Influenza vaccine could also help against severe Covid-19 — but we’re not entirely sure why

The influenza vaccine could help attenuate the adverse effects of SARS-CoV-2 worldwide, according to a new study. Researchers found an 8% drop in hospitalization in Covid-19 positive patients receiving the flu shot, suggesting a benefit for populations struggling to get access to the coronavirus vaccination.

Image credit: Flickr / CAL OES.

The researchers from the University of Miami Milles School of Medicine explored about 75,000 digital medical files of patients from all around the world., especially in the United States and Europe. The study focused on the period before the worldwide rollout of the coronavirus vaccines.

Patients were sampled by age, gender and medical conditions, such as heart disease and diabetes. The researchers looked at the evolution of coronavirus symptoms in the next 30, 60, and 120 days from the time patients were diagnosed, looking for a correlation with the flu vaccine. 

The findings showed a statistical link between the risk of developing Covid-19 symptoms and non-vaccination against the flu, with ICU admissions being lower in patients who received the flu vaccine. Patients who received the influenza vaccine also experienced a decreased risk of sepsis and stroke.

“Even patients who have already received SARS-CoV-2 vaccination may stand to benefit given that the SARS-CoV-2 vaccine does not convey complete immunity, although further research into the relationship and potential interaction between influenza vaccination and SARS-CoV-2 vaccination should be performed,” the researchers wrote.

Influenza and Covid-19

In a study earlier this year, researchers found a potential protective effect of influenza vaccination against adverse outcomes associated with Covid-19. The finding showed a 2.44 greater odds ratio (OR) for hospitalization and 3.29 greater OR for intensive care unit admission on patients who weren’t up to date with their influenza vaccination. 

In fact, several studies have recently suggested a possible protective effect of the influenza vaccine against the novel coronavirus. Although researchers aren’t entirely sure what is offering the protection, they have a few theories.

Influenza and Covid-19 are both contagious respiratory illnesses but are caused by different viruses. While Covid-19 is caused by infection with a coronavirus (SARS-CoV-2), seasonal flu is caused by infection with one of the many influenza viruses that spread annually. Symptoms may be similar and that’s why it’s so important to get tested. 

Influenza is estimated to kill about 2 in every 100,000 people it infects, making it much less severe than the novel coronavirus, with so far 4.55 million deaths on a global scale. Some examples of the flu include the viruses H1N1, which caused the Swine Flu of 2009 and the Spanish Flue of 1918, and the H5N1, which also has pandemic potential. 

Although production and distribution of Covid-19 vaccines continue to increase daily, the fact remains that certain populations in the global community may still have to wait a long period of time before they are vaccinated. This is why researchers are now looking into the possible benefits of more comprehensive influenza vaccination. Researchers also caution that there is still important demographic inequality when it comes to the influenza vaccine.

“Less than half of US adults receive influenza vaccination each year, with Non-Hispanic Black, Hispanic, and American Indian/Alaskan Native individuals having had the lowest influenza vaccination coverage while also being disproportionately affected by SARS-CoV-2,” the researchers wrote.

The study was published in the journal Plos One.

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