When should the third dose of Covid vaccine be given?
A possible third dose of Covid-19 vaccine could be done for two reasons: when there is evidence that the immunological memory induced by the two doses of the vaccine is decaying to such a degree that at some point the vaccinated are no longer protected and are exposed to reinfection or because a variant pi diffusiva of those in circulation that are no longer well recognized by vaccines and people, despite having completed the vaccination course, are exposed to severe forms of the disease. The two reasons can be contemporary.
What is the level of immune response below which the third dose would be necessary?
We do not know the “correlation of protection”, a measurable numerical level of immune response above which one is protected and, below which, one is again susceptible. So how do you know if and when the third dose of vaccine will be needed? It is based on empirical data deriving from the continuous measurement of the immune response over time (both antibody and T lymphocytes) and the number of reinfections in the already fully vaccinated population. We know that the immune response decreases with time, but as written we do not yet know what the point below which you are no longer protected. Based on ongoing clinical trials there is currently no evidence that those who were vaccinated 9-10 months ago with Pfizer, AstraZeneca or Moderna are more susceptible to the virus if exposed to infection. There are no tests that can predict the duration of protection, and this applies to all vaccines in use.
How can you understand the level of the immune response?
With serological tests, quantitative which measure the concentration of antibodies, qualitative which measure the ability of antibodies to neutralize the virus. For the general population, however, they are not recommended because antibodies are only part of the body’s immune response. It is normal for antibodies to decrease over time if there is no other contact with the virus. The “immunological memory” of T lymphocytes also counts, which among other things stimulate the production of antibodies against Sars-CoV-2 when necessary. The search for T lymphocytes for complex ee is generally done only on selected samples of vaccinated, generally it is done for research purposes.
Are there any categories for which the third vaccine might be recommended?
Surely the immunosuppressed (cancer patients, transplant patients) who need more booster doses to mount a barely acceptable immune response. Although there are no precise indications from the regulatory agencies, in many hospitals in Europe, including Italy, the third dose is often administered to this limited category of patients. Elderly or frail people with chronic conditions could also be candidates, but there is currently no clinical evidence to prove this is necessary.
What kind of vaccine will be done for a possible third dose?
Certainly with an mRna-based product even for those who have already been immunized with a viral vector vaccine such as AstraZeneca or Janssen. This is to prevent immunity against adenoviral vectors from neutralizing the effect of the vaccine. Sergio Abrignani, immunologist of the State University of Milan collaborated.
July 10, 2021 (change July 10, 2021 | 08:43)
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