Why Do Epidemics Evolve in Waves?

Year 2025, Volume: 8 Issue: 2, 929 - 941, 12.03.2025

Ayse Peker-dobie , Semra Ahmetolan , Ayşe Hümeyra Bilge , Ali Demirci

https://doi.org/10.47495/okufbed.1525498

Abstract

Multiple epidemic waves have been observed during the “Spanish Flu” (1918-1919) and the Covid-19 (2019-2021) pandemics. The “Spanish Flu” pandemic, characterized by the H1N1 viral strain, was a severe and well documented pandemic that manifested itself in three distinct epidemic waves spanning a period of 18 months. The Covid-19 pandemic has also been characterized by multiple epidemic waves and the its data also include information on the variants of the ancestor virus. In this study, potential factors contributing to the occurrence of multiple waves are discussed by employing the Susceptible- Infected- Removed SIR model. These factors may include seasonality effects and relaxation of control measures. The introduction of a new variant of a pathogen can initiate a new wave, representing a distinct epidemic event. Also, at the end of an epidemic wave, the relaxation of restrictions allows previously protected individuals to re-enter the susceptible population, leading to an increase in susceptible individuals (S). Finally, the easing of restrictions promotes higher interconnections within the susceptible population, resulting in an increase in the basic reproduction number (R_0). By observing the simulations using the Susceptible-Infected-Removed model, we can conclude that the introduction of a new variant seems to be more dominant among causes leading to a new wave.

Keywords

Spanish Flu, COVID-19, Epidemic waves, SIR model, Basic reproduction number, Contact rate

Salgınlar Neden Dalgalar Halinde İlerler?

Abstract

“İspanyol Gribi” (1918-1919) ve Covid-19 (2019-2021) pandemileri sırasında birden fazla salgın dalgası gözlemlenmiştir. H1N1 viral suşu ile karakterize edilen “İspanyol Gribi”, 18 aylık bir döneme yayılan ve üç farklı salgın dalgası ile kendini gösteren şiddetli ve iyi belgelenmiş bir salgındı. Covid-19 pandemisi de çoklu dalgalar ile karakterize edilmiş olup, salgın verisi ata virisün varyantları bilgisini de içermektedir. Bu çalışmada, birden fazla dalganın ortaya çıkmasına yol açan potansiyel faktörler, mevsimsellik etkileri ve kontrol önlemlerinin gevşetilmesi olabilir. Patojenin yeni bir varyantının ortaya çıkışı, yeni bir dalga başlatarak farklı bir salgın başlatabilir. Ayrıca, bir salgın dalgasının sonunda, kısıtlamaların gevşetilmesi, daha önce korunan bireylerin yeniden duyarlı nüfusa girmesine ve duyarlı bireylerin (S) artmasına yol açabilir. Son olarak, kısıtlamaların gevşetilmesi, duyarlı nüfus içinde daha yüksek bağlantılar teşvik ederek temel üreme sayısının (R_0) artmasına neden olabilir. Duyarlı-Enfekte-İyileşen modeli kullanılarak yapılan simülasyonlar göz önüne alındığında, yeni bir varyantın ortaya çıkışının, yeni bir dalgaya yol açan nedenler arasında daha baskın olduğu ihtimali tartışılmıştır.

Keywords

İspanyol Gribi, COVID-19, Epidemi dalgaları, SIR modeli, Temel üreme sayısı, Temas oranı

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