Shifting Patterns of Influenza Circulation during the COVID-19 Pandemic, Senegal

Abstract

Historically low levels of seasonal influenza circulation were reported during the first years of the COVID-19 pandemic and were mainly attributed to implementation of nonpharmaceutical interventions. In tropical regions, influenza's seasonality differs largely, and data on this topic are scarce. We analyzed data from Senegal's sentinel syndromic surveillance network before and after the start of the COVID-19 pandemic to assess changes in influenza circulation. We found that influenza shows year-round circulation in Senegal and has 2 distinct epidemic peaks: during January-March and during the rainy season in August-October. During 2021-2022, the expected January-March influenza peak completely disappeared, corresponding to periods of active SARS-CoV-2 circulation. We noted an unexpected influenza epidemic peak during May-July 2022. The observed reciprocal circulation of SARS-CoV-2 and influenza suggests that factors such as viral interference might be at play and should be further investigated in tropical settings.

Keywords: Africa; COVID-19; Influenza; SARS; SARS-CoV-2; Senegal; coinfection; coronavirus disease; epidemiology; respiratory infections; seasonality; severe acute respiratory syndrome coronavirus 2; surveillance; viral interference; viruses.

Figure 1

Geographic distribution of community and hospital sentinel sites participating in surveillance for shifting patterns of influenza circulation during the COVID-19 pandemic, Senegal. Sites represent the network of sentinelle syndromique du Sénégal (sentinel syndromic surveillance of Senegal), also known as the 4S Network. Enlarged map at left shows detailed view of the Dakar capital region and 4S Network hospitals located in the region. ILI, influenza-like illness; SARI, severe acute respiratory illness.

Figure 2

Prepandemic average epidemic curves used to demonstrate shifting patterns of influenza circulation during the COVID-19 pandemic, Senegal. Graphs show annual and overall average percentage of influenza-positive reverse transcription PCR tests per epidemiology week reported by the sentinelle syndromique du Sénégal (sentinel syndromic surveillance of Senegal), also known as the 4S Network, during January 2013–January 2020.

Figure 3

Average number of cases detected in a study of shifting patterns of influenza circulation during the COVID-19 pandemic, Senegal. Bars indicate number of reverse transcription PCR–positive tests for influenza and SARS-CoV-2 per epidemiology week reported by the sentinelle syndromique du Sénégal (sentinel syndromic surveillance of Senegal), also known as the 4S Network, during January 2013–July.

Figure 4

Average epidemic curves showing shifting patterns of influenza circulation during the COVID-19 pandemic, Senegal. Graphs show percentage of influenza-positive reverse transcription PCR tests per epidemiologic week reported by the sentinelle syndromique du Sénégal (sentinel syndromic surveillance of Senegal), also known as the 4S Network, during January 2020–December 2022.

Figure 5

Number of reverse transcription PCR (RT-PCR)–positive samples per week in a study of shifting patterns of influenza circulation during the COVID-19 pandemic, Senegal. Data represent RT-PCR–positive tests per epidemiologic week reported by the sentinelle syndromique du Sénégal (sentinel syndromic surveillance of Senegal), also known as the 4S Network, including influenza subtypes and SARS-CoV-2 variants. A) Weekly influenza incidence during the prepandemic period, January 2018–2019. B) Weekly influenza and SARS-CoV-2 incidence during the pandemic period, January 2020–July 2022.