People who are vaccinated against influenza may be partially protected against many severe effects of COVID-19, and are less likely to need emergency care, according to a large-scale study.

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An analysis of nearly 75,000 COVID-19 patients from around the world strongly suggests that the annual flu shot reduces the risk of stroke, sepsis and deep vein thrombosis (DVT) in patients with COVID-19. 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), the researchers said.

"This finding is particularly significant because the pandemic is straining resources in many parts of the world," said Devinder Singh, a professor at the University of Miami Miller School of Medicine in the US. "Therefore, our research -- if validated by prospective randomised clinical trials -- has the potential to reduce the worldwide burden of disease," Singh, the study's senior author, said. Several recent studies have suggested that the flu vaccine may provide protection against COVID-19 meaning it could be a valuable weapon in the fight to halt the pandemic.

In the largest study of its kind, the researchers screened de-identified electronic health records held on the TriNetX research database of over 70 million patients to identify two groups of 37,377 patients. The two groups were matched for factors that could affect their risk of severe COVID-19, including age, gender, ethnicity, smoking and health problems such as diabetes, obesity and chronic obstructive pulmonary disease. 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 conducted using patients from countries including the US, UK, Germany, Italy, Israel and Singapore. The research was presented at the European Congress of Clinical Microbiology & Infectious Diseases (ECCMID), held online. The incidence of 15 adverse outcomes, including sepsis, strokes, DVT, and acute respiratory failure, within 120 days of testing positive for COVID-19 was then compared between the two groups. The analysis revealed that those who had not had the flu jab were up to 20 per cent more likely to have been admitted to ICU. They were also up to 58 per cent more likely to visit the Emergency Department, up to 45 per cent more likely to develop sepsis, up to 58 per cent more likely to have a stroke, and up to 40 per cent more likely to have DVT.

The risk of death was not reduced, the researchers said. However, the researchers said it is not known exactly how the flu jab provides protection against COVID-19. Most theories centre around the influenza vaccine boosting the innate immune system -- general defences we are born with that are not tailored to any particular illness, they said. The researchers said their results strongly suggest that the flu vaccine protects against several severe effects of COVID-19. They noted that more research is needed to prove and better understand the possible link but, in the future, the flu shot could be used to help provide increased protection in countries where the COVID-19 vaccine is in short supply.

"Influenza vaccination may even benefit individuals hesitant to receive a COVID-19 vaccine due to the newness of the technology, said Susan Taghioff, of the University of Miami 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, Taghioff added.