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Home » Flu vaccine and COVID-19 infections – some evidence it might lower risk

Flu vaccine and COVID-19 infections – some evidence it might lower risk

Recently, I have been discussing the flu vaccine and COVID-19 infections. First, I debunked anti-vaccine myths. Second, I explained that the seasonal flu vaccine might be helpful in improving outcomes for patients who contract the coronavirus.

However, at that time, I wanted to make it clear that:

Once again, I am not making any claim that the seasonal flu vaccine will prevent a coronavirus infection. It’s just about comorbidities, that is, other health conditions that increase one’s risk for dangerous outcomes from the disease.

Because COVID-19 is a respiratory disease, anything that weakens the respiratory system could (and again, we don’t have solid information on the pathophysiology and comorbidities for the disease) lead to a worse course for the disease. And that would include a higher risk of mortality.

The flu vaccine can reduce the risk of one coronavirus comorbidity since the flu is a respiratory disease. So, the flu vaccine isn’t going to help reduce your risk of coronavirus infection, but it will reduce your risk of complications, including death, from COVID-19.

In addition, preventing the flu may help to reduce hospitalizations and ICU admissions, allowing for more capacity for patients who have contracted COVID-19.

But again, I assumed that the flu vaccine would have little effectiveness against SARS-CoV-2, the virus that causes COVID-19. This seemed biologically plausible because, as opposed to what Donald Trump claims, the flu virus and coronavirus are vastly different organisms. The two viruses are actually in two different phyla, meaning that the influenza virus and coronavirus are as closely related as a human is to a lobster. 

There appeared to be no scientifically supported reason to believe that the flu vaccine actually prevents COVID-19. However, there seems to be some intriguing, preliminary, and potentially convincing evidence that the flu vaccine may have some effect on the risk of COVID-19.

Flu vaccine and COVID-19 – the paper

In an article published on the medRxiv preprint server,  Priya A. Debisarun et al. wrote that the flu vaccine may also trigger the body to produce broad infection-fighting molecules that may attack the pandemic-causing coronavirus. In other words, the flu vaccine may reduce your risk of COVID-19.

Before we continue, I must give my standard warnings:

  1. This is a preprint.
  2. This has not been peer-reviewed.
  3. And it has not been published in any journal. 

The coronavirus pandemic has been a boon to the medRxiv server because there is so much new data that peer-reviewed journals are well behind in publishing important information. There are over 10,000 articles on this pandemic on medRxiv and their sister bioRxiv servers. It’s an important source of new data during these times, but I must caution that experts have not reviewed it.

However, this study gives some very important evidence that maybe this old feathered non-avian dinosaur was wrong – maybe the flu vaccine does help prevent COVID-19. 

The researchers at the Radboud University Medical Center in Nijmegen, the Netherlands (been there, loved it) examined the hospital’s databases to determine the risk of COVID-19 among hospital employees who got the flu vaccine during the 2019-20 flu season. These are some of their results:

  • Workers who received a flu vaccine were 39% less likely to test positive for COVID-19 as of June 1, 2020.
  • 2.23% of non-vaccinated employees tested positive.
  • 1.33% of vaccinated ones tested positive. 

That is a fascinating result. However, there could be a number of reasons why they found these differences beyond a biological effect from the vaccine:

  • Individuals who get the flu vaccine may adhere to COVID-19 prevention guidelines like wearing masks and social distancing.
  • Individuals who get vaccines may take better care of their health than those who don’t.
  • And remember, a correlation may not indicate causation

As I have discussed before, there are scientific methods to establish whether correlation equals causation, and one of the major steps is to establish biological plausibility, that is, whether there is some biological mechanism that could lead us to determine whether the flu vaccine could actually prevent a COVID-19 infection. 

The researchers also conducted an experiment that may show how flu vaccines could prevent COVID-19. First, they purified blood cells from healthy individuals. Then they exposed some of the cells to a flu vaccine, letting the cells proliferate for six days. At that point, the researchers exposed those cells to SARS-CoV-2, and they analyzed them 24 hours later.

They found that cells that had been given the flu vaccine produced higher levels of cytokines compared to the cells that had not been exposed to the vaccine. I know that these molecules have a bad reputation because of the “cytokine storm” which is quite dangerous and can damage organs. In fact, late in the progress of COVID-19, the virus can cause the cytokine storm.

However, cytokines are important to the immune process – they are critical to the immune response to pathogenic infections. 

I’ve discussed this before – vaccines may have nonspecific innate immune system effects on other viruses. In fact, researchers have started several clinical trials to determine if the BCG vaccine (for tuberculosis) may reduce the risk of COVID-19 or improve outcomes. Vaccines may cause this nonspecific immune response by reprogramming stem cells that give rise to cells involved in this early innate immune response.

These researchers are not alone in these types of findings. A paper published in Vaccines in September 2020 and another published in the Journal of Medical Virology in June 2020 found that the rates of COVID-19 were lower in regions of Italy with higher percentages of adults aged 65 and older who had received the flu vaccine. 

Now, both were ecological studies, which are not as well-designed as other types of epidemiological studies, but they do give us even more evidence that there’s some potential link between the flu vaccine and COVID-19.


Let’s be frank – this is very preliminary evidence that the flu vaccine could help prevent COVID-19. The flu vaccine is still not a specific vaccine for the novel coronavirus, but it may reduce the risk of the disease. 

Given that the flu vaccine will reduce respiratory infections that put one at more risk of bad outcomes from COVID-19 and that it will reduce the burden on our healthcare system, there are already very good reasons to get the flu vaccine. 

This new evidence should give you more reasons to get the flu vaccine. 




Michael Simpson
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