As we move into warmer weather in the northern hemisphere, the seasonal influenza outbreak is coming to an end. The relatively short duration and abrupt end of the 2019/2020 flu season are particularly striking when compared to previous years. One of the main contributing factors to this change is the measures taken to contain the COVID-19 pandemic.
This year, the attention of the world has been focused on the novel coronavirus (SARS-CoV-2) the infectious disease it causes, COVID-19. Receiving relatively little attention, is this year’s recently ended flu season. Measures taken to contain the COVID-19 pandemic have had a direct impact on the course of influenza. In this article, we summarize the most important takeaways from this year’s flu season.
Dominant virus types
Influenza viruses are generally divided into three types: A, B and C. This flu season was dominated again by influenza virus type A, the same as last season. Over the course of the 2019/2020 flu season, the Influenza Working Group at the Robert Koch Institute (RKI) identified a total of 916 different influenza viruses. In 45% of all cases, these were influenza A (H3N2) viruses, followed by influenza A (H1N1) pdm09 viruses at 41%, and type B viruses making up the remaining 14%. 
“The classic transmission route for influenza viruses and corona viruses is droplet infection, but smear infection is also possible via contaminated surfaces and then via the hands. This means that hand hygiene also plays a prominent role in preventing these viral infections. In addition to hand washing with soap, hygienic hand disinfection with alcohol-based sanitizers is an effective means of eliminating these viruses from hands. When choosing a hand disinfectant, it is important to ensure that the enveloped viruses are within the spectrum of activity of the sanitizer. In this case, sanitizers must be at least limited virucidal”.Dr. Andreas Glöckner, Medical Director
Across the continent, the European Centre for Disease Prevention and Control (ECDC) found that virus type A was responsible for about three out of four influenza cases.  According to data from the US Centers for Disease Control and Prevention (CDC), this influenza type also dominated in the USA. Of note, only with increasing age did the proportion of type A rise significantly. In the 5-24 years age group, virus type B was dominant. 
Duration of the flu season
The 2019/2020 flu season made only a “brief appearance”. The RKI reports in its Epidemiological Bulletin that the flu epidemic in Germany circulated for only eleven weeks at the beginning of January 2020, ending at least two weeks sooner than in previous years.  It peaked between the end of January to mid-February. Influenza infections in other European countries also collapsed drastically by the end of March, demonstrating an abrupt, premature end to this season’s wave of influenza. 
The reasons for this are obvious. The globally enforced measures introduced to contain COVID-19 – such as implementation of curfews and school closures, cancellations of major events – also deprived the influenza viruses of the “breeding ground” for further spread in the population. In terms of transmission routes, the two infectious diseases are similar, primarily transmitted by droplet infection and also via direct contact between the hands and surfaces contaminated with the viruses.
“The nationwide measures to contain and slow down the COVID-19 pandemic in Germany may have contributed significantly to this reduction […]. Since children play a major role in the spread of the annual flu epidemic, the school closures from the 12th week [mid-March] of 2020 onwards should be mentioned here in particular”.Robert Koch-Institute
Other important infection prevention measures, such as wearing a mask in public and providing sanitizer stands for hand hygiene, slowed down the spread of influenza. Vaccinations for seasonal influenza are also made available each year, as an added protection measure.
However, the effectiveness of vaccine for this season’s influenza virus is estimated to range between 30% and 60% in all age groups, according to results of a recent study for the European Union and Great Britain. 
How many cases of flu have there been?
Despite the comparatively short flu epidemic, the RKI reported 186,919 confirmed cases of the flu in Germany by mid-May. This figure corresponds approximately to the results of the 2018/2019 flu season. About 16% of all infected persons had to be treated in hospital. Another parallel to last year’s season: the total of 4.2 million influenza-related visits to doctors. In addition, the RKI reports more than 500 deaths related to an influenza virus infection. [1,4]
Compared to the 2017/2018 flu epidemic, which claimed over 25,000 lives throughout Germany, this year’s season appears far less serious.
In North America, initial estimates by the CDC indicate that there were at least 24,000 influenza-related deaths and more than 18 million visits to doctors in the United States related to the flu.  In neighbouring Canada, just over 42,500 people were reported to have been infected with the influenza virus. 
Japanese researchers from Kyoto University Hospital also found that the COVID-19 pandemic had a direct impact on seasonal influenza, at least in East Asia, resulting in fewer infections of the virus. The researchers attribute this, among other things, to greater health awareness, including infection prevention measures. 
Hand hygiene remains top priority
Greater awareness and participation in thorough hand hygiene, highlighted as a result of the COVID-19 pandemic, must continue as a long-term counter measure to future seasonal waves of influenza. With the annual influenza virus vaccination only providing partial protection, thorough hand hygiene remains of great importance.
The World Health Organization (WHO) recently published a recommendation to this effect, directed at all public institutions of the UN member states. Though the original aim of the recommendation was to contain COVID-19, greater hand hygiene infrastructure in public institutions will also support prevention of the spread of seasonal flu viruses. 
Since children play a central role in the spread of influenza, further attention should also be paid to the hygiene infrastructure in educational and training institutions, especially to the regular washing of clothes. A 2015 study underlines the relevance of thorough hand hygiene in educational institutions in order to reduce the number of flu cases in children. 
 Robert Koch-Institut – Arbeitsgemeinschaft Influenza “Influenza Wochenbericht”. Kalenderwoche 20/2020
 ECDC Surveillance Report. “Influenza virus characterisation, Summary Europe, April 2020“
 CDC “Age Group Distribution of Influenza Positive Specimens Reported by Public Health Laboratories, National Summary, 2019-20 Influenza Season through the Week Ending May 23, 2020”
 Goerlitz, Luise, et al. “Erste Ergebnisse zum Verlauf der Grippewelle in der Saison 2019/20: Mit 11 Wochen vergleichsweise kürzere Dauer und eine moderate Anzahl an Influenza-bedingten Arztbesuchen.” (2020).
 ECDC. “Weekly influenza update, week 18, May 2020”
 Rose, Angela, et al. “Interim 2019/20 influenza vaccine effectiveness: six European studies, September 2019 to January 2020.” Eurosurveillance 25.10 (2020): 2000153.
 Centers for Disease Control and Prevention. “2019-2020 U.S. Flu Season: Preliminary Burden Estimates”
 Public Health Agency of Canada. “FluWatch report: April 5 to May 9, 2020 (weeks 15 to 19)”
 Itaya, Takahiro, Yuki Furuse, and Kazuaki Jindai. “Does COVID-19 infection impact on the trend of seasonal influenza infection? 11 countries and regions, from 2014 to 2020.” International Journal of Infectious Diseases (2020).
 Weltgesundheitsorganisation “Recommendations to Member States to improve hand hygiene practices to help prevent the transmission of the COVID-19 virus”, 1. April 2020
 Torner, Núria, et al. “Effectiveness of non-pharmaceutical measures in preventing pediatric influenza: a case–control study.” BMC public health 15.1 (2015): 543.