The 2019/20 influenza season was very unusual

Not only was the season unusually mild. It also stopped abruptly due to the measures introduced to curb COVID-19. And COVID-19 “disturbed” the regular influenza surveillance measures.

An unusual season.

That is how Statens Serum Institut characterises the 2019/20 influenza season in the latest issue of EPI-NEWS.

The season was unusual because it was mild compared with the serious influenza epidemics seen in Denmark and the rest of Europe in the past three seasons (2016/17, 2017/18 and 2018/19).

COVID-19 measures broke the influenza curve

The season turned even more unusual when Denmark was hit by the global corona virus pandemic towards the end of February. Initially, the pandemic triggered a brief, but high increase in the number of persons with influenza-like symptoms, but as large parts of Denmark entered lockdown from 12 March, a drastic drop was observed in the already small number of influenza cases.

“In early March, the share of patients with influenza-like symptoms increased. We recorded an increase in the sentinel system from 1% to 6% within a few weeks. This, however, was seen because the sentinel system was expanded to use a slightly broader influenza/COVID-19 disease definition, so that we could follow the spread of COVID-19 in the Danish population. Furthermore, phone consultations and home sampling done by the patients themselves were introduced,” explains Staff Physician Lasse S. Vestergaard from the SSI. He continues:
“In the following weeks, the share of people with influenza-like symptoms decreased once again to near-zero levels when Denmark started its lockdown, initiated social distancing, use of hand disinfectants, etc. to avoid transmission of COVID-19. Influenza virus simply stopped circulating within weeks and the majority of the influenza samples tested were negative”.

Which influenza affected Denmark?

The 2019/20 influenza season was dominated by influenza A. In the beginning of the season, the subtypes (H1N1)pdm09 and (H3N2) were seen with almost the same frequency , but towards the end of the season, influenza A (H3N2) made up the majority of the detected cases.

Only a few cases of influenza B was recorded during this influenza season. Most of these cases were of the Victoria subtype.

How many people were affected by influenza?

A total of 68,530 patients were tested for influenza from week 40 last year through week 20 this year. Among them, 7,575 were positive (6,775 for influenza A and 800 for influenza B).

“This corresponds to an 11% positive rate, which is lower than the previous two years. In 2017/18, we reached a 29% positive rate among those tested, and in 2018/19 the positive rate was 20%,” notes Lasse S. Vestergaard.

Fewer patients admitted to hospital

The low occurrence of influenza is also reflected in the number of patients admitted to hospital due to influenza, which was the lowest seen in the past five years.

In the 2019/20 season, a total of 2,290 patients were admitted to hospital. Their median age was 62 years and the majority (69%) had not received influenza vaccination ahead of the season.
In total, 762,500 at-risk citizens received influenza vaccination. This means that the vaccination coverage among elderly people aged 65 years or older reached 52%, which is in line with the previous year.

And fewer deaths

Finally, far fewer deaths due to influenza were seen in the 2019/20 season than previously.

“The estimated overall excess mortality in the entire season was only 119 persons - most of whom were 65 years or older. This is substantially lower than we have seen in the three preceding seasons which were all serious. Not just in Denmark, but in Europe,” adds Lasse S. Vestergaard.

Thus, an excess mortality of 751 persons was observed in the 2016/17 influenza season, 2,822 persons in 2017/18 and 790 persons in 2018/19.