Key points are not available for this paper at this time.
The COVID-19 pandemic has generated global perturbations in our daily lives that are unprecedented in scale. Since early 2020, changes in contact patterns and mobility have affected the regular seasonal cycles of many infectious diseases globally, including influenza (Figure 1). A better understanding of the effect of these perturbations can shed light on key epidemiological mechanisms that remain unclear despite decades of research. These include the strength and mechanisms that drive seasonality in transmission, the persistence of immunity from natural infection, the evolutionary bottlenecks operating during low transmission seasons, and the impact of non-pharmaceutical interventions (NPI) that could be used in future influenza pandemics.On a global scale, influenza circulation was particularly low in 2020 as lockdown and travel-related quarantines were imposed in many countries. Influenza started to resurge in late 2021, marked by out-of-season activity in the Southern Hemisphere. In the first half of 2022, seasonal patterns had not returned to normal, with unusually late and protracted influenza seasons in the Northern Hemisphere, and early season activity in the Southern Hemisphere. In June 2022, a peak in weekly influenza cases was reported in Australia, dominated by the A/H3N2 subtype (Department of Health and Aged Care. Australian Government 2022Department of Health and Aged Care. Australian Government. Australian Influenza Surveillance Report and Activity Updates. https://www1.health.gov.au/internet/main/publishing.nsf/Content/cda-surveil-ozflu-flucurr.htm (Accessed 30 July 2022).Google Scholar), far exceeding their 5-year average and earlier than is typical. The upsurge of influenza occurred after the Omicron (B.1.1.529) wave peaked in January 2022, with reported co-circulation of SARS-CoV-2 and influenza A since that time. In South America, seasonal influenza arrived in Brazil in the odd summer months of November and December 2021 (Faico-Filho et al., 2022Faico-Filho KS Barbosa GR Bellei N. Peculiar H3N2 outbreak in São Paulo during summer and emergence of the Omicron variant.J Infect. 2022; 85: 90-122https://doi.org/10.1016/j.jinf.2022.04.007Abstract Full Text Full Text PDF PubMed Scopus (1) Google Scholar), between the Delta (B.1.617.2) and Omicron waves (Nott et al., 2022Nott R Fuller TL Brasil P Nielsen-Saines K. Out-of-season influenza during a COVID-19 void in the State of Rio de Janeiro, Brazil: temperature matters.Vaccines (Basel). 2022; 10: 821https://doi.org/10.3390/vaccines10050821Crossref PubMed Scopus (2) Google Scholar). As reported in the latest World Health Organization (WHO)’s update in July 2022 (World Health Organization 2022World Health OrganizationInfluenza Update no. 421. WHO, Geneva2022https://www.who.int/publications/m/item/influenza-update-n-421Google Scholar), influenza activity was decreasing in Argentina and Paraguay and increasing in Chile and Uruguay. South Africa experienced an unusually late influenza season in 2021, followed by a return to normal timing of activity in 2022, with both seasons dominated by A/H1N1. Overall, recent influenza activity in the temperate Southern Hemisphere is not occurring synchronously between countries nor with climatic drivers. What is to be expected of influenza activity patterns in the Northern Hemisphere in the coming months?Seasonality is a hallmark of influenza epidemiology in the inter-pandemic period, a complex phenomenon shaped by the interplay of population contact patterns, virus survival and host immunity (Tamerius et al., 2011Tamerius J Nelson MI Zhou SZ Viboud C Miller MA Alonso WJ. Global influenza seasonality: reconciling patterns across temperate and tropical regions.Environ Health Perspect. 2011; 119: 439-445https://doi.org/10.1289/ehp.1002383Crossref PubMed Scopus (308) Google Scholar). Environmental and climatic influences have been shown to play a role in influenza seasonality but do not fully explain spatio-temporal variability in the occurrence of seasonal outbreaks (Shaman et al., 2010Shaman J Pitzer VE Viboud C Grenfell BT Lipsitch M. Absolute humidity and the seasonal onset of influenza in the continental United States.PLoS Biol. 2010; 8e1000316https://doi.org/10.1371/journal.pbio.1000316Crossref PubMed Scopus (422) Google Scholar; Tamerius et al., 2013Tamerius JD Shaman J Alonso WJ Bloom-Feshbach K Uejio CK Comrie A et al.Environmental predictors of seasonal influenza epidemics across temperate and tropical climates.PLoS Pathog. 2013; 9e1003194https://doi.org/10.1371/journal.ppat.1003194Crossref PubMed Scopus (319) Google Scholar). During the interpandemic period, influenza seasons are well-synchronized in temperate climate zones of Northern and Southern Hemispheres and organized around their respective winters (Wenger and Naumova, 2010Wenger JB Naumova EN. Seasonal synchronization of influenza in the United States older adult population.PLoS One. 2010; 5: e10187https://doi.org/10.1371/journal.pone.0010187Crossref PubMed Scopus (46) Google Scholar; Lam et al., 2019Lam TT Tang JW Lai FY Zaraket H Dbaibo G Bialasiewicz S et al.Comparative global epidemiology of influenza, respiratory syncytial and parainfluenza viruses, 2010-2015.J Infect. 2019; 79: 373-382https://doi.org/10.1016/j.jinf.2019.07.008Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar; Morris et al., 2018Morris SE Freiesleben de Blasio B Viboud C Wesolowski A Bjørnstad ON et al.Analysis of multi-level spatial data reveals strong synchrony in seasonal influenza epidemics across Norway, Sweden, and Denmark.PLoS One. 2018; 13e0197519https://doi.org/10.1371/journal.pone.0197519Crossref Scopus (8) Google Scholar). Yet, peak timing can vary by up to 3-4 months depending on the season (Price et al., 2019Price RHM Graham C Ramalingam S. Association between viral seasonality and meteorological factors.Sci Rep. 2019; 9: 929https://doi.org/10.1038/s41598-018-37481-yCrossref PubMed Scopus (82) Google Scholar; Pica and Bouvier, 2012Pica N Bouvier NM. Environmental factors affecting the transmission of respiratory viruses.Curr Opin Virol. 2012; 2: 90-95https://doi.org/10.1016/j.coviro.2011.12.003Crossref PubMed Scopus (160) Google Scholar). In influenza pandemic seasons, which mark the emergence and dissemination of antigenically novel strains, large departures from regular seasonal cycles can occur. Out-of-season waves have been reported during the 1918 and 2009 influenza pandemics (Andreasen et al., 2008Andreasen V Viboud C Simonsen L. Epidemiologic characterization of the 1918 influenza pandemic summer wave in Copenhagen: implications for pandemic control strategies.J Infect Dis. 2008; 197: 270-278https://doi.org/10.1086/524065Crossref PubMed Scopus (191) Google Scholar; He et al., 2015He D Lui R Wang L Tse CK Yang L Stone L. Global Spatio-temporal Patterns of Influenza in the Post-pandemic Era.Sci Rep. 2015; 5: 11013https://doi.orfg/10.1038/srep11013Crossref PubMed Scopus (51) Google Scholar). These perturbations are typically limited to the first year of pandemic virus circulation. Of note, the evolutionary process generating new pandemic strains is unpredictable, involving cross-species transmission (for instance for swine to humans), a process which can theoretically occur in any season (Zimmer and Burke, 2009Zimmer SM Burke DS. Historical perspective–Emergence of influenza A (H1N1) viruses.N Engl J Med. 2009; 361: 279-285https://doi.org/10.1056/NEJMra0904322Crossref PubMed Scopus (289) Google Scholar).COVID-19 has generated more perturbation in influenza activity than prior influenza pandemics. When COVID-19 first emerged in 2020, little or no influenza activity was recorded in both the Northern (Chan et al., 2020Chan CP Wong NS Leung CC Lee SS. Positive impact of measures against COVID-19 on reducing influenza in the Northern Hemisphere.J Travel Med. 2020; (ePub 28 May 2020)https://doi.org/10.1093/jtm/taaa087Crossref Scopus (9) Google Scholar) and Southern Hemispheres (Kim et al., 2021Kim J Gómez Gómez RE Hong K Yum S Jang J Chun BC Changing influenza activity in the Southern Hemisphere countries during the COVID-19 pandemic.Int J Infect Dis. 2021; 108: 109-111https://doi.org/10.1016/j.ijid.2021.05.039Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar). The absence or abrupt subsidence of influenza circulation is likely the result of reduction of human mobility and contacts in response to COVID-19. While 2020 was a mostly silent year for influenza, the situation in 2021-2022 has been more difficult to interpret. The heterogeneity in magnitude and timing of influenza activity globally can be partly attributed to the diversity of COVID-19 control strategies. The roll-out of COVID-19 vaccines in 2021, with varied coverage between countries, age groups and time periods, resulted in decreased uptake for other vaccines (Maltezou et al., 2022Maltezou HC Medic S Cassimos DC Effraimidou E Poland GA. Decreasing routine vaccination rates in children in the COVID-19 era.Vaccine. 2022; 40: 2525-2527https://doi.org/10.1016/j.vaccine.2022.03.033Crossref PubMed Scopus (3) Google Scholar), though the global impact on influenza coverage has yet been fully evaluated. In the US for instance, COVID-19 vaccination has increased disparities in influenza vaccine coverage between states (Leuchter et al., 2022Leuchter RK Jackson NJ Mafi JN Sarkisian CA. Association between Covid-19 Vaccination and Influenza Vaccination Rates.N Engl J Med. 2022; 386: 2531-2532https://doi.org/10.1056/NEJMc2204560Crossref PubMed Scopus (4) Google Scholar). Further, NPI to mitigate COVID19 have been highly heterogeneous between countries, ranging from containment to achieve elimination on one end, to mitigation with strategic relaxation on the other. This would have likely increased heterogeneity in contacts between locations and in turn in permissiveness for influenza transmission (Oliu-Barton et al., 2022Oliu-Barton M Pradelski BSR Algan Y Baker MG Binagwaho A Dore GJ et al.Elimination versus mitigation of SARS-CoV-2 in the presence of effective vaccines.Lancet Glob Health. 2022; 10: e142-e147https://doi.org/10.1016/S2214-109X(21)00494-0Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar). With the ever-changing COVID-19 crisis as the backdrop, waning population immunity against influenza and reduced genetic diversity of circulating influenza viruses add to the unpredictability of future outbreaks, creating challenges in the development of control strategies (Dhanasekaran et al., 2022Dhanasekaran V Sullivan S Edwards KM Xie R Khvorov A Valkenburg SA et al.Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination.Nat Commun. 2022; 13: 1721https://doi.org/10.1038/s41467-022-29402-5Crossref PubMed Scopus (9) Google Scholar).Experimental data indicate that co-infection between influenza and SARS-CoV-2 viruses is a condition of concern, with co-infected mice progressing to severe disease (Achdout et al., 2021Achdout H Vitner EB Politi B Melamed S Yahalom-Ronen Y Tamir H et al.Increased lethality in influenza and SARS-CoV-2 coinfection is prevented by influenza immunity but not SARS-CoV-2 immunity.Nat Commun. 2021; 12: 5819https://doi.org/10.1038/s41467-021-26113-1Crossref PubMed Scopus (11) Google Scholar). The relentless occurrence of COVID-19 outbreaks fuelled by new variants, along with a return of influenza, could potentially increase the likelihood of influenza/SARS-CoV-2 infection. However, epidemiological studies in England and the USA have shown that influenza infection was associated with a lower risk of SARS-CoV-2 infection (Stowe et al., 2021Stowe J Tessier E Zhao H Guy R Muller-Pebody B Zambon M et al.Interactions between SARS-CoV-2 and influenza, and the impact of coinfection on disease severity: a test-negative design.Int J Epidemiol. 2021; 50: 1124-1133https://doi.org/10.1093/ije/dyab081Crossref PubMed Scopus (50) Google Scholar; et al., in SARS-CoV-2 are influenza virus and Virol. 2020; PubMed Scopus Google Scholar) though to risk of severe disease and (Stowe et al., 2021Stowe J Tessier E Zhao H Guy R Muller-Pebody B Zambon M et al.Interactions between SARS-CoV-2 and influenza, and the impact of coinfection on disease severity: a test-negative design.Int J Epidemiol. 2021; 50: 1124-1133https://doi.org/10.1093/ije/dyab081Crossref PubMed Scopus (50) Google Scholar). be between these viruses the risk of by the A phenomenon has been for influenza and or COVID-19 and et al., S C L R et impact the population of influenza and the 2019; Scopus Google Scholar), and remain the impact of a COVID-19 and influenza outbreak could on a Vaccination and could be for the of influenza in and these interventions the heterogeneous and of influenza circulation the the of influenza circulation the has a global control to be et al., S M A M K M et seasonality and vaccination timing in tropical and of and World Health PubMed Scopus Google Scholar), on vaccine on a The return of influenza activity in 2021-2022 up new for influenza seasonal patterns return to normal after the COVID-19 the timing of influenza vaccination be and influenza be with COVID-19 the for the of the vaccine be have on SARS-CoV-2 and influenza other respiratory This is a to studies and the interplay between seasonal contacts and infection. These studies include and of of the transmission of influenza on influenza/SARS-CoV-2 and of changes in population immunity be a in the COVID-19 crisis globally, which is to new on the epidemiology of other and new for The COVID-19 pandemic has generated global perturbations in our daily lives that are unprecedented in scale. Since early 2020, changes in contact patterns and mobility have affected the regular seasonal cycles of many infectious diseases globally, including influenza (Figure 1). A better understanding of the effect of these perturbations can shed light on key epidemiological mechanisms that remain unclear despite decades of research. These include the strength and mechanisms that drive seasonality in transmission, the persistence of immunity from natural infection, the evolutionary bottlenecks operating during low transmission seasons, and the impact of non-pharmaceutical interventions (NPI) that could be used in future influenza pandemics. a global scale, influenza circulation was particularly low in 2020 as lockdown and travel-related quarantines were imposed in many countries. Influenza started to resurge in late 2021, marked by out-of-season activity in the Southern Hemisphere. In the first half of 2022, seasonal patterns had not returned to normal, with unusually late and protracted influenza seasons in the Northern Hemisphere, and early season activity in the Southern Hemisphere. In June 2022, a peak in weekly influenza cases was reported in Australia, dominated by the A/H3N2 subtype (Department of Health and Aged Care. Australian Government 2022Department of Health and Aged Care. Australian Government. Australian Influenza Surveillance Report and Activity Updates. https://www1.health.gov.au/internet/main/publishing.nsf/Content/cda-surveil-ozflu-flucurr.htm (Accessed 30 July 2022).Google Scholar), far exceeding their 5-year average and earlier than is typical. The upsurge of influenza occurred after the Omicron (B.1.1.529) wave peaked in January 2022, with reported co-circulation of SARS-CoV-2 and influenza A since that time. In South America, seasonal influenza arrived in Brazil in the odd summer months of November and December 2021 (Faico-Filho et al., 2022Faico-Filho KS Barbosa GR Bellei N. Peculiar H3N2 outbreak in São Paulo during summer and emergence of the Omicron variant.J Infect. 2022; 85: 90-122https://doi.org/10.1016/j.jinf.2022.04.007Abstract Full Text Full Text PDF PubMed Scopus (1) Google Scholar), between the Delta (B.1.617.2) and Omicron waves (Nott et al., 2022Nott R Fuller TL Brasil P Nielsen-Saines K. Out-of-season influenza during a COVID-19 void in the State of Rio de Janeiro, Brazil: temperature matters.Vaccines (Basel). 2022; 10: 821https://doi.org/10.3390/vaccines10050821Crossref PubMed Scopus (2) Google Scholar). As reported in the latest World Health Organization (WHO)’s update in July 2022 (World Health Organization 2022World Health OrganizationInfluenza Update no. 421. WHO, Geneva2022https://www.who.int/publications/m/item/influenza-update-n-421Google Scholar), influenza activity was decreasing in Argentina and Paraguay and increasing in Chile and Uruguay. South Africa experienced an unusually late influenza season in 2021, followed by a return to normal timing of activity in 2022, with both seasons dominated by A/H1N1. Overall, recent influenza activity in the temperate Southern Hemisphere is not occurring synchronously between countries nor with climatic drivers. What is to be expected of influenza activity patterns in the Northern Hemisphere in the coming is a hallmark of influenza epidemiology in the inter-pandemic period, a complex phenomenon shaped by the interplay of population contact patterns, virus survival and host immunity (Tamerius et al., 2011Tamerius J Nelson MI Zhou SZ Viboud C Miller MA Alonso WJ. Global influenza seasonality: reconciling patterns across temperate and tropical regions.Environ Health Perspect. 2011; 119: 439-445https://doi.org/10.1289/ehp.1002383Crossref PubMed Scopus (308) Google Scholar). Environmental and climatic influences have been shown to play a role in influenza seasonality but do not fully explain spatio-temporal variability in the occurrence of seasonal outbreaks (Shaman et al., 2010Shaman J Pitzer VE Viboud C Grenfell BT Lipsitch M. Absolute humidity and the seasonal onset of influenza in the continental United States.PLoS Biol. 2010; 8e1000316https://doi.org/10.1371/journal.pbio.1000316Crossref PubMed Scopus (422) Google Scholar; Tamerius et al., 2013Tamerius JD Shaman J Alonso WJ Bloom-Feshbach K Uejio CK Comrie A et al.Environmental predictors of seasonal influenza epidemics across temperate and tropical climates.PLoS Pathog. 2013; 9e1003194https://doi.org/10.1371/journal.ppat.1003194Crossref PubMed Scopus (319) Google Scholar). During the interpandemic period, influenza seasons are well-synchronized in temperate climate zones of Northern and Southern Hemispheres and organized around their respective winters (Wenger and Naumova, 2010Wenger JB Naumova EN. Seasonal synchronization of influenza in the United States older adult population.PLoS One. 2010; 5: e10187https://doi.org/10.1371/journal.pone.0010187Crossref PubMed Scopus (46) Google Scholar; Lam et al., 2019Lam TT Tang JW Lai FY Zaraket H Dbaibo G Bialasiewicz S et al.Comparative global epidemiology of influenza, respiratory syncytial and parainfluenza viruses, 2010-2015.J Infect. 2019; 79: 373-382https://doi.org/10.1016/j.jinf.2019.07.008Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar; Morris et al., 2018Morris SE Freiesleben de Blasio B Viboud C Wesolowski A Bjørnstad ON et al.Analysis of multi-level spatial data reveals strong synchrony in seasonal influenza epidemics across Norway, Sweden, and Denmark.PLoS One. 2018; 13e0197519https://doi.org/10.1371/journal.pone.0197519Crossref Scopus (8) Google Scholar). Yet, peak timing can vary by up to 3-4 months depending on the season (Price et al., 2019Price RHM Graham C Ramalingam S. Association between viral seasonality and meteorological factors.Sci Rep. 2019; 9: 929https://doi.org/10.1038/s41598-018-37481-yCrossref PubMed Scopus (82) Google Scholar; Pica and Bouvier, 2012Pica N Bouvier NM. Environmental factors affecting the transmission of respiratory viruses.Curr Opin Virol. 2012; 2: 90-95https://doi.org/10.1016/j.coviro.2011.12.003Crossref PubMed Scopus (160) Google Scholar). In influenza pandemic seasons, which mark the emergence and dissemination of antigenically novel strains, large departures from regular seasonal cycles can occur. Out-of-season waves have been reported during the 1918 and 2009 influenza pandemics (Andreasen et al., 2008Andreasen V Viboud C Simonsen L. Epidemiologic characterization of the 1918 influenza pandemic summer wave in Copenhagen: implications for pandemic control strategies.J Infect Dis. 2008; 197: 270-278https://doi.org/10.1086/524065Crossref PubMed Scopus (191) Google Scholar; He et al., 2015He D Lui R Wang L Tse CK Yang L Stone L. Global Spatio-temporal Patterns of Influenza in the Post-pandemic Era.Sci Rep. 2015; 5: 11013https://doi.orfg/10.1038/srep11013Crossref PubMed Scopus (51) Google Scholar). These perturbations are typically limited to the first year of pandemic virus circulation. Of note, the evolutionary process generating new pandemic strains is unpredictable, involving cross-species transmission (for instance for swine to humans), a process which can theoretically occur in any season (Zimmer and Burke, 2009Zimmer SM Burke DS. Historical perspective–Emergence of influenza A (H1N1) viruses.N Engl J Med. 2009; 361: 279-285https://doi.org/10.1056/NEJMra0904322Crossref PubMed Scopus (289) Google Scholar). COVID-19 has generated more perturbation in influenza activity than prior influenza pandemics. When COVID-19 first emerged in 2020, little or no influenza activity was recorded in both the Northern (Chan et al., 2020Chan CP Wong NS Leung CC Lee SS. Positive impact of measures against COVID-19 on reducing influenza in the Northern Hemisphere.J Travel Med. 2020; (ePub 28 May 2020)https://doi.org/10.1093/jtm/taaa087Crossref Scopus (9) Google Scholar) and Southern Hemispheres (Kim et al., 2021Kim J Gómez Gómez RE Hong K Yum S Jang J Chun BC Changing influenza activity in the Southern Hemisphere countries during the COVID-19 pandemic.Int J Infect Dis. 2021; 108: 109-111https://doi.org/10.1016/j.ijid.2021.05.039Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar). The absence or abrupt subsidence of influenza circulation is likely the result of reduction of human mobility and contacts in response to COVID-19. While 2020 was a mostly silent year for influenza, the situation in 2021-2022 has been more difficult to interpret. The heterogeneity in magnitude and timing of influenza activity globally can be partly attributed to the diversity of COVID-19 control strategies. The roll-out of COVID-19 vaccines in 2021, with varied coverage between countries, age groups and time periods, resulted in decreased uptake for other vaccines (Maltezou et al., 2022Maltezou HC Medic S Cassimos DC Effraimidou E Poland GA. Decreasing routine vaccination rates in children in the COVID-19 era.Vaccine. 2022; 40: 2525-2527https://doi.org/10.1016/j.vaccine.2022.03.033Crossref PubMed Scopus (3) Google Scholar), though the global impact on influenza coverage has yet been fully evaluated. In the US for instance, COVID-19 vaccination has increased disparities in influenza vaccine coverage between states (Leuchter et al., 2022Leuchter RK Jackson NJ Mafi JN Sarkisian CA. Association between Covid-19 Vaccination and Influenza Vaccination Rates.N Engl J Med. 2022; 386: 2531-2532https://doi.org/10.1056/NEJMc2204560Crossref PubMed Scopus (4) Google Scholar). Further, NPI to mitigate COVID19 have been highly heterogeneous between countries, ranging from containment to achieve elimination on one end, to mitigation with strategic relaxation on the other. This would have likely increased heterogeneity in contacts between locations and in turn in permissiveness for influenza transmission (Oliu-Barton et al., 2022Oliu-Barton M Pradelski BSR Algan Y Baker MG Binagwaho A Dore GJ et al.Elimination versus mitigation of SARS-CoV-2 in the presence of effective vaccines.Lancet Glob Health. 2022; 10: e142-e147https://doi.org/10.1016/S2214-109X(21)00494-0Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar). With the ever-changing COVID-19 crisis as the backdrop, waning population immunity against influenza and reduced genetic diversity of circulating influenza viruses add to the unpredictability of future outbreaks, creating challenges in the development of control strategies (Dhanasekaran et al., 2022Dhanasekaran V Sullivan S Edwards KM Xie R Khvorov A Valkenburg SA et al.Human seasonal influenza under COVID-19 and the potential consequences of influenza lineage elimination.Nat Commun. 2022; 13: 1721https://doi.org/10.1038/s41467-022-29402-5Crossref PubMed Scopus (9) Google Scholar). data indicate that co-infection between influenza and SARS-CoV-2 viruses is a condition of concern, with co-infected mice progressing to severe disease (Achdout et al., 2021Achdout H Vitner EB Politi B Melamed S Yahalom-Ronen Y Tamir H et al.Increased lethality in influenza and SARS-CoV-2 coinfection is prevented by influenza immunity but not SARS-CoV-2 immunity.Nat Commun. 2021; 12: 5819https://doi.org/10.1038/s41467-021-26113-1Crossref PubMed Scopus (11) Google Scholar). The relentless occurrence of COVID-19 outbreaks fuelled by new variants, along with a return of influenza, could potentially increase the likelihood of influenza/SARS-CoV-2 infection. However, epidemiological studies in England and the USA have shown that influenza infection was associated with a lower risk of SARS-CoV-2 infection (Stowe et al., 2021Stowe J Tessier E Zhao H Guy R Muller-Pebody B Zambon M et al.Interactions between SARS-CoV-2 and influenza, and the impact of coinfection on disease severity: a test-negative design.Int J Epidemiol. 2021; 50: 1124-1133https://doi.org/10.1093/ije/dyab081Crossref PubMed Scopus (50) Google Scholar; et al., in SARS-CoV-2 are influenza virus and Virol. 2020; PubMed Scopus Google Scholar) though to risk of severe disease and (Stowe et al., 2021Stowe J Tessier E Zhao H Guy R Muller-Pebody B Zambon M et al.Interactions between SARS-CoV-2 and influenza, and the impact of coinfection on disease severity: a test-negative design.Int J Epidemiol. 2021; 50: 1124-1133https://doi.org/10.1093/ije/dyab081Crossref PubMed Scopus (50) Google Scholar). be between these viruses the risk of by the A phenomenon has been for influenza and or COVID-19 and et al., S C L R et impact the population of influenza and the 2019; Scopus Google Scholar), and remain the impact of a COVID-19 and influenza outbreak could on a Vaccination and could be for the of influenza in and these interventions the heterogeneous and of influenza circulation the the of influenza circulation the has a global control to be et al., S M A M K M et seasonality and vaccination timing in tropical and of and World Health PubMed Scopus Google Scholar), on vaccine on a The return of influenza activity in 2021-2022 up new for influenza seasonal patterns return to normal after the COVID-19 the timing of influenza vaccination be and influenza be with COVID-19 the for the of the vaccine be have on SARS-CoV-2 and influenza other respiratory This is a to studies and the interplay between seasonal contacts and infection. These studies include and of of the transmission of influenza on influenza/SARS-CoV-2 and of changes in population immunity be a in the COVID-19 crisis globally, which is to new on the epidemiology of other and new for
Lee et al. (Thu,) studied this question.