Covid: How the corona pandemic changed the flu situation

The measures against the Covid-19 pandemic showed an unexpected side effect early on: when air traffic almost came to a standstill and social contacts were severely restricted, the annual flu epidemic stopped in many countries. Switzerland also experienced a historically low number of influenza infections in the 2020/2021 season.

An international team of researchers has now published in the specialist magazine «Science» traces how much the measures against the corona pandemic have influenced the global circulation of flu pathogens to date.

The scientists showed that flu infections fell massively almost everywhere in the world at the beginning of the pandemic. Tests for influenza viruses gave positive results much less often than before: this so-called test positive rate was more than 95 percent lower in the first year of the pandemic than in previous years. At the same time, the influenza viruses had less genetic diversity during the peak phase of the pandemic than before and after this period.

However, the analysis of epidemiological and viral genome data showed that influenza pathogens continued to circulate in parts of South Asia and West Africa – favored by the less strict corona measures there and a tropical climate in which the microorganisms are active all year round.

This meant that after the end of the Corona measures, the pathogens were able to spread around the globe again – and “remarkably quickly,” as lead author Zhiyuan Chen from the University of Oxford and Fudan University in Shanghai said in a press release. As early as March 2023, flu viruses were circulating worldwide again to the same extent as before the pandemic – and thus demonstrated their resistance even to changes in the environment over several years, as the article says.

There was one exception, however. While the influenza A subtypes H1N1 and H3N2 as well as the B viruses of the so-called Victoria lineage came back with full force, the B viruses of the Yamagata lineage appear to have fallen by the wayside. According to the study, fewer than 20 infections with these pathogens have been registered since the beginning of the pandemic. It is not even certain whether these are actually natural infections or infections that can be traced back to the live vaccine. Reporting errors are also possible. This season’s vaccines are therefore no longer directed against this pathogen variant. It is not yet possible to predict how their disappearance will affect the other influenza viruses.

The authors also cannot say why the Yamagata line did not appear to survive the pandemic period. A possible explanation is that this line triggered larger outbreaks in the years before the pandemic and therefore there was high immunity in the population. At the same time, it apparently had less genetic adaptability than other influenza pathogens.

More influenza epidemics could occur in the future

On the one hand, the study underlines that non-pharmaceutical interventions “can be incredibly effective and are probably more effective than vaccination campaigns alone,” write US researchers Pejman Rohani and Justin Bahl in one accompanying commentary.

At the same time, the two authors point out that the pandemic has also fundamentally changed immunity against influenza viruses. On the one hand, because many people’s immune systems did not encounter the pathogens for several years, and on the other hand, because in parts of the world flu vaccinations fell victim to the chaos of the Corona period. This situation could lead to larger influenza epidemics occurring again in the future.

The authors of the “Science” publication also warn of possible more severe flu outbreaks, which in turn could promote the emergence of new virus strains. This means that the events of the pandemic may have an impact on the flu situation for a longer period of time.

Found an error? Report now.

1 Comment