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The Times of India claims that:

With intramuscular vaccines (most of the COVID-19 vaccine shots are delivered through this mean right now), rubbing, pinching or massaging the injection site may interfere with the effectiveness of the vaccine. While it may seem to relieve and quench the soreness that follows inoculation, massaging at the point may seldom cause the drug to back up through the subcutaneous tissue present in the deepest layer of your skin, so, as a safety measure, massages or hard rubs are best avoided for the time being.

Is this any true? (e.g. Does it matter if I hardly press the arm or mildly?)

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  • If someone much smarter than I could read this Canadian paper specifically Section 7, what does reactogenicity mean? (FYA, this article deals with pain reduction in childhood vaccinations and talks about rubbing.)
    – CGCampbell
    Commented Dec 14, 2021 at 16:17
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    @CGCampbell "Reactogenicity" would be the "tendency to produce a reaction"; in this case, the sort of inflammatory reaction you get when you give a vaccine. By itself, it isn't specific about the type of reaction, so either an increase in soreness or a full-on life-threatening response could be considered "increased reactogenicity". The linked article they cite could be summarized "if you rub it too hard it hurts". Commented Dec 14, 2021 at 17:00
  • If this were true then it would be a standard practice of vaccination to tell you so, since nobody wants to impede the effectivity of a vaccination. Commented Dec 27, 2021 at 11:38

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Leakage through the injection canal does occur (Ignaut 2012), without manipulation and for short insulin needles (which are quite comparable to vaccination needles) it is about 0.35%, so quite negligible.

There are injection MO that help against leakage (special way of inserting needle, slow injection, leaving the needle in for 10 seconds, not massaging the site)

The Z-track method of injecting into the muscle relies on pulling on the skin, injecting , then releasing the skin, which will shift over the injection site, sealing the canal. Massaging the site might reopen the canal and will introduce additional pressure, so it might realistically lead to leakage.

Z-track injection by pulling skin sideways, injecting, then releasing. From https://www.pharmapproach.com/intramuscular-route-of-drug-administration-advantages-and-disadvantages/

Note that without Z-track injection, a massage might close the canal (via the same mechanism), so older manuals might encourage massage.

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