With the arrival of autumn, vaccination campaigns against influenza and Covid began and, in particular, a joint vaccination against the two viruses for those most at risk. But like (almost) every year, there is a fear that people will not rush to get vaccinated.
Recently, I remembered an exchange with one of my acquaintances. Arnaud, owner of a bar in the city center of Lille, asked me: “Do you think I should get vaccinated? » Before I even answered him, he started: “Finally, Covid is like the flu, it happens in winter, it causes the same illness and we can get vaccinated”.
I did not have time to explain to Arnaud in detail what differentiates these two viruses and what they may have in common. I will try to do it in this article… hoping that he will read it. And that he will understand that these viruses must not be taken lightly.
Two viruses that kill every year
From one year to the next, flu epidemics are not the same, in particular because the viral strains involved differ. On the other hand, there is one constant: the flu kills.
On average, there are 9,000 deaths every year due to this infection, according to Public Health France. The agency recalls that “the weight of seasonal flu is therefore considerable on the healthcare system but also on society in general (absenteeism, morbidity and mortality)”.
The annual toll paid to Covid-19 since its emergence in 2020 is even higher: in 4 years, this virus has caused at least 116,000 deaths in hospitals, nursing homes or other care structures, reports the INSEE. And we remember how the different waves of Covid-19 had undermined our entire health system, before vaccines were made available.
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Two airborne viral diseases
If the flu and Covid-19 are indeed two viral diseases, they are caused by very different viruses. Flus are caused by Influenza virus type A, B and – very rarely – C and D, while Covid-19 is caused by a coronavirus, SARS-CoV-2, and its numerous variants, which are subtypes of viruses whose genome differs by one or more mutations of that of the original virus.
The flu virus, like that of Covid-19, however, shares one thing in common: their mode of transmission. Both are transmitted by contact with contaminated nasal and salivary secretions, through the air (the viruses are found in the particles of secretions that we produce when sneezing, coughing, or even simply speaking or breathing).
In the case of the flu virus, the same for Covid, transmission is influenced by ambient temperature and humidity: the colder it is and the more humid the ambient air, the better the transmission. This occurs through close contact, but it can also occur several meters away. In addition, aerosol microparticles containing infectious viruses can persist for several hours in the ambient air.
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All these elements explain why wearing a mask and ventilating premises remain very effective tools in the fight against these two respiratory infections.
Both viruses can also persist on contaminated surfaces (door handles, soiled tissues not thrown in the trash, etc.). If the Covid virus only remains infectious for a few hours on surfaces, the flu virus can remain infectious for up to 24 hours, hence the importance of washing your hands well, particularly during epidemic periods.
Finally, let us point out that the reproduction number of seasonal influenza viruses (R0, which corresponds to the number of people that an infected individual contaminates on average) is around 1.5, while that of SARS variants- CoV-2 can exceed 2.5.
This greater contagiousness partly explains why SARS-CoV-2 is also the cause of epidemic waves during the summer, unlike the flu which is more slowed down by the summer period.
Similar symptoms in common forms
The duration of incubation (i.e. the time between contamination and the appearance of symptoms) is short in both infections: 24 to 72 hours on average, and up to 5 days after contamination by the influenza virus, 72 hours for the omicron variant of SARS-CoV-2.
In the case of both Covid and the flu, it is considered that infected people can be contagious during the incubation phase. Once symptoms are declared, it is estimated that people are contagious for up to 7 days for the flu and for at least 7 days for Covid.
In the vast majority of cases, people infected with one or other of these viruses will suffer from fatigue, fever, cough, headache and throat pain, as well as a runny nose. However, symptoms vary from person to person, and a significant number of infected people remain asymptomatic (they do not show symptoms of the disease), while remaining contagious.
In the case of Covid, it is estimated that 30 to 60% of people carrying the virus are asymptomatic (especially young children). When it comes to the flu, almost half of infections may be asymptomatic.
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Risks of complications to take into account
The influenza virus enters our body through the nose or mouth and then multiplies in the respiratory epithelium. This viral multiplication is localized, which explains why the flu results in respiratory damage. However, although the infection is localized, said damage can lead to the decompensation of other pathologies, that is to say disrupt diseases that were previously well controlled (diabetes or renal failure for example).
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The consequences of SARS-CoV-2 infection can be very different.
Despite a pronounced tropism for the respiratory system, this virus not only infects the cells of the respiratory tract but also those of other organs (nose, lungs, brain, intestine, kidneys, etc.). The potential long-term consequences of infections are not yet well understood, although it is known that some people will develop long-term forms of the disease.
Some specialists are particularly concerned about cognitive losses that could be associated with the infection. Work published at the beginning of October 2024 showed that one year after mild Covid, young and healthy people presented mild cognitive disorders. Another recent study found that the brains of older people with severe Covid were more severely affected.
This work echoes previous research which had already revealed that infection with SARS-CoV-2 accelerated brain aging, and caused microruptures of vessels in the brain.
Populations predisposed to severe forms of Covid and flu
Certain situations predispose to serious forms of these two infections: obesity, chronic diseases (diabetes, cardiovascular, pulmonary, renal diseases, etc.), pregnancy and the period following childbirth (postpartum).
The risk of developing a severe form is also higher in so-called immunocompromised people, whether this immunodepression is of genetic origin, linked to an illness such as cancer or HIV infection or whether it results from treatment with corticosteroids. long-term or chemotherapy.
People affected by these situations must protect themselves in particular, as do individuals around them who could be unintentional sources of contamination.
We also have to think about the children. Indeed, if they are very often infected by respiratory viruses and generally develop mild forms, they can transmit these viruses as well as adults… Depending on their age, we can make them aware of the risks of transmitting these viruses, teach them to put on a mask, ventilate the premises, etc.
Limited and ineffective antiviral treatments
Today, whether for flu or “simple” Covid, symptoms are treated with medications intended to reduce fever (antipyretics), sometimes with a nasal decongestant, rest and work stoppage if necessary. . Only serious forms (or situations presenting a risk of serious forms) require special care.
Regarding the treatment of influenza with antiviral drugs, only oseltamivir, better known under the brand name Tamiflu, is still reimbursed by Health Insurance. Zanamivir has not been used since 2020, as it is considered too ineffective.
In addition, these molecules are difficult to use because they must be administered within 48 hours after the onset of the disease, which makes their use difficult and reduces their effectiveness. However, oseltamivir remains used in hospitals to treat immunocompromised people, while waiting for more effective molecules to be available.
Against SARS-CoV-2, the first strategy consisted of testing the antiviral power of molecules already present on the market. Although this choice did not prove very effective, it nevertheless made it possible to identify a molecule, remdesivir, initially developed against the Ebola virus, as having a certain effect against SARS-CoV-2. Remdesivir is currently reserved for patients at high risk of developing a severe form, such as immunocompromised people.
Another molecule, nirmatrelvir, is also used, in combination with another drug, ritonavir, which prevents its degradation by the liver and prolongs its duration of action. The nirmatrelvir/ritonavir combination (known under the brand name Paxlovid) is today the first-line treatment for people at high risk of developing a severe form of Covid.
In short, the limited supply of antiretroviral drugs against these two infectious diseases also calls for vaccination which remains, we cannot repeat it enough, the best weapon to prevent serious forms of the flu like Covid.
