Posted at 8:00 a.m.
After how long after injection do vaccines lose their effectiveness against hospitalizations and deaths?
Early studies showed great effectiveness, lasting up to several years. “According to the most recent data, efficacy against complications seems rather to persist for about six months in healthy people,” says Dr.D Maryse Guay, professor in the Faculty of Medicine and Health Sciences at the University of Sherbrooke. For some populations, such as the elderly and immunocompromised people, levels of protection may decline more rapidly. The duration can also vary according to the variants.
Why does vaccine immunity last so short compared to other vaccines?
“It is due to the virus itself, explains Alain Lamarre, professor and researcher specializing in immunology and virology at the National Institute for Scientific Research (INRS). The virus mutates frequently and manages to circumvent immunity, which is not the case with many other viruses. »
To infect a person, the virus must attach itself to receptors in human cells, in order to inject its genetic material into them. Current vaccines develop antibodies that recognize the surface protein of the virus, preventing it from attaching to cells. “Since the surface protein changes according to the different variants, the virus can evade the immune response,” explains Lamarre.
Note: all viruses mutate, but some do so much more slowly. “This is the case for measles, for example, which is fairly stable over time. The immune response is long lasting and there are not many variants that develop,” he says. The stability of the virus therefore allows the measles vaccine to be effective for life in most people fully vaccinated with two doses.
Is there a limit to the number of doses one can receive before it becomes harmful to the body?
No, answer the experts consulted by The Press. “There are no contraindications to receiving the same vaccine several times over a certain period,” says Mr. Lamarre. “The messenger RNA contained in vaccines is abolished very quickly by our system. There is no accumulation that is done, ”adds the DD Guy.
“What could happen, potentially, is that the vaccination is less effective, because you create a kind of tolerance, but that’s theoretical,” says Lamarre. For the moment, this is far from being the case, he observes. “Even after a fourth dose, the immune system still reacts and develops antibodies again,” he says.
How long after being infected with the virus should one go for a new dose of vaccine?
“It is clear that immediately after the infection, there is no point in being vaccinated, since we already have very good immunity”, says the DD Guy. The ideal would be to wait at least three months, she says. “A little longer is perhaps even better. »
According to data compiled by the United States Centers for Disease Control and Prevention (CDC), people who have already been infected with SARS-CoV-2 are at low risk of subsequent infection for at least six months. . “We are still waiting for the results of the next studies, but it is clear that getting vaccinated in less than three months is a waste of time”, summarizes the doctor.
Now that the fourth dose is available to everyone aged 18 and over, should we rush to get it?
“The usefulness of the fourth dose is currently poorly documented for people under 60,” says Mr. Lamarre. For the moment, there is still no recommendation from the Quebec Committee on Immunization (CIQ) for a 4and dose in healthy 18 to 60 year olds. “For those who received their third dose less than six months ago, they are quite well protected against severe disease. There is no rush to get a fourth,” he said. He recommends waiting until the end of the summer to get it instead. “Chances are good that there will be a seventh wave in the fall, so it could be a good time,” he says.
Should I get vaccinated every six months, all the time?
For the moment, specialists do not yet know how many boosters will be necessary or how often they will have to be administered. One thing is certain, “it is not desirable or sustainable for our health system to vaccinate the entire population every month”, argues the DD Guy. Further studies will be required before coming to a conclusion. “Perhaps we will go for an annual vaccination, but it will take more hindsight to determine it,” she concludes.