HONG KONG — Influenza could more than double the transmission rate of the coronavirus, according to new European research.

A joint study by the Max Planck Institute in Germany and Pasteur Institute in France showed that on average, a patient infected with the Sars-CoV-2 virus could pass Covid-19 to two other persons.

But if he or she also had the flu, it would help the coronavirus jump to four or five people, it said.

“The results were unequivocal,” said the researchers, led by German epidemiologist Dr Matthieu Domenech de Celles, in a non-peer-review paper posted on Medrxiv.org on Wednesday (Sept 9).

There is growing concern that a second wave of the pandemic may hit the northern hemisphere this autumn, possibly making the current coronavirus crisis even worse.

“I don’t know… nor can anyone know what the fall is going to bring,” said top US infectious disease expert Dr Anthony Fauci in an interview with the American medical publication Jama last month.

Dr Domenech de Celles and colleagues developed a model that could simulate the co-circulation of seasonal flu and Covid-19. They analysed the Covid-19 data in European countries, including Belgium, Italy, Spain and Norway to decouple the impact of the two different infectious diseases.

Debate surrounds how the flu affects the pandemic. Some scientists believe the infection caused by influenza strains would produce a cross-immune response that could provide partial protection against the novel coronavirus.

But others say it would only make the situation worse, because a co-infected patient has to fight two different viruses at the same time. Furthermore, some typical flu symptoms such as coughing and sneezing would help spread Covid-19.

The study led by Dr Domenech de Celles found “consistent evidence that, during the period of co-circulation, influenza was associated with an average 2-2.5-fold population-level increase in Sars-CoV-2 transmission”, the French and German researchers said.

The decline of new cases after the first wave in spring was not only caused by lockdown and social distancing measures, but also the end of flu season, according to their analysis.

The researchers also found that 30 to 50 per cent of the co-infections were likely not detected.

One possible reason, according to Dr Domenech de Celles, was the different time it took for the onset of the two illnesses. The coronavirus usually takes more than five days to start showing symptoms, longer than flu incubation period of one or two days. So when a patient tested for coronavirus, the flu might have already disappeared from the patient.

A pandemic caused by a variant of the H1N1 virus in 1918 infected about a third of the world’s population and killed 50 million people. It had come in several waves, and some scientists believed these waves were driven and made worse by the seasonal flu.

A study by Google and Cold Spring Harbor Laboratory in May found that flu infection could significantly increase the amount of angiotensin-converting enzyme 2 (ACE2) in the human body. ACE2 is a main receptor used by the coronavirus to bind with host cells.

Dr Robert Redfield, director of the US Centres for Disease Control and Prevention, said last month that autumn could “be the worst we’ve seen” if Americans did not follow mitigation guidelines, such as avoiding crowds or wearing face masks.

Dr Wang Chen, president of the Chinese Academy of Medical Sciences in Beijing and a senior adviser to the Chinese government on pandemic response, said 
everyone should get a flu shot before the influenza season hits in November.

The flu vaccination could cut the risk of being infected by different viruses that caused similar symptoms circulating at the same time, he told a national conference of the China Association for Science and Technology last month.

It “must be done before the end of September, or in October the latest”, he said. SOUTH CHINA MORNING POST