The emergence of SARS-CoV-2 was followed by public health restrictions to inhibit the spread and limit the quantity of severe outcomes from COVID-19 disease. With possible knock-on effects of these interventions on the transmission of other respiratory viral infections, the situation posed questions as to the effects on upcoming influenza seasons. We have outlined our use of a previously developed seasonal influenza transmission model, calibrated to historical influenza seasons in England [10], to quantify the epidemiological implications on seasonal influenza (in England) of modified social interaction patterns and a broadened vaccination programme. We conducted these analyses prior to the 2020/2021 and 2021/2022 influenza seasons, respectively, contributing to the evidence base being compiled by scientific advisory groups in the UK that were aiding the COVID-19 pandemic response.

We forecasted that the combined interaction of NPIs (targeted at reducing COVID-19 transmission but also impacting influenza transmission) and a wide-spread and intensive vaccination campaign was likely to substantially reduce the scale of seasonal influenza epidemics over the winter - not only in terms of raw cases, but also in terms of hospitalisations and mortality. The combined action of NPIs and extended vaccination we anticipated may reduce influenza by over 95%. For the lead in to the 2020/2021 influenza season, with the intended coverage of the vaccination programme and interventions in place at the time the modelling suggesting that seasonal influenza was likely to pose a low burden to the health services was then borne out in England [8] and other regions in the temperate Northern Hemisphere [13, 14]; in the 2020/2021 influenza season the European Centre for Disease Prevention and Control stated that there were no hospitalised cases of influenza or fatalities from influenza reported by EU (European Union)/EEA (European Economic Area) countries [13], whilst in the USA the CDC reported that the cumulative rate of laboratory-confirmed influenza-associated hospitalisations was the lowest recorded since that type of data collection began [14].

Although a dramatic drop in harms from influenza was predicted in all ages, those aged 2-17 were predicted to experience the lowest decline. Consequently, it was suggested that any spare vaccine capacity (after vaccinating the eldest and those at-risk of complications if infected) should be targeted towards school-aged children; in the context of England, supplementing the current primary-school programme with vaccination in secondary schools.

Our result suggesting a compensatory influenza season in 2021/2022 due to a suppressed influenza season in 2020/2021 has also been demonstrated in other mathematical model frameworks. One such example was by Krauland et al. [15], who used an agent-based model and also investigated the relationship between the potential magnitude of a resurgent influenza epidemic to cross-immunity from past infection and transmissibility of strains. In another example, applied to the United States, Lee et al. [16] used a more parsimonious, non-age structured compartmental influenza transmission model that included immunity propagation. They also showed that higher influenza vaccine uptake would reduce the projected resurgence in influenza.

Public health interventions against SARS-CoV-2 have had a profound impact on the worldwide circulation of other respiratory viruses, including seasonal influenza [17]. As a result of non-pharmaceutical interventions in place for COVID-19 (such as mask-wearing, reduced social interactions and international travel) influenza activity levels were extremely low globally in 2020 to 2021 [18]. In the temperate Northern Hemisphere, during the warmer summer months of 2022 when rates of respiratory viral infections would normally be very low or absent, some countries saw unusual seasonal changes in the prevalence of influenza [19], respiratory syncytial virus (RSV) [20] and parainfluenza [21]; these observations coincided with a reduction in COVID-19 mitigation measures. In England, there has been indications of an early onset of the 2022/2023 influenza season, with the hospital admission rate for confirmed cases of influenza in mid-October being greater in 2022 than seen in preceding years [22].

Looking ahead, there is uncertainty as to the size of future seasonal influenza epidemics. The potential elimination of the B/Yamagata lineage [23] may have as yet unforeseen ramifications on the evolution of influenza strains and the multi-strain transmission dynamics. A multi-nation analysis by Ali et al. [24], using a data-driven mechanistic predictive modelling framework, suggested the potential for substantial increases in infection burden in upcoming influenza seasons worldwide. Meanwhile, results by Felix Garza et al. [25] suggest that the risks posed by seasonal influenza viruses remained largely unchanged during the first two years of the COVID-19 pandemic and that the sizes of future seasonal influenza virus epidemics will likely be similar to those observed before the pandemic.

Another area of doubt is the robustness of pathogen surveillance and data streams in perpetuity. The COVID-19 pandemic has impacted surveillance systems [26–28] and existing vaccination programmes [29–31], leading to uncertainty in how both influenza and COVID-19 surveillance can be jointly and durably implemented over prolonged time horizons. We must understand and assess any changes that impact influenza surveillance in order to inform public health for future actions.

Our choice of model structure and parameterisation inherently has associated assumptions. Thus, it is important our findings are viewed with due consideration of the limitations. We summarise here four of particular note. First, our model takes a national perspective, but regional variations in adherence to NPIs and/or vaccine uptake may be crucial. For example, areas where uptake of the seasonal influenza vaccine in target groups has been historically below the national average might also have differing NPI usage and uptake of an expanded vaccination programme compared to the national average. Second, and as previously mentioned, the framework did not explicitly include healthcare workers or care-home residents and staff, who can be particularly at risk of being exposed to seasonal influenza. Third, in the first piece of analysis undertaken prior to the 2020/2021 influenza season, to include the impact of NPIs on the age-structure of contacts in the relevant scenarios, we used estimates that included schools being open, with this amended contact structure then being used throughout the duration of the influenza season. We did not incorporate temporally varying contact patterns to capture school holidays, or the possibility of social mixing patterns changing around dates tied to special occasions (the Christmas and New Year period, for example). Finally, given the relative novelty of the SARS- CoV-2 virus in humans, there remains a great amount of uncertainty in our understanding of the interaction of SARS-CoV-2 with other respiratory infections in circulation, such as influenza [32]. We did not consider the consequences of interaction between SARS-CoV-2 and seasonal influenza, such as coinfections. Our knowledge on processes such as viral interference and coinfection will continue to improve as new information is accumulated.

It is not yet apparent when a more predictable pattern of respiratory virus circulation, including seasonal influenza, will return; over time we will develop an improved understanding of how circulatory patterns of respiratory viruses (and other pathogens) have altered in the context of the COVID-19 pandemic. Ongoing consideration of preventive measures to reduce influenza virus infection risks in the community is therefore prudent, including strengthening influenza vaccination programmes. Scenario assessments using epidemiological models and mathematical models of infectious disease dynamics can continue to provide quantitative evidence for the potential impact of different epidemiological characteristics and public health policies on the course of seasonal influenza outbreaks, particularly following receivership of new epidemiological and behavioural data.