Some disturbing postings on social media have occurred lately. They have implied or outright stated that Flu shots may increase one’s risk of contracting COVID-19 or increase the likelihood of having a severe case if one does develop such an infection.
One such posting, and likely reposted by many, referred to two studies: one published in 2012 and the other this past January. It is worthwhile to quickly review these from an epidemiologist’s point of view to show why their conclusions cannot be used to extrapolate to the effect of Flu shots upon the risk of contracting COVID-19. Also, it is worth presenting several subsequent studies and reviews that will allay this concern.
The 2012 study (Increased risk of noninfluenza respiratory virus infections associated with receipt of inactivated influenza vaccine) looked at 115 children (69 vaccinated, and 46 not vaccinated). They ranged in age from 6-15 years old. The vaccine used (a trivalent inactivated influenza vaccine) is not the one currently used. They found a subsequent increased risk of non-influenza viral infections in children who received the shot. Coronaviruses were listed as one of the types found but they accounted for only a small portion of the non-influenza viruses evaluated, and, most important of all were very different from the coronavirus causing COVID-19 . Upon reviewing their data, the possibility of an association is not statistically significant- in other words the same finding could have happened by chance alone and not due to any cause and effect! Even if there had been an association found, the fact that the study had so few participants would make the findings weak. Also, for numerous reasons, conclusions from studies in children are often not generalizable to the adult population.
The other often-quoted study, published this past January (Influenza vaccination and respiratory virus interference among Department of Defense personnel during the 2017–2018 influenza season), looked at ~9,000 people, predominantly male, active duty service members between the ages of 18-35 years old. Some of their data has been used to show that inactivated influenza vaccine increases the risk of a coronavirus infection. The author of the study (Dr. Wolff) was disturbed enough by this misuse/misinterpretation of his data that he wrote a letter, this past June, to the Editor of Vaccine ,which published his study, stating the following:
“…..the results of this study cannot and should not be interpreted to represent any sort of relationship or association of influenza vaccination receipt and COVID-19 illness. Results from this study DO NOT support the anti-vaccination viewpoint of avoiding seasonal influenza vaccination, and in fact should be interpreted in the opposite manner, since significant protection against influenza was associated with vaccination receipt, and a slight decrease in the odds of infection from other respiratory viruses was also noted. Results from this study should not be applied to or interpreted with COVID-19 in any way.”
Unfortunately, there were problems in the method used to account for the results- particularly as pertains to coronaviruses. Based upon “corrected” statistical methods of data analysis, as suggested by other researchers, the findings do not really show any statistically increased risk of coronavirus-positive infections.
A paper published at the end of this May ( Influenza Vaccine Does Not Increase the Risk of Coronavirus or Other Noninfluenza Respiratory Viruses: Retrospective Analysis From Canada, 2010–2011 to 2016–2017 ), looking at nearly 11,000 Canadians seen by their primary care physicians, concluded that influenza vaccination had no effect upon coronaviruses or other non-influenza viruses in so far as increased or decreased risk of infection by them.
A preprint paper published online this September 3 (Could seasonal influenza vaccination influence COVID-19 risk?), looked through the medical literature and identified and analyzed 18 studies. To summarize their conclusions, it appears that the probability of an influenza vaccine causing an increased risk of a non-influenza respiratory infection is neither more nor less likely. The findings were weak for either conclusion. They did recommend that further studies be done. However, it is important to note that some of the papers reviewed were not necessarily looking at COVID-19 but, rather, at a large number of non-influenza viruses, and that close to half of the studies they reviewed involved children.
Finally, a recent Cleveland Clinic study (Safety of Influenza Vaccine during COVID-19) pre-published this September, looked at over 18,000 patients from this past influenza season (Fall of 2019-Winter of 2020) and compared those who received to those who didn’t receive an influenza vaccine. They found that influenza vaccination was not related to an increased incidence of COVID-19, nor did it result in an increased risk of hospitalization, ICU admission or in-hospital mortality in those who developed COVID-19.
Consequently, in contrast to what social media may be saying, getting a Flu shot does not appear to increase your risk of contracting COVID-19 or to increase its severity or complications. So, get your flu shot! There is also some evidence that a flu shot may be beneficial above and beyond just preventing influenza- but that was the subject of another blog.