Influenza vaccine may offer some protection against severe COVID-19 infection, studies suggest
A study conducted on more than 92,000 patients in Brazil suggests that an inactivated trivalent flu shot may protect you from getting severe COVID-19 by improving your innate immunity.
By now, most of us understand that those with chronic health conditions and the elderly are at higher risk of complications from COVID-19 compared to others. The risk of contracting the infection though is equal among all people, no matter the age.
In the absence of a treatment or a vaccine, human beings are relying on public health measures such as social distancing, face masks and hand washing for prevention.
Now, a study conducted on more than 92,000 patients in Brazil suggests that an inactivated trivalent flu shot may protect you from getting a severe COVID-19 infection by improving your innate immunity.
The study is still in the pre-print phase and has not yet been peer-reviewed.
Not first study
At least two other studies have indicated that a flu shot provides some protection against severe COVID-19.
One of these studies conducted in Italy indicated that areas in which more people took a quadrivalent flu vaccine had a lower death rate from COVID-19. The study suggested that the flu vaccine may train the immune system of a person to quickly recognise and clear away any harmful organisms invading the lungs.
However, this kind of response is more likely in a live attenuated vaccine (vaccine in which the live but weak form of the disease-causing pathogen is used) not in an inactivated vaccine (vaccines in which a dead virus or bacteria is used to generate immunity against the disease it causes).
An inactivated vaccine does not create as strong an immune response as a live attenuated vaccine does.
Another study on the elderly population in the US found that even a 10 percent increase in the flu vaccine coverage in an area can reduce the COVID-19 mortality rate in the area by 28 percent.
As per the study, unvaccinated people are at risk of persistent viral infections that decline their T-cell diversity, in a way suppressing their immune system. T-cell diversity helps fight infections better as it provides a bigger pool of T-cells to fight against a new pathogen and it also provides more flexible T-cell receptors to fight the pathogen even if it mutates.
An inactivated vaccine, on the other hand, does not induce virus-specific T-cells in a person and so this drawback of this vaccine may be coming handy now in protecting against COVID-19.
Flu virus specifically impairs the ability of T cells to kill virus-infected cells, Hence, it impairs a person’s immune system to fight against other pathogens including the COVID-19. Unvaccinated people are likely to have flu specific T cells in their blood, which can cause a strong inflammatory response and cytokine production and hence severe COVID-19.
The latest study
In the latest study, the researchers in Brazil found that a flu shot can reduce mortality by 20 percent when taken before the onset of COVID-19 and about 27 percent when the vaccine was given after the onset of COVID-19.
The study also suggested the same mechanism as the one pointed out in the Italian study - non-specific stimulation of the innate immune system. Non-specific immunity refers to the immunity that a vaccine generates against pathogens other than it was originally targeted against. This immunity has been seen with other vaccines like MMR and BCG, both of which are also being suggested for reducing the severity of COVID-19.
Also, this study mentioned certain similarities between SARS-CoV-2, the causative agent of COVID-19, and the flu virus. Both are RNA viruses for one (have RNA as their genetic material) and then they both have similar pathogenesis and transmission. So, chances are that our immune system recognises both these viruses through similar receptors.
The flu shot
Influenza or simply the flu is a viral disease caused by the influenza viruses. The disease affects the respiratory system and causes symptoms like cough, fever, sore throat, tiredness, muscle pain and runny nose. Pneumonia is one of the complications of the flu, along with sinus and ear infections.
Every year, the World Health Organisation recommends mandatory flu shots for the elderly, children and pregnant women, the high-risk population for influenza. The shots have to be taken yearly since the flu virus changes its genetic material that quickly. A new or updated vaccine that targets the new strains of the virus is needed to provide continued protection from the disease.
The flu shot for 2019-2020 was a trivalent (containing three strains of flu virus) or tetravalent (containing four strains of flu virus) vaccine containing either 2 strains of influenza A and one strain of influenza B virus or two strains each of influenza A and B virus. There are four types of influenza: A, B, C and D. Only influenza A and B can cause severe disease.
A flu shot can be either a live attenuated vaccine or an inactivated vaccine. The live vaccine can be given to anyone above the age of six years, while the inactivated vaccine is given to only healthy people (excluding pregnant women) in the age group of two to 49.
For more information, read our article on Mild vs Severe COVID-19 symptoms
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