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Covid mRNA vaccines are injected into the deltoid. What is the process in which they spread from there to the rest of the body? Would there be a better immunization reaction if the second dose is injected elsewhere?

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    $\begingroup$ Why do you think the vaccine would need to spread to the rest of the body? $\endgroup$
    – Bryan Krause
    Jun 2 at 16:17
  • $\begingroup$ Mostly because of reports from Israel, US army and UK on miocardatis which may be linked to the vaccine in younger patients (although in the UK it is in low numbers compared with the others, but the age of the patients is older). If it is related, I'm trying to ascertain whether it could be from the mRNA instructions $\endgroup$ Jun 2 at 17:31
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    $\begingroup$ meta.stackexchange.com/questions/66377/what-is-the-xy-problem $\endgroup$
    – Bryan Krause
    Jun 2 at 17:43
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    $\begingroup$ @KyuriOseetea The vaccine doesn't spread; instead, the immune system spreads its responsive elements. $\endgroup$
    – jakebeal
    Jun 2 at 19:25
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The short answer to how vaccines are spread around the body is, quickly.

What is the process in which they spread from there to the rest of the body?

The reason vaccines are often delivered into muscle tissue is because muscle tissue is highly vascularized with both blood and lymphatic vessels. Immediately after a vaccine injection, the delivery vehicle (probably lipid nanoparticles or a viral vector) will quickly start diffusing into the blood, where it will be carried up to the heart, and from there begin spreading around the body (a lot of it ends up in the liver). Somewhat less quickly, in the lymph it will essentially start to generate an immune response immediately as it drains into lymph nodes and begins circulating throughout the lymphatic system (note the the desired immune response won't start until the genetic component of a vaccine has entered nucleated cells and has started being translated into the desired viral antigens).

Would there be a better immunization reaction if the second dose is injected elsewhere?

Don't know if I can definitively justify saying "no" to this question, but probably not. Muscle tissue seems to be the best injection site for vaccines. Deltoids are probably the best muscle because there's a lower chance of missing the intended injection site compared with some other muscles (e.g. glutes). And the systemic spread of vaccine components starts happening pretty fast after injection. In one animal study, researchers surgically removed a leg muscle 1 to 10 minutes after injection and still didn't prevent the related antibody response, or even really reduce it compared to animals with the muscle left intact.

Assorted References:

mRNA Vaccines: What Happens

Inadvertent subcutaneous injection of COVID-19 vaccine

The importance of injecting vaccines into muscle Different patients need different needle sizes

Unmodified mRNA in LNPs constitutes a competitive technology for prophylactic vaccines

Differential dependence on target site tissue for gene gun and intramuscular DNA immunizations (1997)

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