In a new study, researchers used a mathematical model to study the first months of the corona pandemic in Europe.
They found that the decrease of COVID-19 cases in spring was not only related to countermeasures but also to the end of the flu season.
Influenza may have increased transmission of the coronavirus by an average of 2.5-fold.
The results suggest that the coming flu epidemic will have an amplifying impact on the COVID-19 pandemic.
The researchers emphasize the potential importance of flu vaccinations as possible extra protection against COVID-19.
The research was conducted by scientists at the Max Planck Institute for Infection Biology in Berlin and elsewhere.
Data from earlier experiments showed the effects of a co-infection with coronavirus and flu.
The team focused on the effectiveness of vaccination programs and the seasonality of infectious diseases such as influenza.
When the first COVID-19 cases occurred in Europe at the beginning of the year, they wanted to apply his knowledge in epidemiological models to the new pandemic.
They developed a mathematical model of coronavirus transmission and mortality to decipher the influence of the flu season on the COVID-19 pandemic.
The researchers modeled the course of the pandemic in Belgium, Norway, Italy, and Spain. Four European countries in which the pandemic was differently pronounced during the first half of the year.
To approach the real infection events, the model was based on known disease parameters like the “generation interval,” i.e. the time needed for an infected person to infect another person.
The researchers also took non-pharmaceutical countermeasures into account, since lockdowns and social distancing had an extensive impact on the pandemic.
This was measured by the so-called Stringency Index, a value developed by Oxford University, which indicates the “strictness” of government anti-coronavirus measures.
After mathematically recreating the pandemic, the researchers were able to test various assumptions about the impact of the flu season.
They checked if the model was more realistic under the assumption that influenza either reduces, increases, or does not influence the transmission rate of coronavirus.
The team found that influenza may have increased coronavirus transmission at the population level by 2 to 2.5 times, on average during the period of co-circulation.
They checked their model against data on daily deaths in the four countries. This enabled them to demonstrate that their model is consistent with the observed pandemic mortality data.
Without the amplifying impact of influenza, the model explained the observed data substantially less well–with significantly lower COVID-19 infection rates.
It remains open whether influenza patients are more likely to transmit coronavirus to others or whether flu makes people more susceptible to corona–though the latter seems more probable according to the researchers.
Other research groups recently showed that flu viruses may increase susceptibility to COVID-19 in patients: Influenza viruses cause a higher production of the receptors that are used by the coronavirus to dock to human respiratory cells.
The team says vaccinating against influenza may be essential in the coming flu season.
Not only to relieve hospitals but also to contain the potential effect of influenza on the transmission of coronavirus.
One author of the study is mathematician Matthieu Domenech de Cellès.
The study is published in medRxiv.
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