Researchers from the University of Miami in Florida, US, found that patients with Covid-19 who had been vaccinated against flu were also less likely to visit the emergency department and be admitted to the intensive care unit (ICU).
On the other hand, those who had not had the flu jab were significantly more likely (up to 20 per cent more likely) to have been admitted to ICU; to visit the Emergency Department (up to 58 per cent more likely), to develop sepsis (up to 45 per cent more likely), to have a stroke (up to 58 per cent more likely) and a DVT (up to 40 per cent more likely). The risk of death was not reduced.
It isn't known exactly how the flu jab provides protection against Covid-19 but most theories centre around it boosting the innate immune system -- "general" defences we are born with that are not tailored to any particular illness, said the researchers.
The study shows that flu shot could be used to help provide increased protection in countries where the Covid-19 vaccine is in short supply, however it is not a substitute for Covid vaccinations, they added.
"Influenza vaccination may even benefit individuals hesitant to receive a Covid-19 vaccine due to the newness of the technology," added Susan Taghioff, from the University's Miller School of Medicine.
"Despite this, the influenza vaccine is by no means a replacement for the Covid-19 vaccine and we advocate for everyone to receive their Covid-19 vaccine if able to," she noted.
For the study, the team screened de-identified electronic health records of more than 70 million patients to identify two groups of 37,377 patients from countries including the US, UK, Germany, Italy, Israel and Singapore.
Members of the first group had received the flu vaccine between two weeks and six months before being diagnosed with Covid-19. Those in the second group also had Covid-19 but were not vaccinated against flu.
The study was presented at the ongoing European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) taking place online between July 9 and 12.