The annual influenza vaccine may reduce the risk of stroke, sepsis, blood clots and several other severe effects in patients with COVID-19, according to the largest study of its kind.

The researchers from the University of Miami Miller School of Medicine, US, also found that patients with COVID-19 who had been vaccinated against the flu were significantly less likely to visit the emergency department (ED) and be admitted to the intensive care unit.

"Only a small fraction of the world has been fully vaccinated against COVID-19 to date, and with all the devastation that has occurred due to the pandemic, the global community still needs to find solutions to reduce morbidity and mortality," said senior study author Devinder Singh, professor at the Miller School.

"My team has been able to observe an association between the flu vaccine and reduced morbidity in COVID-19 patients," said Singh, who conducted the study with lead authors Susan Taghioff and Benjamin Slavin.

The study, published in the journal PLoS One on August 3, analysed patient records from a number of countries, including the US, the UK, Germany, Italy, Israel and Singapore.

The researchers screened de-identified electronic health records on the TriNetX research database for more than 70 million patients to identify two groups of 37,377 patients.

Members of the first study group had received the flu vaccine two weeks and six months prior to being diagnosed with COVID-19.

Those in the second group also had a positive COVID-19 diagnosis but were not vaccinated against the flu.

The incidence of 15 adverse outcomes, including sepsis, strokes, deep vein thrombosis (DVT), emergency department visits and death, within 30, 60, 90 and 120 days of testing positive for COVID-19 were then compared between the two groups.

The analysis revealed that those who had not had the flu shot were significantly more likely (up to 20 per cent) to have been admitted to the ICU.

They were also significantly more likely to visit the emergency department (up to 58 per cent), to develop sepsis (up to 45 per cent), to have a stroke (up to 58 per cent) and a DVT (up to 40 per cent).

The risk of death among patients immunised with the flu vaccine was not reduced, the researchers said.

They were also able to calculate how many COVID-19-positive patients would need to receive an influenza vaccine to avoid one adverse outcome.

The team found that only 176 patients needed to have received a flu vaccine to prevent one ED visit within 120 days of testing positive for COVID-19.

Also, only 286 patients needed to have received their flu vaccine to prevent one case of sepsis.

For every 440 patients who were up to date on their flu shot, one ICU admission was prevented, according to the researchers.

Although it is not exactly known yet how the influenza vaccine provides protection against COVID-19, the researchers said, most theories speculate that the flu shot may boost the innate immune system -- general defences we are born with that do not protect against any one specific illness.

The results, they said, strongly suggest that the flu vaccine may protect against several severe effects of COVID-19.

However, the researchers strongly recommend that people receive COVID-19 vaccines as well their annual influenza vaccine.

The researchers added that more studies, in the form of randomised control trials, are needed to prove and better understand the possible link.

However, they said the flu shot could be used to help provide increased protection in countries where the COVID-19 vaccine is in short supply or even aid in the ongoing struggle against breakthrough cases in those individuals already vaccinated against COVID-19.

"Continued promotion of the influenza vaccine also has the potential help the global population avoid a possible 'twindemic' -- a simultaneous outbreak of both influenza and coronavirus," Taghioff added.