experts warn that seasonal forcing is shaped by many factors, and even if SARS-CoV-2 exhibits a seasonal decline, it may persist for a long time by infecting susceptible people.
What is the importance of seasonality for public health?
Beyond the pressing question of what to expect with the ongoing pandemic, a better understanding of seasonality may inform disease surveillance, epidemiological predictions, and the optimal timing for vaccination campaigns. Sufficient and reliable data on the underlying mechanisms promoting and restricting the infectious diseases may support public health officials in the development of more efficient and innovative interventions. It can also be operationalized for the development of more precise and extensive predictions and models ahead of anticipated epidemics and disease seasons. Finally, a deeper understanding of disease seasonality could be help public health officials determine the optimal timing for vaccination campaigns and other preventive measures in order to ensure maximum immunity before outbreaks.
- Fisman, D. N. (2007, April 1). Seasonality of Infectious Diseases. Annual Review of Public Health, 28(1), 127–143. https://doi.org/10.1146/annurev.publhealth.28.021406.144128
- Kandeel, A., Dawson, P., Labib, M., Said, M., El-Refai, S., El-Gohari, A., & Talaat, M. (2016, September 8). Morbidity, Mortality, and Seasonality of Influenza Hospitalizations in Egypt, November 2007-November 2014. PLOS ONE, 11(9), e0161301. https://doi.org/10.1371/journal.pone.0161301
- Martinez, M. E. (2018, November 8). The calendar of epidemics: Seasonal cycles of infectious diseases. PLOS Pathogens, 14(11), e1007327. https://doi.org/10.1371/journal.ppat.1007327
- Price, R. H. M., Graham, C., & Ramalingam, S. (2019, January 30). Association between viral seasonality and meteorological factors. Scientific Reports, 9(1). https://doi.org/10.1038/s41598-018-37481-y
- Zhang, X., Zhang, T., Young, A. A., & Li, X. (2014, February 5). Applications and Comparisons of Four Time Series Models in Epidemiological Surveillance Data. PLoS ONE, 9(2), e88075. https://doi.org/10.1371/journal.pone.0088075