Surveillance, prevention of COVID-19 and seasonal influenza in Europe

In the northern hemisphere, annual seasonal influenza activity usually causes a high burden of disease during the winter months. Since March 2020, however, the virus has been circulating at unprecedented low levels. What does this mean for the coming season and influenza surveillance? In its 'Spotlight on influenza 2021', Eurosurveillance presents a series of articles that describe various aspects of the disease, including surveillance and prevention.

In the accompanying editorial for the 'Spotlight on influenza 2021' series, Amparo Larrauri and Katarina Prosenc stress 'a considerable uncertainty on the influenza virus behavior in the upcoming 2021/22 season' as on the one hand the influenza virus might not have evolved antigenically, and could thus be recognized by people's immune systems. On the other hand, more people could get infected or experience more severe influenza as they were not exposed to the virus in the previous season.

Critical elements: vaccination and integrated surveillance

Coinciding with the start of the 2021/2022 European influenza surveillance season, several governments have already lifted or are easing COVID-19 restrictions and with that societies are beginning to open up again – which could lead to a surge in influenza activity in the coming season and a co-circulation with SARS-CoV-2 viruses.

In this context, Larrauri and Prosenc highlight that 'influenza vaccination and the strengthening of an integrated surveillance of influenza viruses and SARS-CoV-2 are critical elements in preparing for a possible resurgence of influenza in the 2021/22 season'.

The five articles of the 'Spotlight on influenza 2021' present different aspects of surveillance and prevention of influenza ahead of the winter season in the northern hemisphere.

Adlhoch et al. give an epidemiological and virological overview of the 2019/2020 influenza season and the dominant influenza viruses that circulated at the time. The article highlights that reduced reporting across the WHO European Region coincided with the onset of widespread circulation of SARS-CoV-2. The authors hypothesize that the COVID-19 pandemic contributed to an earlier end to the 2019/2020 influenza season.

Retrospectively studying surveillance data from Denmark over three seasons before and two seasons after wide introduction of molecular point-of-care tests (mPOC), Benedetti et al. demonstrate how point-of-care testing can be successfully integrated into regular influenza surveillance. The authors found an increase of tested specimens while the percentage of positive results overall stayed the same before and after introduction of mPOC.

Baum et al. estimate influenza vaccine effectiveness in the elderly population using data from Finnish population-based registries. Controlling for potential confounders, they show that seasonal influenza vaccination reduces the risk of severe influenza in those aged 65 years and older. The authors report that in eight consecutive seasons, vaccination reduced the hazard of severe influenza disease in those vaccinated by 16% to 48%.

Severe acute respiratory infections (SARI) are an important cause of hospitalization and mortality, and the influenza virus is a well-recognized cause of SARI. Looking at potential benefits of early testing for other non-influenza respiratory viruses (NIRV), Subissi et al. show that one third of the SARI cases were positive for NIRV – in children under the age of 5 years this proportion exceeded that of influenza. The authors conclude that SARI cases should also be tested for NIRV.

In a broader view that includes all the countries of the World Health Organization's (WHO) European Region, a meta-analysis by Belazi et al. finds that laboratory-confirmed influenza accounts for approximately one third of all acute respiratory infections for which medical care was sought in the Region during the influenza season.

As pointed out by Larrauri and Prosenc, 'in this possible scenario, influenza vaccination and the strengthening of an integrated surveillance of influenza viruses and SARS-CoV-2 are critical elements in preparing for a possible resurgence of influenza in the 2021/22 season'.