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August 04, 2021

Flu shots may offer some protection against COVID-19's most severe complications, study finds

The influenza vaccine does not prevent people from contracting the disease caused by the coronavirus; doctors stress the importance of getting both inoculations

Now there is one more reason to get your annual flu shot – it can reduce the risks of stroke, sepsis and deep vein thrombosis in patients battling COVID-19, new research suggests.

Patients infected with SARS-CoV-2 who had been vaccinated against influenza also were less likely to visit the emergency department or need to be admitted to the intensive care unit.

In an analysis of almost 75,000 patients, University of Miami researchers discovered several potential benefits that the influenza vaccine offers against the coronavirus.

Other studies have also found similar protective benefits against COVID-19 illness and severe complications, but this is the largest to look at these connections so far.

"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," Devinder Singh, chief of plastic surgery and professor of clinical surgery at the University of Miami's Miller School, said.

The international study included patient records from the U.S., United Kingdom, Germany, Italy, Israel and Singapore. Patients were grouped by whether or not they had received the flu vaccine prior to their positive COVID-19 diagnosis. The two groups were then matched for factors that could increase their risk for severe COVID-19, including age, gender, ethnicity, smoking and comorbidities such as diabetes, obesity and chronic obstructive pulmonary disease.

The researchers particularly looked at the rate of 15 adverse outcomes in both groups. These outcomes included sepsis, strokes, deep vein thrombosis, pulmonary embolism, acute respiratory failure, acute respiratory distress syndrome, joint pain, renal failure, anorexia, heart attack, pneumonia, emergency department visits, hospital admission, intensive care unit admission and death within 30, 60, 90 and 120 days of testing positive for COVID-19.

Overall, they found that the patient group that wasn't vaccinated against influenza was up to 20% more likely to have been admitted to intensive care. They are also up to 58% more likely to visit a hospital's emergency department.

While the risk of death remained the same in both groups, the unvaccinated patients were up to 45% more likely to develop sepsis and up to 58% more likely to have a stroke during their COVID-19 illness. They are also up to 40% more likely to develop deep vein thrombosis.

How the flu vaccine provides this protection isn't completely understood by scientists. The most prevailing theory is that the flu shot may give a general boost to the immune system, which is then able to provide a stronger defense against SARS-CoV-2.

The researchers said randomized controlled trials are needed to better understand the connection between the flu vaccine and fewer severe effects of COVID-19. For now though, they emphasized the importance of getting both the flu shot and the COVID-19 vaccine for the strongest protection against COVID-19.

It can also prevent a dual surge of both COVID-19 and influenza cases at the same time, they said.

"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," said Susan Taghioff, a medical student at the University of Miami and one of the study's authors.

"Regardless of the degree of protection afforded by the influenza vaccine against adverse outcomes associated with COVID-19, simply being able to conserve global health care resources by keeping the number of influenza cases under control is reason enough to champion continued efforts to promote influenza vaccination worldwide."

The study was published in the journal PLoS One. Preliminary data from the study was released at the European Congress of Clinical Microbiology & Infectious Disease.

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