Resurgence of Respiratory Syncytial Virus in Children: An Out-of-Season Epidemic in Portugal


  • Ana Rita Torres Departamento de Epidemiologia. Instituto Nacional de Saúde Doutor Ricardo Jorge. Lisboa.
  • Raquel Guiomar Departamento de Doenças Infeciosas. Instituto Nacional de Saúde Doutor Ricardo Jorge. Lisboa.
  • Nuno Verdasca Departamento de Doenças Infeciosas. Instituto Nacional de Saúde Doutor Ricardo Jorge. Lisboa.
  • Aryse Melo Departamento de Doenças Infeciosas. Instituto Nacional de Saúde Doutor Ricardo Jorge. Lisboa.
  • Ana Paula Rodrigues Departamento de Epidemiologia. Instituto Nacional de Saúde Doutor Ricardo Jorge. Lisboa.
  • on behalf of Rede Portuguesa de Laboratórios para o Diagnóstico da Gripe



Child, Palivizumab; Respiratory Syncytial Virus Infections/epidemiology, Respiratory Syncytial Virus Infections/prevention and control, Respiratory Syncytial Virus, Human


Introduction: An out-of-season increase in respiratory syncytial virus (RSV) incidence was observed in Portugal from June 2021 onwards, revealing a continuing surge in cases throughout 2021/2022 autumn/winter. We aimed to describe this out-of-season epidemic and define its epidemic period, by analysing RSV incidence from week 40 of 2020 (2020-W40) to week 18 of 2022 (2022-W18).
Material and Methods: Surveillance data on weekly RSV laboratory confirmed cases, in Portugal, was used to monitor RSV incidence using CUSUM test methodology for count data.
Results: In 2021-W23, the CUSUM score identified a significant increase in the risk of RSV. By that time, the percentage of RSV positive tests rose from 1% in 2021-W22 (3/265) to 6% in 2021-W23 (18/298). Despite a sharp decrease in RSV incidence on 2021-W33 and on 2022-W02, the CUSUM score stayed over the limit up to 2022-W07, indicating that the RSV activity remained at an epidemic level. Distinct peaks of RSV cases were observed between 2021-W30 and 2021-W32 (average of 77 RSV cases per week) and between 2021-W39 and 2021-W41 (average of 79 RSV cases per week) with positivity rates around 60%.
Conclusion: An out-of-season RSV epidemic was identified, with a longer epidemic period compared with previous seasons. Possible reasons include relaxation of COVID-19 physical distancing measures and a greater proportion of population susceptible to disease. As several factors may change the pattern of RSV activity, countries should implement year-round surveillance RSV surveillance systems. These findings might have an impact on public health planning regarding future RSV surges, namely, on the palivizumab prophylaxis period for high-risk infants.


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Author Biography

on behalf of Rede Portuguesa de Laboratórios para o Diagnóstico da Gripe

Rede Portuguesa de Laboratórios para o Diagnóstico da Gripe:
Luísa MOTA-VIEIRA, Raquel MONIZ, Eliana COSTA, Carlos CALDAS, Rita CÔRTE-REAL, Sofia SANTOS, Adriana PEDROSA, Ana Cristina SILVA, Fátima VALE, Rita TAVARES, Mário CUNHA, Luís MARTINS, Aurélio MESQUITA, Fernando BRANCA, Fernando FONSECA, Nádia MARTINS, Sandra REBELO, Filipa BAZENGA, Cristina TOSCANO, Ana Paula DIAS, José ALVES, Ludivina FREITAS, Joana LEAL, Jorge LOUREIRO, José António FREITAS, Sandra Margarida VIEIRA, Maria João PERES, Maria Regina VISEU, Paula MOTA, Ana FERREIRA, Maria do Rosário BARRETO, Teresa Maria MEIRA, Raquel MARQUES, João LEBRE, Paulo PEREIRA, Ana Rita COUTO, Jacome BRUGES-ARMAS


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How to Cite

Torres AR, Guiomar RG, Verdasca N, Melo A, Rodrigues AP, on behalf of Laboratórios para o Diagnóstico da Gripe. Resurgence of Respiratory Syncytial Virus in Children: An Out-of-Season Epidemic in Portugal. Acta Med Port [Internet]. 2023 Jan. 27 [cited 2023 Mar. 29];. Available from: