Individuals who received an influenza vaccine and subsequently developed COVID-19 have the potential for less severe side effects from the disease, according to a new study published in the Plos One medical journal on Wednesday. 

What You Need To Know

  • A new study published Wednesday found individuals who received an influenza vaccine and subsequently developed COVID-19 tended to develop less severe side effects from the disease

  • The study specifically found patients who received the influenza vaccine had a mitigated risk of sepsis, deep vein thrombosis, and both emergency department and intensive care unit (ICU) admissions after contracting COVID-19 

  • The study, however, found “no association between influenza vaccination and risk of death” in COVID-19 patients 

  • Researchers do not yet have a concrete answer as to why the influenza vaccine might offer protection against severe cases of COVID-19

Researchers examined the records of over 73 million patients from around the world, namely the United States, the United Kingdom, Italy, Germany, Israel and Singapore. 

Those records were pared down to two groups of 37,377 COVID-19 patients: Members of one group had received an influenza vaccine anywhere from six months to two weeks prior to contracting COVID-19, and the other group did not get a flu vaccine. 

Researchers say two weeks is the minimum amount of time for a person to develop sufficient antibodies after receiving a flu vaccine, while six months is when protection against a virus can start to fade. 

The study specifically found patients who received the influenza vaccine had a mitigated risk of sepsis, deep vein thrombosis, and both emergency department and intensive care unit (ICU) admissions after contracting COVID-19. 

COVID-19 patients who had not gotten a flu vaccine were 20% more likely to be admitted to the ICU, around 58% more likely to make an emergency room visit, 58% more likely to have a stroke and up to 45% more likely to develop sepsis.

The findings “suggest a potential protective effect that could benefit populations without readily available access to SARS-CoV-2 vaccination,” the study’s authors wrote in part, adding: “Thus, further investigation with future prospective studies is warranted.” 

The study, however, found “no association between influenza vaccination and risk of death” in COVID-19 patients. 

The Plos One study is not the first to proffer a link between the flu vaccine and less severe side effects from the SARS-CoV-2 virus. 

One study, published in the Journal of the American Board of Family Medicine in January, examined 2,005 patients at the University of Florida who tested positive for COVID-19. Patients who had not received an influenza vaccination before contracting COVID-19 had both greater odds of hospitalization and greater odds of ICU admission compared to COVID-19 patients who had gotten their influenza vaccine.

“Our analysis suggests that the influenza vaccination is potentially protective from moderate and  severe cases of COVID-19 infection,” the study concluded in part. “This protective effect holds regardless of comorbidity.” 

While researchers do not yet have a concrete answer as to why the influenza vaccine might offer protection against severe cases of COVID-19, they do have a number of theories. One possible answer is that the influenza vaccine triggers additional killer cells in the human body, which can in turn help to fight off the SARS-CoV-2 virus.

Another theory, originally proposed in the International Immunopharmacology Sept. 2020 issue, suggests a certain oil-in-water squalene emulsion present in influenza vaccines also helps to create an immune response against SARS-CoV variants. 

The authors of the study published Wednesday say not only is the discovery important in “guarding against a possible ‘twindemic’ of simultaneous outbreaks of influenza and SARS-CoV-2,” but it can also be used to slow the tide of severe COVID-19 cases in area with little access to COVID-19 vaccines. 

"Influenza vaccination may even benefit individuals hesitant to receive a COVID-19 vaccine due to the newness of the technology,” wrote Susan Taghioff, co-author of the study and a research assistant at the University of Miami Miller School of Medicine, wrote in a statement announcing the results. "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.”