The upcoming flu season may worsen the novel coronavirus pandemic, experts have warned, after studies revealed that coinfection with influenza and COVID-19 could lead to a higher risk of death.

Researchers at the University of Liverpool found that symptoms in mice with coinfection of influenza virus and COVID-19 were worse than the individual infections. The results, which were published this week on the preprint server bioRxiv, follow on from a United Kingdom study released last month that found human patients with coinfection had up to six times higher risk of death than the general population.

Last week, the British government issued new guidelines for securing flu shots, after the Royal College of General Practitioners, or RCGP, wrote to Health Secretary Matt Hancock highlighting the struggles doctors have had securing enough vaccines.

James Stewart, who is a professor of molecular virology at Liverpool, said that there is "growing concern "about the interactions between COVID-19 and other respiratory infections this winter.

"Our study highlights the urgent need to maintain flu vaccination," said Stewart, who is co-author on the new study.

The research found that there was a larger inflammatory response in mice infected with both influenza and COVID-19. Inflammation is a key driver for severe COVID-19 infection in humans and plays a significant role in mortality, according to the study.

Infected mice "exhibited significantly more rapid mortality compared with mice infected with either virus alone", researchers found.

"Seasonal influenza virus can overwhelm the National Health Service in normal years," said Julian Hiscox, who is chair in infection and global health at Liverpool and coauthor on the study. "Both SARSCoV-2 and influenza are likely to co-circulate and present a risk. Our work shows how infection with both is dangerous and we can at least do something about mitigating the risk of flu through vaccination."

A separate study, which was released on bioRxiv in late September by Public Health England, said there is "strong evidence of an interaction effect compared to SARS-CoV-2 and influenza acting independently".

The study looked at hospital data from between January and April this year, and found that the odds of ventilator use, intensive care admission and death were all greatest among coinfection patients. The study found that patients with a coinfection had a risk of death of six times greater than the general population.

The study also found that the chance of contracting influenza is lower when a person has COVID-19 and in the Liverpool study, mice with influenza showed lower viral replication of the novel coronavirus. The Public Health England study posited that there may be a "possible pathogenic competition between the two viruses".

Last Friday, the UK government issued guidelines over its flu vaccination program directly to doctors, and announced that over 10 million flu vaccines have been delivered, with a further 20 million scheduled for delivery between now and December.

The announcement came after a letter from the RCPG in early October sought assurances that there would be enough vaccine to meet demand.